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2011年第43届国际奥林匹克化学竞赛预备题

时间:2011-02-01


Preparatory Problems
43 International Chemistry Olympiad
Editor: Saim ?zkar
rd

Department of Chemistry, Middle East Technical University

Tel +90 312 210

3203, Fax +90 312 210 3200 e-mail icho2011@metu.edu.tr

January 2011 Ankara

2

Preparatory Problems

Problem Authors

O. Yavuz Ataman Sezer Aygün Metin Balc? ?zdemir Do?an Jale Hacalo?lu Hüseyin ???i Ahmet M. ?nal ?lker ?zkan Saim ?zkar Cihangir Tanyeli

Department of Chemistry, Middle East Technical University, 06531 Ankara, Turkey.

3

Preparatory Problems

Preface
We have provided this set of problems with the intention of making the preparation for the 43rd International Chemistry Olympiad easier for both students and mentors. We restricted ourselves to the inclusion of only a few topics that are not usually covered in secondary schools. There are six such advanced topics in theoretical part that we expect the participants to be familiar with. These fields are listed explicitly and their application is demonstrated in the problems. In our experience each of these topics can be introduced to well-prepared students in 2-3 hours. Solutions will be sent to the head mentor of each country by e-mail on 1 st of February 2011. We welcome any comments, corrections or questions about the problems via e-mail to icho2011@metu.edu.tr. Preparatory Problems with Solutions will be on the web in July 2011. We have enjoyed preparing the problems and we hope that you will also enjoy solving them. We look forward to seeing you in Ankara.

Acknowledgement
I thank all the authors for their time, dedication, and effort. All the authors are Professors in various fields of chemistry at Middle East Technical University. I also thank Dr. Murat Sümbül, Dr. Salih ?z?ubuk?u and Yunus Emre Türkmen for their ideas and valuable contributions regarding the preparatory problems for 43rd IChO. In both preparation and testing of practical problems, we are most grateful to Professor ?ahinde Demirci and the laboratory team members, our assistants, P?nar Akay, Seylan Ayan, Derya ?elik, Melek Din?, ?a?atay Dengiz, Zeynep ?nci Günler, Tu?ba Orhan, Suriye ?zlem, Burak Ural, and Emrah Y?ld?r?m. Ankara, 26 January 2011 Editor Prof. Dr. Saim ?zkar 4

Preparatory Problems

Contents
Physical constants, symbols, and conversion factors Fields of Advanced Difficulty 7 8 9 9 9 10 11 13 14 15 16 17 18 19 20 21 21 23 23 24 25 26 27 29

Theoretical problems
Problem 1 Problem 2 Problem 3 Problem 4 Problem 5 Problem 6 Problem 7 Problem 8 Problem 9 Problem 10 Problem 11 Problem 12 Problem 13 Problem 14 Problem 15 Problem 16 Problem 17 Problem 18 Problem 19 Problem 20 Problem 21 Superacids Stabilization of high-valent transition metal ions Colemanite mineral as boron source Magnesium compounds Nitrogen oxides and oxoanions Ferrochrome Xenon compounds Structure of phosphorus compounds Arsenic in water Amphoteric lead oxide Analyzing a mixture of calcium salts Breath analysis Decomposition kinetics of sulfuryl dichloride Clock reaction Mixing ideal gases Kinetics in gas phase Chemical Equilibrium Iodine equilibrium Molecular weight determination by osmometry Allowed energy levels and requirements for absorption of light Rotational and vibrational energy levels of a diatomic molecule

5

Preparatory Problems

Problem 22 Problem 23 Problem 24 Problem 25 Problem 26 Problem 27 Problem 28 Problem 29 Problem 30

Particle in a box: Cyanine dyes and polyenes Radioactive decay Enzyme-substrate interaction Amides NMR Spectroscopy Cyclitols Antiviral antibiotic Acyclic β-amino acids Life of Ladybug

31 33 34 35 36 40 42 45 47 50 51 53 56 59 62 64 66

Practical Problems, Safety
Problem 31 Problem 32 Problem 33 Problem 34 Problem 35 Problem 36 Problem 37 Preparation of trans-dichlorobis(ethylenediamine)-cobalt(III)chloride and kinetics of its acid hydrolysis Analysis of calcium salts Potassium bisoxalatocuprate(II) dihydrate: Preparation and analysis Synthesis and analysis of aspirin Determination of iron and copper by iodometric titration Phenol propargylation: Synthesis of 1-nitro-4-(prop-2-ynyloxy)benzene and (prop-2-ynyloxy)benzene Huisgen dipolar cycloaddition: Copper(I)-catalyzed triazole formation

6

Preparatory Problems

Physical constants, symbols, and conversion factors
Avogadro's constant, NA = 6.0221×1023 mol–1 Boltzmann constant, kB = 1.3807×10-23 J·K–1 Gas constant, R = 8.3145 J·K–1·mol –1 = 0.08205 atm·L·K–1·mol–1 Faraday constant, F = 96485 C·mol–1 Speed of light, c = 2.9979×108 m·s–1 Planck's constant, h = 6.6261×10-34 J·s Standard pressure, P° = 1 bar = 105 Pa Atmospheric pressure, Patm = 1.01325×105 Pa Zero of the Celsius scale, 273.15 K Mass of electron, me = 9.10938215×10-31 kg 1 nanometer (nm) = 10-9 m 1 micrometer (?m) = 10-6 m 1 electronvolt (eV) = 1.602×10-19 J

Periodic Table of Elements with Relative Atomic Masses
1
1 H 1.008 3 Li 6.941 11 Na 22.99 19 K 39.10 37 Rb 85.47 55 Cs 132.91 87 Fr (223)

18 2
4 Be 9.012 12 Mg 24.31 20 Ca 40.08 38 Sr 87.62 56 Ba 137.33 88 Ra 226.0

13
5 B 10.81 13 Al 26.98 31 Ga 69.72 49 In 114.82 81 Tl 204.38

14
6 C 12.01 14 Si 28.09 32 Ge 72.64 50 Sn 118.71 82 Pb 207.2

15
7 N 14.01 15 P 30.97 33 As 74.92 51 Sb 121.76 83 Bi 208.98

16
8 O 16.00 16 S 32.07 34 Se 78.96 52 Te 127.60 84 Po (209)

17
9 F 19.00 17 Cl 35.45 35 Br 79.90 53 I 126.90 85 At (210)

3
21 Sc 44.96 39 Y 88.91 57 La 138.91 89 Ac (227) 58 Ce 140.12 90 Th 232.04

4
22 Ti 47.87 40 Zr 91.22 72 Hf 178.49 104 Rf (261) 59 Pr 140.91 91 Pa 231.04

5
23 V 50.94 41 Nb 92.91 73 Ta 180.95 105 Ha (262) 60 Nd 144.24 92 U 238.03

6
24 Cr 52.00 42 Mo 95.96 74 W 183.84

7
25 Mn 54.94 43 Tc [98] 75 Re 186.21

8
26 Fe 55.85 44 Ru 101.07 76 Os 190.23

9
27 Co 58.93 45 Rh 102.91 77 Ir 192.22

10
28 Ni 58.69 46 Pd 106.42 78 Pt 195.08

11
29 Cu 63.55 47 Ag 107.87 79 Au 196.97

12
30 Zn 65.38 48 Cd 112.41 80 Hg 200.59

2 He 4.003 10 Ne 20.18 18 Ar 39.95 36 Kr 83.80 54 Xe 131.29 86 Rn (222)

61 Pm (145) 93 Np 237.05

62 Sm 150.36 94 Pu (244)

63 Eu 151.96 95 Am (243)

64 Gd 157.25 96 Cm (247)

65 Tb 158.93 97 Bk (247)

66 Dy 162.50 98 Cf (251)

67 Ho 164.93 99 Es (254)

68 Er 167.26 100 Fm (257)

69 Tm 168.93 101 Md (256)

70 Yb 173.05 102 No (254)

71 Lu 174.97 103 Lr (257)

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Preparatory Problems

Fields of Advanced Difficulty

Theoretical
Kinetics: Integrated first order rate equation; analysis of complex reaction mechanisms using the steady state approximation; determination of reaction order and activation energy. Thermodynamics: Relationship between equilibrium constant, electromotive force and standard Gibbs free energy; the variation of equilibrium constant with temperature. Quantum Mechanics: Energetics of rotational, vibrational, and electronic transitions using simple model theories. Molecular Structure and Bonding Theories: The use of Lewis theory, VSEPR theory and hybridization for molecules with coordination number greater than four. Inorganic Chemistry: Stereochemistry and isomerism in coordination compounds. Spectroscopy: Interpretation of relatively simple 13C- and 1H-NMR spectra; chemical shifts, multiplicities, coupling constants and integrals.

Practical
Column chromatograpy. Thin layer chromatography.

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Preparatory Problems, Theoretical

Theoretical problems
Problem 1 Superacids

The acids which are stronger than pure sulfuric acid are called superacids. Superacids are very strong proton donors being capable of protonating even weak Lewis acids such as Xe, H2, Cl 2, Br2, and CO2. Cations, which never exist in other media, have been observed in superacid solutions. George Olah received the Nobel Prize in Chemistry in 1994 for the discovery of carbocation generation by using superacids. The enhanced acidity is due to the formation of a solvated proton. One of the most common superacids can be obtained by mixing SbF5 and HF. When liquid SbF5 is dissolved in liquid HF (in molar ratio of SbF5/HF greater than 0.5) the SbF6- and Sb2F11- anions are formed, and the proton released is solvated by HF. a) Write balanced chemical equations to show the species formed when HF and SbF5 are mixed. b) Draw the structures of SbF6- and Sb2F11- (in both ions the coordination number of antimony is 6 and in Sb2F11- there is a bridging fluorine atom). c) Write the chemical equations for the protonation of H2 and CO2 in HF/SbF5 superacid solution. d) Draw the Lewis structure of HCO2+ including the resonance forms and estimate the H?O?C bond angle in each resonance form.

Problem 2

Stabilization of high-valent transition metal ions

Relatively few high-valent transition metal oxide fluoride cations are known. OsO3F+, OsO2F3+ and ?-F(OsO2F3)2+ are some of these, where ?-F indicates the F- ion bridging the two Os units. In a recent study (Inorg. Chem. 2010, 49, 271) the [OsO2F3][Sb2F11] salt has been synthesized by dissolving solid cis-OsO2F4 in liquid SbF5, which is a strong Lewis acid, at 25
o

C, followed by removal of excess SbF5 under vacuum at 0 oC. The crystal structure of

[OsO2F3][Sb2F11] determined by XRD reveals the existence of OsO2F3+ cation and fluoride bridged Sb2F11- anion. Under dynamic vacuum at 0 °C, the orange, crystalline [OsO2F3][Sb2F11] loses SbF5, yielding [?-F(OsO2F3)2][Sb2F11] salt. In both salts osmium is six-

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Preparatory Problems, Theoretical
coordinate in solid state, but in liquid SbF5 solution, both cation. a) Write balanced chemical equations for the formation of [OsO2F3][Sb2F11] and [?F(OsO2F3)2] [Sb2F11]. b) Draw all the possible geometrical isomers of trigonal bipyramidal OsO2F3+ cation. c) What is the oxidation number of Os in the OsO2F3+ and ?-F(OsO2F3)2+ cations? d) When we assume a free rotation around Os-F(bridging) bond, ?-F(OsO2F3)2+ cation complex can be represented as a mononuclear octahedral complex of osmium, [OsO2F3X]+, where X = F-OsO2F3. Assuming that X is a monodentate ligand, draw all possible geometrical isomers of [OsO2F3X]+ complex ion. Is there any optical isomer of [OsO2F3X]+?
19

F-NMR and Raman data are

consistent with the presence of five-coordinate osmium in the trigonal bipyramidal OsO2F3+

Problem 3

Colemanite mineral as boron source

Boron is an important element in the world from both strategic and industrial points of view. Although the element is not directly used, its compounds have a wide range of applications almost in all manufacturing areas, except food. Boron is oxophilic and, therefore, occurs primarily as oxide (borates) in nature. Borate minerals occur in a few locations in the world. The largest reserves of boron minerals are in the western part of Turkey. One of the most important borate minerals is colemanite with the formula 2CaO?3B2O3?5H2O. Boric acid (H3BO3) is produced in Turkey and Europe mainly from the reaction of colemanite with sulfuric acid. The reaction is carried out at temperatures above 80 °C. Calcium sulfate dihydrate (Gypsum, CaSO4·2H2O) crystallizes from the reaction solution and the crystals are filtered out from the hot solution. Subsequently, boric acid crystallizes from the solution when it is cooled down to room temperature. Filtration of gypsum crystals from the reaction solution is a crucial process in the boric acid production for achieving high purity and high efficiency, as the subsequent crystallization of boric acid from the supernatant solution is substantially affected by contaminations. The reaction of sulfuric acid with colemanite takes place in two steps: In the first step colemanite is dissolved in sulfuric acid forming the calcium(II) ion and boric acid. In the second step, calcium sulfate, formed from Ca2+ and SO42? ions, precipitates as gypsum

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Preparatory Problems, Theoretical
crystals. In an experiment, 184.6 g colemanite containing 37.71% wt. B2O3 and 20.79% wt. CaO is dissolved in aqueous sulfuric acid yielding initially 1.554 M boric acid at 80 ?C. The reaction is carried out in a closed system so that the volume of the solution remains essentially constant. The saturation concentration of calcium ion in this solution is [Ca2+]sat = 0.0310 M at 80 °C. a) Write a balanced equation for the dissolution of colemanite in sulfuric acid. b) Calculate the amount of gypsum obtained from the crystallization. c) Calculate the mass of calcium ion remained in the solution. d) Calculate the theoretical amount of boric acid that can be obtained in this experiment. e) After hot filtration of gypsum crystals, boric acid is obtained by crystallization when the solution is cooled down to room temperature. The boric acid obtained is still contaminated by sulfate ions. The sulfur contamination is not desired in industrial use of boric acid, such as production of borosilicate glasses. Can recrystallization of boric acid in water remove the sulfate contamination of the product?

Problem 4

Magnesium compounds

Magnesium is one of the important elements in human body. Hundreds of biochemical reactions that drive energy metabolism and DNA repair are fueled by magnesium. Over 300 different enzymes rely on magnesium to facilitate their catalytic action. Magnesium maintains blood pressure and relaxes blood vessels and arteries. Magnesium deficiency leads to physiological decline in cells setting the stage for cancer. Among the numerous available magnesium dietary supplements, magnesium citrate has been reported as more bioavailable than the most commonly used magnesium oxide. Magnesium is a highly flammable metal. Once ignited, it is difficult to extinguish as it is capable of burning in water, carbon dioxide, and nitrogen. a) Write a balanced equation for the formation of magnesium oxide by reaction of magnesium with i. oxygen, O2 ii. carbon dioxide, CO2 b) Magnesium hydroxide is formed by reaction of Mg or MgO with H2O. Write a balanced equation for the formation of magnesium hydroxide from the reaction of H2O with

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Preparatory Problems, Theoretical
i. Mg ii. MgO c) When magnesium metal is heated in N2 atmosphere the white-yellow compound A is formed. Hydrolysis of A yields the colorless gas B which has basic character when dissolved in water. The reaction of B with aqueous solution of hypochlorite ion generates chloride ion, water, and the molecular compound C which is soluble in water. The reaction of B with hydrogen peroxide also produces the compound C and water. When the colorless gas B is heated with sodium metal, a solid compound D and hydrogen gas are produced. The reaction of compound D with nitrous oxide produces gaseous ammonia, solid sodium hydroxide, and a solid compound E. When the solid E is heated it decomposes to sodium metal and nitrogen gas. Write balanced equations for the formation of each compound A, B, C, D, and E. d) Draw the Lewis structure of the anion present in compound E. Choose the most stable resonance structure. e) Compound C was first used as rocket fuel during World War II. Today, it is used as a lowpower propellant in spacecrafts. In the presence of certain catalysts such as carbon nanofibers or molybdenum nitride supported on alumina, one of the decomposition reactions of C involves production of ammonia and nitrogen gas. Write a balanced equation for the decomposition reaction of compound C generating ammonia and nitrogen gas. f) Estimate the energy associated with the decomposition of compound C into ammonia and nitrogen gas and standard enthalpy of formation of NH3 at 298 K. Standard enthalpy of formation of liquid and gaseous C are 50.6 and 95.4 kJ·mol-1, respectively, at 298 K. Average bond energies of N≡N, N=N, N-N and N-H are 946, 418, 163, and 389 kJ·mol-1, respectively, at 298 K. g) In an experiment, 2.00 mL of C is placed in a 1.00 L evacuated reaction vessel containing a suitable catalyst at 298 K. After decomposition, the reaction vessel is cooled down to 298 K. Calculate the final pressure inside the vessel (density of liquid C is 1.0045 g·cm-3). h) Calculate the work done if isothermal expansion of the reaction vessel discussed in part (g) occurs against the atmospheric pressure of 1 atm.

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Preparatory Problems, Theoretical

Problem 5

Nitrogen oxides and oxoanions

Nitrogen occurs mainly in the atmosphere. Its abundance in Earth`s Crust is only 0.002% by mass. The only important nitrogen containing minerals are sodium nitrate (Chile saltpeter) and potassium nitrate (saltpeter). Sodium nitrate, NaNO3, and its close relative sodium nitrite, NaNO2, are two food preservatives with very similar chemical formulae, but different chemical properties. Sodium nitrate helps to prevent bacterial colonization of food. Sodium nitrite is a strong oxidizing agent used as a meat preservative. As in the case of almost any food additive or preservative, sodium nitrate is linked to several adverse reactions in susceptible people. Consuming too much sodium nitrate can cause allergies. Excessive ingestion of the preservative can also cause headaches. a) Draw the Lewis structures for the anions of these two salts including all possible resonance forms. Which one of these two anions has shorter N-O bond distance? b) Zn reduces NO3- ions to NH3 in basic solution forming tetrahydroxozincate(II) ion. Write a balanced equation for the reaction between zinc and ammonia in basic solution. c) When a strong base is gradually added to a solution containing Zn2+ ions a white precipitate of Zn(OH)2 first forms (Ksp = 1.2×10-17 for Zn(OH)2). To a 1.0 L solution of 5.0×10-2 mol Zn2+ ions, 0.10 mol OH- is added. Calculate the pH of this solution. d) When more base is added to the solution, the white precipitate of Zn(OH)2 dissolves forming the complex ion Zn(OH)42-. The formation constant for the complex ion is 4.6×1017. Calculate the pH of the solution in part (c) when 0.10 mol OH- ion is added (assuming the total volume does not change). e) A mixture containing only NaCl and NaNO3 is to be analyzed for its NaNO3 content. In an experiment, 5.00 g of this mixture is dissolved in water and solution is completed to 100 mL by addition of water; then a 10 mL aliquot of the resulting solution is treated with Zn under basic conditions. Ammonia produced during the reaction is passed into 50.0 mL of 0.150 M HCl solution. The excess HCl requires 32.10 mL of 0.100 M NaOH solution for its titration. Find the mass % of NaNO3 in the solid sample. f) Both NaCl and NaNO3 are strong electrolytes. Their presence in solution lowers the vapor pressure of the solvent and as a result freezing point is depressed. The freezing point depression depends not only on the number of the solute particles but also on the solvent itself. The freezing point depression constant for water is Kf = 1.86 °C/molal. Calculate the freezing point of the solution prepared by dissolving 1.50 g of the mixture

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Preparatory Problems, Theoretical
described in (d) consisting of NaCl and NaNO3 in 100.0 mL water. Density of this solution is d = 0.985 g·cm-3. g) N2H4 is one of the nitrogen compounds which can be used as a fuel in hydrazine fuel cell. Calculate the standard free energy change for the fuel cell reaction given below. N2H4(g) + O2(g) → N2(g) + 2 H2O(l) The standard potentials are given below: O2(g) + 2H2O(l) + 4e- → 4OH-(aq) N2(g) + 4H2O(l) +4e- → N2H4(g) + 4OH-(aq) E° = 1.23 V E° = - 0.33 V

h) The free energy change is related to the maximum amount of work that can be obtained from a system during a change at constant temperature and pressure. The relation is given as -?G = wmax. Calculate the maximum amount of work that can be obtained from the fuel cell which consumes 0.32 g N2H4(g) under standard conditions.

Problem 6

Ferrochrome

Chromium is one of the most abundant elements in Earth’s Crust and it is mined as chromite mineral, FeCr2O4. South Africa, Kazakhstan, India, Russia, and Turkey are substantial producers. For the production of pure chromium, the iron has to be separated from the mineral in a two step roasting and leaching process. 4 FeCr2O4(s) + 8 Na2CO3(s) + 7 O2(g) → 8 Na2CrO4(s) + 2 Fe2O3(s) + 8 CO2(g) 2 Na2CrO4(s) + H2SO4(aq) → Na2Cr2O7(s) + Na2SO4(aq) + H2O(l) Dichromate is converted to chromium(III) oxide by reduction with carbon and then reduced in an aluminothermic reaction to chromium. Na2Cr2O7(s) + 2 C(s) → Cr2O3(s) + Na2CO3(s) + CO(g) Cr2O3(s) + 2 Al(s) → Al 2O3(s) + 2 Cr(s)
a) Calculate the mass of Cr that can be theoretically obtained from 2.1 tons of ore which

contains 72.0% FeCr2O4 mineral. b) Chromium, due to its strong corrosion resistance, is an important alloying material for steel. A sample of certain steel is to be analyzed for its Mn and Cr content. Mn and Cr in a 5.00 g steel sample are oxidized to MnO4- and Cr2O72-, respectively, via a suitable treatment to yield 100.0 mL solution. A 50.0 mL portion of this solution is added to BaCl2

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Preparatory Problems, Theoretical
and by adjusting pH, chromium is completely precipitated as 5.82 g BaCrO4. A second 50.0 mL portion of the solution requires exactly 43.5 mL of 1.60 M Fe2+ for its titration in acidic solution. The unbalanced equations for the titration reactions are given below. MnO4-(aq) + Fe2+(aq) + H+(aq) → Mn2+(aq) + Fe3+(aq) Cr2O72-(aq) + Fe2+(aq) + H+(aq) → Cr3+(aq) + Fe3+(aq) Balance the equations for the titration reactions. c) Calculate the % Mn and % Cr in the steel sample.

Problem 7

Xenon compounds

Xenon, although present in the earth atmosphere in trace level, has several applications. It is used in the field of illumination and optics in flash and arc lamps. Xenon is employed as a propellant for ion thrusters in spacecraft. In addition, it has several medical applications. Some of xenon isotopes are used in imaging the soft tissues such as heart, lung, and brain. It is used as a general anesthetic and recently its considerable potential in treating brain injuries, including stroke has been demonstrated. Xenon being a member of noble gases has extremely low reactivity. Yet, several xenon compounds with highly electronegative atoms such as fluorine and oxygen are known. Xenon reacts with fluorine to form three different xenon fluorides, XeF2, XeF4 and XeF6. All these fluorides readily react with water, releasing pure Xe gas, hydrogen fluoride and molecular oxygen. The oxide and oxofluorides of xenon are obtained by partial or complete hydrolysis of xenon fluorides. Xenon trioxide can be obtained by the hydrolysis of XeF4 or XeF6. The hydrolysis of XeF4 yields XeO3, Xe, HF, and F2. However, hydrolysis of XeF6 produces only XeO3 and HF. When partially hydrolyzed, XeF4 and XeF6 yield XeOF2 and XeOF4, respectively, in addition to HF.

a) Write balanced equations for the generation of
i. XeO3 by hydrolysis of XeF4

ii. XeO3 by hydrolysis of XeF6 iii. XeOF2 by partial hydrolysis of XeF4 iv. XeOF4 by partial hydrolysis of XeF6

b) Draw the Lewis structures and give the hybridization at the central atom of

15

Preparatory Problems, Theoretical
i. XeF2

ii. XeF4 iii. XeO3 iv. XeOF2 v. XeOF4

Problem 8

Structure of phosphorus compounds

Phosphorus is very reactive and, therefore, never found in the native elemental form in the Earth's Crust. Phosphorus is an essential element for all living organisms. It is the major structural component of bone in the form of calcium phosphate and cell membranes in the form of phospholipids. Furthermore, it is also a component of DNA, RNA, and ATP. All energy production and storage, activation of some enzymes, hormones and cell signaling molecules are dependent on phosphorylated compounds and phosphorylation. Compounds of phosphorus act as a buffer to maintain pH of blood and bind to hemoglobin in red blood cells and affect oxygen delivery. Phosphorus has five valence electrons as nitrogen, but being an element of the third period, it has empty d orbitals available to form compounds up to six coordination number. One allotrope of phosphorus is the white phosphorus which is a waxy solid consisting of tetrahedral P4 molecules. White phosphorus is very reactive and bursts into flame in air to yield the phosphorus(V) oxide P4O10. Its partial oxidation in less oxygen yields the phosphorus(III) oxide P4O6. Disproportionation of white phosphorus in basic solution yields the gaseous phosphine, PH3 and hypophosphite ion, H2PO2-. Phosphorous acid, H3PO3 and phosphoric acid, H3PO4 can be produced by the reaction of P4O6 or P4O10 with water, respectively. White phosphorus reacts with halogens to yield halides with general formulae PX3 and PX5. Oxidation of PCl3 forms phosphoryl trichloride, POCl 3. Reaction of PCl5 with LiF yields LiPF6 which is used as an electrolyte in lithium-ion batteries.

a) Write balanced equations for the preparation of
i. ii. iii. PH3 PCl3 PCl5

iv. P4O6

16

Preparatory Problems, Theoretical
v. P4O10

vi. H3PO3 vii. H3PO4 viii. POCl3 ix. LiPF6

b) Draw the Lewis structures of the following molecules or ions, including the resonance
forms if any. i. ii. iii. v. PCl3 PCl5 PO4-3 PF6-

iv. POCl3

c) Draw the structures of the phosphorus oxides P4O6 and P4O10, starting with tetrahedral P4
skeleton. Each of six oxygen atom will be bridging two phosphorus atoms on an edge. An additional oxygen atom will be bonded to each phosphorus atom as terminal oxogroup in the case of P4O10.

d) Using the Valence Shell Electron Pair Repulsion model determine the geometry of the
following molecules or ions. i. PCl3

ii. POCl3 iii. PCl5 iv. PF6-

e) What is the hybridization at phosphorus atom in the following molecules or ions?
i. ii. PCl3 POCl3

iii. PCl5 iv. PF6-

Problem 9

Arsenic in water

Arsenic is known as a pollutant in environment and a toxic element. However, in December 2010 researchers of the National Aeronautics and Space Administration (NASA) of USA

17

Preparatory Problems, Theoretical
reported a species of bacterium in Mono Lake, California, that can use arsenic instead of phosphorus in biological molecule structures. It seems that monitoring concentration and identities of arsenic species in water will become even more important in near future. In natural waters, arsenic is present in the form of oxoacids: Arsenous or arsenic acid with oxidation states of +3 and +5, respectively. The source of arsenic in natural waters is often of geological origin. Arsenous acid and arsenic acid have the following dissociation constants. H3AsO3 H3AsO4 Ka1 = 5.1×10-10 Ka1 = 5.8×10-3 Ka2 = 1.1×10-7 Ka3 = 3.2×10-12

In aqueous systems, oxidation state of arsenic is dependent on the presence of oxidants and reductants, dissolved oxygen plays an important role. World Health Organization (WHO) has established a maximum total arsenic concentration of 10 ?g/L in drinking water; this value has been adapted by many countries. In a water sample obtained from a river as a potential source of drinking water, pH value is found to be 6.50. Using atomic absorption spectrometry, speciation analysis is also performed and arsenic(III) and arsenic(V) concentrations are found to be 10.8 ?g/L and 4.3 ?g/L, respectively. a) Calculate the total molar concentration for arsenic(III) and arsenic(V) inorganic species in the system, assuming that these are the only arsenic species present. b) What will be the predominant molecular or ionic species for arsenic(III) at pH = 6.50? Write the formula(s). c) What will be the predominant molecular or ionic species for arsenic(V) at pH = 6.50? Write the formula(s). d) Calculate the molar concentration(s) of arsenic(III) species suggested in (b). e) Calculate the molar concentration(s) of predominant arsenic(V) species suggested in (c). f) arsenic(III) is known to be significantly more toxic to human as compared to arsenic(V). Is it advantageous or disadvantageous to have oxidizing agents such as dissolved oxygen in water?

Problem 10

Amphoteric lead oxide

In aqueous media, Pb2+ ions form a precipitate, PbO, which is an amphoteric oxide. In acidic medium, only Pb2+ species is present; with increasing pH, PbO and Pb(OH)3- are formed in

18

Preparatory Problems, Theoretical
appreciable quantities. The important equilibria for lead species in aqueous medium are given below: Reaction 1 Reaction 2 PbO(s) + H2O(l) PbO(s) + 2H2O(l) Pb2+(aq) + 2 OH-(aq) Pb(OH)3-(aq) + H3O+(aq) Ksp = 8.0×10-16 Ka = 1.0×10-15

a) The amphoteric PbO completely dissolves when pH is sufficiently low. When initial
concentration of Pb2+ is 1.00×10-2 mol·L-1, what is the pH at which PbO starts to precipitate?

b) Starting from the value in (b), when pH is increased to a certain value, precipitate is
redissolved. At what pH value does the precipitate dissolve completely?

c) Write a general expression for molar solubility, s, of PbO. d) Theoretically, the minimum solubility is achieved when pH is 9.40. Calculate the molar
concentration of all the species and the molar solubility at this pH.

e) Calculate the pH range where the solubility is 1.0×10-3 mol·L-1 or lower.

Problem 11

Analyzing a mixture of calcium salts

When a 5.000 g mixture of CaCO3, Ca(HCO3)2, CaCl2 and Ca(ClO3)2 is heated at elevated temperature gaseous CO2, H2O, and O2 are evolved. The gases evolved exert a pressure of 1.312 atm in an evacuated 1.000 L cylinder at 400.0 K. When the temperature inside the cylinder is decreased to 300.0 K, the pressure drops to 0.897 atm. The vapor pressure of water at this temperature is 27.0 torr. The gas in the cylinder is used to combust an unknown amount of acetylene C2H2. The enthalpy change during the combustion process is determined as -7.796 kJ with the use of a calorimeter. ?fH° 2H2(g)) = 226.8 kJ.mol-1; ?fH° (C (CO2(g)) = -393.5 kJ.mol-1; ?fH° 2O(g)) = -241.8 kJ.mol-1; ?vapH° (H2O(l)) = 44.0 kJ.mol -1 (H 298K a) Write balanced equations for the possible decomposition reactions generating gases. b) Write a balanced equation for the combustion of C2H2. c) Calculate the total number of moles of gases produced in the cylinder. d) Calculate the number of moles of O2 that was present in the cyclinder. e) Calculate number of moles of CO2 and H2O produced. f) Calculate the weight percentage of CaCO3 and CaCl2 in the original mixture.

19

Preparatory Problems, Theoretical

Problem 12

Breath analysis

Ethanol is dissolved in blood and distributed to organs in the body. As a volatile compound, ethanol can be vaporized quite easily. In lungs, ethanol can change its phase from liquid to gaseous and, hence, it can be exhaled with air. Since the concentration of alcohol vapor in lungs is directly related to its concentration in blood, blood alcohol concentration can be measured using a device called a breathalyzer. In one of the older versions of breathalyzer, a suspect breathes into the device and exhaled air is allowed to pass through a solution of potassium dichromate which oxidizes ethanol to acetic acid. This oxidation is accompanied by a color change from orange to green and a detector records the change in intensity, hence, the change in color, which is used to calculate the percentage of alcohol in breath. When the oxidation of alcohol by potassium dichromate is carried out in an electrochemical cell, either the electrical current generated by this reaction or the change in the electromotive force can be measured and used for the estimation of alcohol content of blood. a) Write a balanced equation for the oxidation of ethanol by the dichromate ion in acidic solution. b) If the standard potential for the reduction of Cr2O72- to Cr3+ is 1.330 V and reduction of acetic acid to ethanol is 0.058 V, calculate the standard electromotive force E° for the overall reaction and show that overall reaction is spontaneous at 25 °C and 1.0 bar. c) In a breathalyzer which uses oxidation of ethanol, the volume of solution is 10.0 mL. When a suspect breathes into the device, 0.10 A of current flow is recorded for 60 s. Calculate the mass of alcohol per volume of exhaled breath. d) In calculating the alcohol content of blood from the amount of alcohol in a breath, the “2100:1 partition ratio” needs to be considered. The ratio states that 2100 mL of expired air (breath) contains the same amount of ethanol as 1 mL of blood. Alternatively, each milliliter of blood has 2100 times the amount of ethanol as each milliliter of expired air. If the volume of expired air described in part (c) is 60.0 mL, calculate the amount of alcohol per mL of blood. e) Cr3+ precipitates in basic solution as Cr(OH)3. The solubility product of chromium(III) hydroxide is 6.3×10-31 at 25 °C. Calculate the standard potential for the reduction of Cr(OH)3 to Cr. Standard potential for the reduction of Cr3+ to Cr is -0.74 V.

20

Preparatory Problems, Theoretical

Problem 13

Decomposition kinetics of sulfuryl dichloride

Sulfuryl dichloride (SO2Cl2) is a compound of industrial, environmental and scientific interest and widely used as chlorinating/sulfonating agent or as component of the catholyte system in batteries. At room temperature, SO2Cl 2 is a colorless liquid with a pungent odor; its boiling point is 70 ° It decomposes to SO2 and Cl2 when heated to or above 100 ° C. C. SO2Cl2(g) → SO2(g) + Cl2(g) An empty container is filled with SO2Cl 2. Its decomposition to SO2 and Cl2 is followed by monitoring the change in total pressure at 375 K. The following data are obtained. Time (s) P total (atm) 0 2500 5000 1.105 7500 1.152 10000 1.197

1.000 1.053

a) By graphical approach, show that the decomposition is a first order reaction and calculate the rate constant at 375 K. b) When the same decomposition reaction is carried out at 385 K, the total pressure is found to be 1.55 atm after 1 h. Calculate the activation energy for the decomposition reaction. c) There will be a negligible amount of SO2Cl2(g) in the reaction vessel after a long period of time. Therefore, the content of the vessel might be considered to be a mixture of SO2 and Cl2 gases. SO2(g) is separated from Cl 2(g) as H2SO4 and Cl2(g) is used to construct a Cl2/Cl- electrode. This electrode is combined with a Cu2+/Cu electrode to make a Galvanic cell. Which electrode is the cathode? E°(Cu2+/Cu) = +0.36 V and E°(Pt/Cl2, Cl-) = +1.36 V d) Calculate the ?G° for the cell reaction given in (c). e) A possible way for separating SO2 and Cl2 from each other is to pass the mixture over solid CaO which will convert all SO2 to CaSO3, a strong electrolyte. Calculate the pH of a 0.020 M CaSO3 solution. For H2SO3 Ka1 = 1.7×10-2 and Ka2 = 6.4×10-8.

Problem 14

Clock reaction

The iodine clock reaction is a classical chemical clock demonstration experiment to display chemical kinetics in action. In this reaction two clear solutions are mixed and after a short time delay, the colorless liquid suddenly turns to a shade of dark blue. The iodine clock

21

Preparatory Problems, Theoretical
reaction has several variations. One of them involves the reaction between

peroxydisulfate(VI) and iodide ions: Reaction A: S2O82-(aq) + 3 I-(aq) → 2 SO42-(aq) + I3-(aq)

The I3- ion formed in Reaction A reacts immediately with ascorbic acid (C6H8O6) present originally in the solution to form I- ion (Reaction B). Reaction B: C6H8O6(aq) + I3-(aq) → C6H6O6(aq) + 3 I-(aq) + 2 H+(aq)

When all the ascorbic acid present in the solution is consumed, the I3- ion generated in Reaction A forms a blue colored complex with starch present in solution (Reaction C). Reaction C: I3-(aq) + starch → Blue-complex.

Thus, the time t elapsed between mixing the reactants and the appearance of the blue color depends on the amount of I3- ion formed. Therefore 1/t can be used as a measure of reaction rate. At 25 °C, 25.0 mL (NH4)2S2O8, 25.0 mL KI, 5.0 mL 0.020 M C6H8O6, and 5.0 mL starch solutions are mixed with different initial concentrations of (NH4)2S2O8 and KI, and the elapsed time t for the appearance of blue color is measured. All the data are tabulated below.

Experiment No 1 2 3 4

[(NH4)2S2O8]o (mol/L) 0.200 0.100 0.050 0.200

[KI]o (mol/L) 0.200 0.200 0.200 0.100

t (s) 20.5 41.0 82.0 41.0

a) Find the rate law for Reaction A using the data given in Table. b) Using the data for the experiment 1, find the initial rate of Reaction A in mol·L-1·s-1. c) Calculate the rate constant for Reaction A at 25 °C. d) The following mechanism is proposed for Reaction A: I-(aq) + S2O82-(aq) IS2O83-(aq) I+(aq) + I-(aq) IS2O83-(aq)

2 SO42-(aq) + I+(aq) I2(aq)

22

Preparatory Problems, Theoretical
I2(aq) + I-(aq) I3-(aq)

Derive an equation for the rate of formation of I3-(aq) assuming that the steady-state approximation can be applied to all intermediates. Is the given mechanism consistent with the rate law found in part (a)? e) Ascorbic acid is a weak diprotic acid. In order to find its first acid dissociation constant, Ka1, 50.0 mL of 0.100 M ascorbic acid solution is titrated with 0.200 M NaOH solution. The pH of solution is measured as 2.86 after addition of 1.00 mL NaOH solution. Calculate acid dissociation constant Ka1 for ascorbic acid. f) Give the predominant species present at pH = 7.82 if Ka2 for ascorbic acid is 2.5×10-12.

Problem 15

Mixing ideal gases

Two rigid containers in thermal equilibrium at 298 K connected by a valve are isolated from the surroundings. In one of the containers, 1.00 mol of He(g) and 0.50 mol of A(g) are present at 1.00 atm. In the other container, 2.00 mol of Ar(g) and 0.50 mol of B2(g) are present at 1.00 atm. a) Predict whether the entropy will increase or decrease when the valve separating the two containers is opened assuming that no chemical reaction takes place. b) Predict whether the entropy will increase or decrease, stating all factors that will have contribution, if a chemical reaction takes place according to the following equation when the valve separating the two containers is opened. A(g) + ?B2(g) → BA(g) c) ?H° = - 99.0 kJ 298

Assuming that all the gases present are ideal, calculate the final pressure at the end of the reaction. The total heat capacity of two containers is 547.0 J/° C.

Problem 16

Kinetics in gas phase

The gas phase reaction A2(g) + 2 B(g) → 2 AB(g) is accelerated by catalyst C. The overall rate constant is found to increase linearly with the catalyst concentration. Following measurements are done at 400 K with [C] = 0.050 mol·L-1:

23

Preparatory Problems, Theoretical
Experiment No 1 2 3 [A2] (mol·L-1) 0.010 0.010 0.100 [B] (mol·L-1) 0.10 0.20 0.20 Initial rate ( mol·L-1·s-1) 1.600×10-10 3.200×10-10 1.012×10-9

a) What is the rate law of this reaction? b) Calculate the numerical value of koverall at 400 K. c) For this hypothetical reaction following mechanism was proposed.
A2(g)
k1

2 A(g)

fast equilibrium ABC(g) slow step

k-1

A(g) + B(g) +C(g) ABC(g)

AB(g) + C(g)

Check that the suggested mechanism gives the equation for the overall reaction.

d) Show that the suggested mechanism is consistent with the rate law determined
experimentally.

e) Calculate the dissociation enthalpy of A2 bond using the following information:
? ? At 400 K, when [A2] is 1.0×10-1 mol·L-1, [A] is 4.0×10-3 mol·L-1. When the first experiment is repeated at 425 K, the initial reaction rate increases to a three-fold value. ? Activation energy of the slowest step is 45.0 kJ.

Problem 17

Chemical Equilibrium

Iodine is an essential trace element for life and is the heaviest element commonly needed by living organisms. At high temperatures an equilibrium between I2(g) and I(g) takes place. The following table summarizes the initial pressure of I2(g) and the total pressure when the equilibrium is reached at the given temperatures. T (K) P(I2) (atm) Ptotal (atm) 1073 0.0631 0.0750 1173 0.0684 0.0918

a) Calculate ?H°, ?G° and ?S° at 1100 K. (Assume that ?H° and ?S° are independent of
temperature in the temperature range given.)

24

Preparatory Problems, Theoretical
b) Calculate the mole fraction of I(g) in the equilibrium mixture when the numerical value of
Kp is the half of the total pressure.

c) Assuming ideal gas behavior for I2(g) and I(g), calculate the bond energy of I2 at 298 K. d) Calculate the wavelength of radiation that must be used to dissociate I2(g) at 298 K. e) In an experiment, when a sample of I2(g) is irradiated by a laser beam of λ =825.8 nm, at
a rate of 20.0 J·s-1 for 10.0 s, 1.0×10-3 mol of I(g) is produced. Calculate the quantum yield for the dissociation process (i.e., the number of moles of I2 dissociated per mole of photons absorbed by the system).

Problem 18

Iodine equilibrium

Drinking water may contain small amount of some contaminants that are harmful to living organisms. Iodine is used as a disinfectant for drinking water for the International Space Station Alpha. Aqueous I2 forms a number of inorganic derivatives, such as hypoiodous acid, HOI; iodate, IO3-; iodide, I- and triiodide, I3-. An equilibrium reaction takes place involving I2, Iand I3- in water according to the following equation;

I2(aq) + I-(aq)

I3-(aq)

When dichloromethane, CH2Cl2 is added to aqueous solution of iodine, I2 is distributed in water and CH2Cl 2 phases according to the following equilibrium process. The equilibrium constant for the distribution is 150.

I2 (aq)

I2 (CH2Cl2)

a) For the homogenous equilibrium reaction which species acts as a Lewis acid? b) One method for determining the concentration of I2 and I3- in a solution is the titration with
a standard solution of S2O32-. An oxidation-reduction reaction takes place when I2 or I3interacts with S2O32- yielding I- and S4O62-. Write the balanced equations for chemical reactions that take place during the titration of I2 and I3- with S2O32-. Indicate the oxidant and the reductant in each reaction? Give the oxidation state of S in Na2S2O3.

c) In order to determine the equilibrium constant of the reaction involving I2, I- and I3- in
water the following experiments are performed at 298 K. When 50.0 mL of 0.010 M KI aqueous solution is added to 25.0 mL solution of I2 in CH2Cl2, two separate phases, aqueous and organic, are formed. Assume that there is no volume change upon mixing.

25

Preparatory Problems, Theoretical
In order to determine concentrations of I2 distributed in CH2Cl2 and aqueous phases, a 5.00 mL aliquot of the CH2Cl2 phase is diluted to 100.0 mL by addition of the solvent, CH2Cl2. The visible spectrum of I2 in the diluted solution, recorded in a 1.00 cm-cell, had a band with a maximum absorbance of 0.516 at 510.0 nm. The molar absorption coefficient, ε of I2 in CH2Cl2 at 510 nm is 858 L·mol -1·cm-1. Calculate equilibrium concentrations of I2 in CH2Cl2 and aqueous phases.

d) In order to determine the equilibrium concentrations of I- and I3-, a 25.0 mL aliquot is
taken from the aqueous phase. To this solution, an excess amount of KI, namely 10.0 mL of 0.100 M KI solution, is added to avoid evaporation of I2. Then, the final solution is titrated with a 0.0100 M solution of Na2S2O3. The end point is reached upon addition of 3.10 mL of Na2S2O3 solution. Calculate the equilibrium concentrations of I-, and I3- in the aqueous phase and the equilibrium constant at 298 K.

e) Calculate ?fG°[I2(CH2Cl2)], if ?fG°[I2(aq)] is 16.4 kJ·mol-1.

Problem 19

Molecular weight determination by osmometry

Measurement of osmotic pressure is one of the techniques used to determine the molecular weight of large molecules, like polymers. The device, osmometer, used to measure the osmotic pressure, consists of a semipermeable membrane that separates pure solvent from a solution. The flow of solvent from pure solvent side to solution side, due to concentration gradient, across the semipermeable membrane is called osmosis. Polyvinylchloride, PVC, is one of the most widely used plastics and can be prepared via chain polymerization. In chain polymerization monomers are added to a growing polymer chain. A typical chain polymerization involves three main steps named as initiation, propagation, and termination. In termination reaction two growing chains combine to form either one dead polymer chain (termination by combination) or two dead polymer chains (termination by disproportionation). In an attempt to determine the molecular weight of PVC via osmotic pressure measurement, a PVC solution is prepared by dissolving 7.0 g of PVC in cyclohexanone (C6H10O) to make a 1.0 L solution at 295 K. One arm of the osmometer is filled with this solution of density 0.980 g·cm-3 and the other arm is filled with pure solvent cyclohexanone to the same level. After a certain time, the height of liquid in the solution side arm increases and at equilibrium a 5.10 cm level difference between two arms is recorded.

26

Preparatory Problems, Theoretical
a) Calculate the osmotic pressure and average molecular weight of PVC. (density of Hg = 13.6 g·cm-3, g = 9.81 m·s-2). b) The kinetic chain length ν is the ratio of the number of monomer units consumed per activated center produced in the initiation step and used to estimate the mode of termination. In the chain polymerization of vinyl chloride to produce PVC, the concentration of active centers produced in the initiation step and the change in the concentration of monomer is found to be 1.00×10-8 mol/L and 2.85×10-6 mol/L, respectively. Calculate the kinetic chain length,ν. c) Predict whether the termination is by combination or by disproportionation. d) The vapor pressure of pure solvent cyclohexanone is 4.33 torr at 25 °C. Calculate the vapor pressure of the PVC solution. e) For pure solvent cyclohexanone, normal freezing point is -31.000 οC. If the freezing point of the PVC solution is -31.003 οC, find the molal freezing point depression constant of cyclohexanone.

Problem 20

Allowed energy levels and requirements for absorption of light

It is a well-established experimental fact that the internal energies of a given atom or a molecule M are confined to discrete values, so called “quantized energies”. “Internal” energy is the total energy of M excluding its translational energy. Translational energy of M is equal to the kinetic energy of a free particle that has the mass of M, which is moving along a straight line with a constant speed. It is not quantized, and it does not play a role in absorption of light by M. Internal energy of an atom is the energy associated with the motion of its electrons around the nucleus. In molecules, there are additional contributions from rotational and vibrational motions. The “allowed” internal energies of M can be numbered as E1, E2, E3, ... in increasing order of energy. These are called the “energy levels” of M. The lowest energy level, E1, is called the “ground” level, and when M has this lowest possible energy, M is said to be in its ground state. All the other, higher energy levels are referred to as “excited” levels of M, and if M is in a state with one of these higher energies it is said to be in an excited state. There is one and only one ground level whereas there are infinitely many excited states of M. Each atom or molecule has its own characteristic set of energy levels.

27

Preparatory Problems, Theoretical
When a sample of M molecules is exposed to a beam of monochromatic light with wavelength λ there may be an energy exchange between the light and the M molecules. In its interaction with M, the monochromatic light beam is considered to consist of identical “photons”, with all photons moving in parallel and along the direction of the beam with the speed of light. Each photon carries an energy given by Ephoton = hν, where h is Planck’s constant and ν is the frequency of the light, related to its wavelength by: ν = c/λ. Since c is a constant, a given monochromatic light may be characterized by stating either λ or ν. A molecule M may take energy from an external source such as light, thereby changing its initial energy level Einitial to a final level Efinal. For example, consider a case, where M is initially in its ground state with the lowest energy E1. Its final energy (Efinal) can only be one of E2, E3, E4, ... As a consequence, the amount of energy that M can accept from the external source is restricted to the values: ?E = En-E1, where n=2, 3, ... Conservation of total energy requires that if M gains energy equal to one of these allowed ?E values, the external source must provide precisely the same amount of energy. When light is used as the energy source, a photon from the light beam may or may not be absorbed by an M molecule in the sample, depending on the frequency ν of light used. Only when the photon energy is exactly equal to one of the allowed ?E values of M, the energy of the photon may be accepted by M. The fundamental condition for absorption of light by M is expressed as hν = ?E. This is a minimum requirement for absorption of light. Depending on whether M is an atom or molecule, and the nature of the energy levels involved in the transition, additional conditions called “selection rules” may have to be concurrently satisfied. A closed test tube containing gaseous H atoms is irradiated by monochromatic light. Six experiments are done, differing from each other only by the wavelength, λ, of light employed. Experiment No. λ (nm) 1 129.6 2 121.6 3 101.6 4 97.25 5 94.97 6 94.11

Find out the experiments in which light will be absorbed by the H atoms in the sample, and describe the transitions involved. Additional Data: The allowed energy levels for the electron in a hydrogen atom are given (in SI units) by

28

Preparatory Problems, Theoretical
RH n2

En = ?

,

n = 1, 2, 3, K

where RH = 2.1787×10-18 J is a constant; i.e. RH is same for all values of the “n” quantum number. Assume that initially all of the hydrogen atoms in the sample are in their ground electronic states. Conservation of total energy is the only requirement for absorption of a photon by a H atom; i.e. there are no extra selection rules regarding the “n” quantum number.

Problem 21

Rotational and vibrational energy levels of a diatomic molecule

Part A: Rotational Energies
Within the rigid-rotor approximation, the list of allowed rotational energies of a diatomic molecule, AB(g) in the gas phase, are given by: Erotation = B J(J+1), where B = J = 0, 1, 2, … is a characteristic property of the molecule called the “rotational constant”

h2 8π 2 I

of the molecule. The expression for B is in SI units; h is Planck’s constant, and I is the moment of inertia of the molecule defined by: I = ?R2, where R is the bond length, and ? is called the “reduced mass” of the diatomic molecule. The latter quantity is defined in terms of the masses, mA and mB, of the atoms in the diatomic molecule AB.

?=

m AmB m A + mB

Theory indicates that the bond length, R, does not change when either A or B is replaced by other isotopes of the atoms A or B. When a sample of gaseous molecules is exposed to microwave radiation, a molecule in the sample that is initially in a rotational energy level with J = Ji may absorb a photon, ending up in a higher energy level with J=Jf. It may be shown that only those rotational transitions in which Jf = Ji+1 can occur in absorption of light that changes the rotational state. The rotational constant of the
12 12

C16O molecule has been experimentally determined as B =
16

23.115 J·mol-1. The isotopic masses of the two atoms in this molecule are known: mass of C = 12 amu by definition, and that of O = 15.994915 amu. The longest wavelength of

29

Preparatory Problems, Theoretical
electromagnetic radiation that causes a transition between the rotational levels of molecule in a sample has been observed to be λ = 0.25876 cm.
12

C16O

a) What are the values of Ji and Jf for a molecule that absorbs a photon with a wavelength
of 0.25876 cm?

b) Calculate the moment of inertia and the bond length of the carbon monoxide molecule. c) Predict the values of the rotational constants, B, for each of the following three
molecules:
13 12

C18O,

13

C18O, and

13

C16O. (Additional data: masses of

18

O=17.999159 and

C=13.003355 amu.)

d) Calculate the longest wavelengths of microwave radiation that may be absorbed by each
of
12

C18O, 13C18O, and 13C16O molecules.

Part B: Rotational plus Vibrational Energies
Within the harmonic oscillator approximation, the list of allowed vibrational energies of a diatomic molecule, AB(g) in the gas phase, are given by:

1 E vibration = (v + ) ε, 2

v = 0, 1, 2 , K

where ε is a characteristic vibrational property of the molecule defined by

ε=

h k 2π ?

In this expression h is Planck’s constant, k is called the “force constant “of the molecule, and ? is the reduced mass of the diatomic molecule. In SI units, ε is in joules, k in N·m-1, and ? in kg. Theory shows that the force constant k is independent of isotopic substitution in the molecule. When a sample of gas molecules is exposed to infrared (IR) radiation, a molecule in the sample that is initially in a vibrational energy level with v = v i may absorb a photon, ending up in a higher energy vibrational level with v = v f. It may be shown that only those transitions in which v f = vi+1 can occur in absorption of light that changes the vibrational state. Absorption of light in the IR region changes not only the vibrational state, but also the rotational state; i.e., a simultaneous change in v and J is involved now. This is because the allowed vibrational plus rotational energies of a molecule are given by

30

Preparatory Problems, Theoretical
Erot.+vib.= Erotation + Evibration

a) The force constant of the carbon monoxide molecule is 1901.9 N·m-1. Find ε in kJ·mol-1
(to 4 significant figures) for each of the following isotopically related CO molecules: i. ii. iii. iv.
12 12 13 13

C16O C18O C18O C16O

b) Find the wavelengths (to 4 significant figures) of IR radiation that may be absorbed by a
molecule in making a transition from an initial state with (v,J) = (0,0) to a final state with (v,J) = (1,1) for each of the following isotopically related CO molecules: i. ii. iii. iv.
12 12 13 13

C16O C18O C18O C16O

Problem 22

Particle in a box: Cyanine dyes and polyenes

In quantum mechanics, particle in a one dimensional box model describes a particle moving between two impenetrable walls separated by a distance L. The allowed energies for a particle in one dimensional box are En = for n = 1, 2, 3, ....

where h is Plank’s constant, m is the mass of the particle, and L is the box length. The electronic absorption spectra of conjugated linear molecules can be simulated by the one dimensional particle in a box model. The delocalized π electrons are treated as free electrons and they are distributed into the allowed energy levels obeying the Pauli Exclusion Principle. If the molecule contains N delocalized π electrons, then the levels from n = 1 to n = N/2 are occupied in the ground state. Figure given below exhibits the energy levels for a conjugated molecule with N = 8.

31

Preparatory Problems, Theoretical
n=5

n = 4 = N/2

energy

n=3 n=2 n=1

The energy levels for a system with 8 free electrons, N=8 The lowest energy electronic transition for such a system involves the excitation of one of the electrons in level n = 4 (N/2) to the level n = 5 (N/2 + 1). For this transition to be affected by light absorption, the wavelength λ of light must be such that

?E =

λ

,

?E =

[(

+1)2 - (

)2]=

(N +1)

Cyanine, pinacyanol, and dicarbocyanine shown below, are dye molecules that have a conjugated chain between the two ends.
H3C N H3C C H C H C H N CH 3 CH 3 cyanine

H 3C N H 3C C H C H C H 2 N 2

CH 3 pinacyanol CH 3

H 3C N H 3C C H C H C H 3 N

CH 3 dicarbocyanine CH 3

32

Preparatory Problems, Theoretical
a) Draw the resonance forms of these three molecules. b) The delocalized electrons can move freely along the central chain of the molecule,
between the two terminal nitrogen atoms but not more than one bond length beyond the nitrogen atom. Particle in a box model can be applied to calculate the quantized energy levels of these delocalized electrons. The box length can be taken as the distance between the two nitrogen atoms, measured along the carbon-carbon bonds, plus one bond length on either side of each nitrogen atom. Determine the number N of delocalized electrons in each dye molecule.

c) Experimentally, the electronic absorption band maxima of these molecules, λmax, are
recorded at 525, 605 and 705 nm for cyanine, pinacyanol and dicarbocyanine, respectively. Calculate ?E for cyanine, pinacyanol and dicarbocyanine.

d) Predict the chain length where the electrons can move freely in these molecules. e) As the conjugated π electrons are free to move along the polyene carbon backbone, but
not allowed to leave the molecule, they can be viewed as particles in a box defined by the carbon backbone of a linear polyene. The average carbon-carbon bond length in a hydrocarbon chain of alternating single and double bonds can be approximated to 140 pm. The length of carbon chain, the box length, is approximately L= 2j×140 pm, where j is the number of double bonds in the polyene chain. Determine the number N of delocalized electrons and the box length L for 1,3-butadiene and 1,3,5-hexatriene.

f) Estimate the frequencies and wavelengths of the lowest electronic transition for 1,3butadiene and 1,3,5-hexatriene.

Problem 23
46 31

Radioactive decay
24

Although twenty three isotopes of phosphorus are known (all the possible isotopes from half-integer spin and high abundance of
31

P

up to P), only P, with spin 1/2, is stable and is therefore present at 100% abundance. The P make it useful for nuclear magnetic resonance
32

(NMR) spectroscopic studies of biomolecules, particularly DNA. Two radioactive isotopes of phosphorus have half-lives that make them useful for scientific experiments. life of 14.3 days and
33 33

P has a half32

P has a half-life of 25.3 days. Both radioisotopes of phosphorous,

P

and P, are beta emitters and they decay as shown in the following nuclear reactions; P→ S+ P→ S+

33

Preparatory Problems, Theoretical

Isotope
32 33 32 33

isotopic masses (amu) 31.97390727 32.9717255 31.97207100 32.97145876

P P S S

a) Calculate the energy of β particles emitted in the decay reactions of 32P and 33P. b) To shield beta radiation usually lead is used. However, secondary emission of X-rays
takes place via a process known as Bremsstrahlung in the case of high energy β emission, Therefore shielding must be accomplished with low density materials, e.g.

Plexiglas , Lucite, plastic , wood, or water. During shielding of β -emission from 32P, Xray photons of λ = 0.1175 nm are produced. Calculate the energy of X-ray photons in eV.

c) Find the mass of 32P which has activity of 0.10 Ci (1 Ci = 3.7×1010 disintegration/s). d) A sample containing both radioisotopes 32P and 33P has an initial activity of 9136.2 Ci. If
the activity decreases to 4569.7 Ci after 14.3 days, calculate the present in the sample.
32

P/33P ratio initially

Problem 24

Enzyme-substrate interaction

In biological systems, it is very common for proteins such as enzymes or receptors to bind to multiple ligands or substrates at the same time. Binding of the first ligand usually affects the binding of the second ligand either positively or negatively. In this question, imagine that there is a protein P which can bind to two different ligands L and MH+ as shown in the figure given below.

34

Preparatory Problems, Theoretical
For simplicity, assume that the binding of these two ligands are independent of each other; i.e. binding of the first ligand does not change the binding constant (complex formation constant) for the second ligand.

a) Equal volumes of 100 ?M solutions of ligand L and protein P are mixed in a buffer
solution of pH 9.50. The formation constant is Kf(P-L) = 2.22×104. Calculate molar concentration for all the species present in this solution. What percentage of the protein P is complexed with the ligand L?

b) Ligand M has a free amine group and only its protonated form, i.e. MH+, can bind to
protein P. What percentage of the ligand M is protonated at pH 9.50? pKa(MH+) = 10.00.

c) Equal volumes of 100 ?M solutions of ligand M and protein P are mixed in a buffer
solution of pH 9.50. Calculate molar concentration for all the species present in this solution. What percentage of the protein P is complexed with the ligand MH+? Kf(P-MH+) = 5.26×105.

d) 100 ?L of 100 ?M protein P, 50 ?L of 200 ?M of ligand L and 50 ?L of 200 ?M of ligand M
are mixed in a buffer solution of pH 9.50. What percentage of the protein P is bound to (i) only L, (ii) only MH+ and (iii) both L and MH+? Calculate molar concentration for all the species present in this solution.

Problem 25

Amides

The amide functional group is one of the most fundamental motifs found in chemistry and biology. Typical acyclic amides 1 are planar and stable, while the cyclic amides (bridgehead lactams) are unstable.

O R C NH 2 1

N O 2-quinuclidone 2 O norcamphor 3

2-Quinuclidone (2), a typical example of bridgehead lactams, is very unstable due to the improper alignment of nitrogen lone pair and carbonyl group for π-interaction. As a consequence, the amide group resembles an amine as evidenced by the ease of salt formation. The organic synthesis of the tetrafluoroborate salt of 2-quinuclidone (2) is a six-

35

Preparatory Problems, Theoretical
step synthesis starting from norcamphor (3) and the final step being an azide-ketone Schmidt reaction.

a) The reaction of enantiopure norcamphor (3) with m-chloroperbenzoic acid (m-CPBA)
gives A and B. A is formed as the major product in 78% yield, whereas the second isomer B is formed as the minor product. The reaction of A with lithium aluminum hydride results in the formation of C, whereas the reduction of B yields optically inactive compound D. Give the structures of the compounds A, B, C, D and determine the absolute configurations (R/S) of the stereogenic carbon atoms in A, B, C and D.
m-CPBA, NaHCO3 O norcamphor 3
Cl

CH 2Cl2, 20 o C

A + B

LiAlH 4 Et2O, 20 oC

C + D

O C OOH

m-CPBA =

b) Treatment of C with 1 equiv. of tosyl chloride (TsCl) yields the compound E, which is
converted into F by reaction with 1 equiv. of NaN3 in dimethylformamide. Give the structures of E and F.

C

1 equiv. TsCl, NEt 3 CH2Cl2, rt, (yield 74%)

E

NaN3 DMF, 70 oC, (yield 92%)

F

c) F is subjected to pyridinium chlorochromate (PCC) oxidation reaction to form the
precursor G of the target compound. Finally, G is exposed to tetrafluoroboric acid (HBF4). The target compound H is isolated as its tetrafluoroborate salt. Beside this product a second isomer I is formed as the minor product. Give the structures of G, H and I.

F

Pyridinium chlorochromate CH2Cl2, 20 oC, (yield 93%)

G

HBF4 Et2 O, 20 o C

H

+

I

Problem 26
1

NMR Spectroscopy

H-NMR Spectroscopy

The 1H-NMR spectroscopy allows the identification of hydrogen atoms in organic molecules. From the position of the signals (chemical shift) and the splitting of the signals, the type and

36

Preparatory Problems, Theoretical
connectivity of the hydrogen atoms can be recognized. Some characteristic hydrogen atom resonances are given below.

O C H

H C=C H H C

sp3

10
1H-NMR

9

8

7

6

5

4

3

2

1

0

Resonance ranges of typical functional groups in ppm relative to TMS

13

C-NMR Spectroscopy
13

The

C-NMR is analogous to 1H-NMR and allows the identification of carbon atoms in
13

organic molecules. The

C-NMR spectrum of a given compound shows as many signals

(singlets) as the number of the different carbon atoms. The relative intensities of all kinds of carbon (primary, secondary, tertiary, and quaternary) signals will be assumed to be equal. Some characteristic carbon resonances are given below.

R

o c

R

R

o c

R OR R C C

R R

RO C

R C

200
13

175

150

125

100

75

50

25

0

C-NMR Resonance ranges of typical functional groups in ppm relative to TMS

Description of six isomeric (constitutional) compounds (A, B, C, D, E and F) having the formula C5H10O2 is as follows ? ? ? None of the compounds is branched. There is no O-H absorbance in the IR spectra of the isomers. In each compound, one oxygen atom is sp2- and the other is sp3-hybridized.

Determine the structures of all isomers by using the information given above and analyzing the 1H- and 13C-NMR spectra given below. Abbreviations: s = singlet, d = doublet, t = triplet, q = quartet, qui = quintet, h = hextet.

37

Preparatory Problems, Theoretical
Compound A
3* (s)
1

H-NMR spectrum of A

3 (t) 2 (t) 2 (h)

4

3

2 PPM

1

0

*) The num bers on the signals give the number of the resonating protons

13C-NMR

spectrum of A

180

160

140

120

100 PPM

80

60

40

20

0

Compound B
3 (t)
1

3 (t)

H-NMR spectrum of B 2 (q)

2 (q)

5

4

3 PPM

2

1

0

13

C-NMR spectru m of B

180

160

140

120

100 PPM

80

60

40

20

0

Compound C
2 (s) 3 (s)

1

H-NMR spectrum of C 3 (t)

2 (q)

5

4

3 PPM

2

1

0

38

Preparatory Problems, Theoretical

13C-NMR

spectrum of C

220

200

180

160

140

120 PPM

100

80

60

40

20

0

Compound D
3 (s)

1

H-NMR spectrum of D
3 (t)

2 (t) 2 (h)

5

4

3 PPM

2

1

0

13

C-NMR spectrum of D

180

160

140

120

100 PPM

80

60

40

20

0

Compound E
3 (s) 2 (s)
1 H-NMR

spectrum of E
3 (t)

2 (q)

5

4

3 PPM

2

1

0

13

C-NMR spectrum of E

220

200

180

160

140

120 100 PPM

80

60

40

20

0

39

Preparatory Problems, Theoretical
Compound F
3 (t) 2 (d)
1

H-NMR spectrum of F

2 (t)

1 (t)

2 (h)

10

8

6 PPM

4

2

0

Two signals overlap

13

C-NMR spectrum of F

220

200

180

160

140

120 100 PPM

80

60

40

20

0

Problem 27

Cyclitols

Cyclitols have recently attracted a great deal of attention due to their diverse biological activities and their versatilities as synthetic intermediates. Polyhydroxy cyclohexanes, such as inositols, quercitols, and conduritols, belong to the family of cyclitols. These compounds can exist in a number of different stereoisomers; inositols, quercitols, and conduritols have 9, 16, and 6 possible stereoisomers, respectively. Cyclohexa-1,3-diene (1) is an important key compound for the synthesis of versatile cyclitol derivatives. The total synthesis of isomeric cyclitols having the molecular formula C6H12O4 is shown below.
O 2(1?g )

A

LiAlH 4 Ether

B

m-CPBA CHCl3

E

+

F
H2 O cat. H2 SO4

1

OsO4, NMO Acetone/H 2O rt

C
?

+

D

G

The reaction of cyclohexa-1,3-diene (1) with singlet oxygen O2(1?g), generated in situ by irradiation of oxygen molecule in the presence of a sensitizer, yields an unstable bicyclic

40

Preparatory Problems, Theoretical
compound A. The reaction of A with LiAlH4 in ether results in the formation of B, whose
13

C-NMR spectrum shows 3 resonances. One of them appears in the sp2 region.

?

Osmylation of B in the presence of an excess of NMO (N-morpholine oxide) (more than 2 equiv.) at room temperature in acetone/H2O leads to the isomeric mixture of C and D where the compound C is formed as the major product.

?

Oxidation of B with m-chloroperbenzoic acid (m-CPBA) gives a diastereomeric mixture of E and F. Treatment of this mixture with H2O in the presence of a catalytic amount of H2SO4 affords only compound G as a racemic mixture having the molecular formula of C6H12O4.

a) Draw the structures of A, B, C, D, E, F, and G using dashed-wedged line notation to indicate the relative configurations. b) Osmylation of 1 in the presence of an excess of NMO (N-morpholine oxide) (more than 2 equiv.) at room temperature in acetone/H2O leads to the formation of a diastereomeric mixture of H and D where the compound H is formed as the major product.
OsO4 , NMO Aceton/H 2O rt

H

+

D

1

Draw the structure of H using dashed-wedged line notation. c) Reaction of cyclohexa-1,3-diene (1) with 1 equiv. m-chloroperbenzoic acid (m-CPBA) gives a single product I, which is reacted with H2O in the presence of a catalytic amount of H2SO4 to provide J (There is no rearranged product). Osmylation of J in the presence of excess of NMO (N-morpholine oxide) (more than 1 equiv.) at room temperature in acetone/H2O leads to formation of isomeric G and K.
m-CPBA, 1 equiv. CHCl3, rt

I

H2O/H + rt

J

OsO4 , NMO Aceton/H 2O rt

G

+

K

1

Draw the structures of I, J, and K using dashed-wedged line notation. d) Reaction of cyclohexa-1,3-diene (1) with 2 equiv. of m-chloroperbenzoic acid (m-CPBA) gives a diastereomeric mixture of L and M. Reaction of this mixture (L and M) with H2O in the presence of a catalytic amount of H2SO4 yields a mixture of N and C.

41

Preparatory Problems, Theoretical
m-CPBA, 2 equiv. CHCl3, rt H 2O cat. H 2SO4 , rt

L + M

N

+

C

1
Draw the structures of L, M, and N using dashed-wedged line notation.

Problem 28

Antiviral antibiotic
Ascochlorin (antiviral antibiotic)

O H
7 8 1

OH
9 3 5

23

20 13 14 16 21

22

11

O
18

OH

Cl Part B Part A

Ascochlorin is an antiviral antibiotic obtained from the filter cake of the fermented broth of Ascochyta viciae Libert. It has a strong inhibitory effect on viral growth in cultured cells. The absolute stereochemistry of this antibiotic was determined by X-ray analysis. Due to high biological activity, the ascochlorin family has attracted the attention of synthetic organic chemists. Part A (Synthesis of right side of ascochlorin) Synthesis of cyclohexanone unit (right side of ascochlorin) starts with Diels-Alder reaction which is a [4+2] cycloaddition reaction. Reaction of 1,3-butadiene and chiral dienophile 1 gives the cycloadduct A with the desired stereochemistry of cyclohexanone unit. Basic hydrolysis of A affords the sultam 2 and the chiral carboxylic acid B. This carboxylic acid B undergoes iodolactonization reaction (initial step is the formation of an iodonium ion intermediate) to form the δ-iodolactone C. Treatment of C with DBU (non nucleophilic base) forms the compound D. Reduction of D with LiAlH4 and then hydrolysis with water produces the diol E. Selective oxidation of E with pyridinium chlorochromate (PCC) leads to the product F. Subsequent protection of the hydroxy group with (CH3)3SiCl produces the compound G.

42

Preparatory Problems, Theoretical

CH 3 O N S O O CH 3AlCl2 -30 oC

CH 3

A

NaOH H 2 O, H2O 2 O S

NH O

+

B

1

2

NaHCO 3 KI, I 2

F (C9H14O 2)
(CH 3) 3SiCl Et3 N OHC H3C H 3C OSi(CH3) 3

PCC

E (C9H16O2)

1. LiAlH4 2. H 2O

D (C9H12O2)
N N DBU

DBU

C (C9H13IO2)

G

1,8-diazabicyclo[5.4.0]undec-7-ene

Besides [4+2] cycloadditon reaction, the compound 1 can also undergo [2+2] cycloadditon reactions. For example, the dienophile 1 undergoes a cycloaddition reaction with ketene (general formula, R2C=C=O). When the dienophile 1 is reacted with 2,2-dichloroacetyl chloride (ketene equivalent) in the presence of a base, an isomeric mixture of H and I is formed.
CH3 N O O S O O Cl2HC Et3N Cl

H

+

I

1
a) Draw the structures of compounds A, B, C, D, E, and F with the correct stereochemistry. b) Draw the structure of ketene obtained in situ from 2,2-dichloroacetyl chloride. c) Draw the structures of H and I. Part B (Synthesis of left side of ascochlorin) In order to synthesize the left side of the molecule (Part B), the aromatic compound J is used as starting material (PG is a protecting group for OH). Reaction of J with strong base

43

Preparatory Problems, Theoretical
(butyllithium, BuLi) forms the lithiated compound K. Treatment of this intermediate with copper(I) iodide and then with epoxide gives compound L. Reaction of this compound with thionyl chloride (SOCl 2) produces compound M. In order to combine part A and part B, Wittig reaction is planned. For this purpose compound M is reacted with triphenylphosphine (PPh3) and then the product of this reaction is treated with BuLi to get the intermediate Wittig reactant N. Finally coupling is achieved by reaction of N with aldehyde G, which yields O, the main skeleton of ascochlorine. In order to remove (CH3)3Si group, O is treated with dilute acid solution which gives compound P. The ascochlorine synthesis is completed by five more steps.
O H3CO H 3C Cl OPG OPG BuLi -78 C
o

O H3CO H 3C

OPG Li OPG Cl 1. CuI 2. O H3CO

O

OPG

CH3 OH OPG

H 3C Cl

L
SOCl2

J
OHC H 3C H3 C OSi(CH3 )3 G

K

N(C32H 28ClO4P(PG) 2)
H3O+

1. PPh 3 2. BuLi

M (C 14H 14Cl2O4(PG)2)

O

P (C23H 27ClO5(PG)2)

PG = Protecting group H

O

OH

CH3 H C CH 3 3
19

O

Me Cl

OH

H 3C

ascochlorine

a) Propose a mechanism for the conversion of K to L. b) Draw the structures of compounds M, N, O, and P. c) The ascochlorin synthesis from compound P is completed by performing five more steps: i) ii) Oxidation of OH group on the cyclohexyl unit. Introduction of the methyl group to C19 (do not worry about the correct stereochemistry).

44

Preparatory Problems, Theoretical
iii) Conversion of ester group on the aromatic unit to aldehyde. iv) Selective reduction of α,β-unsaturated double bond of cyclohexenone unit. v) Removal of protecting groups (PG) which can be achieved by using Bu4NF.

Write the reagents used for the steps i, ii, and iii.

Problem 29

Acyclic β -amino acids

In recent years, acyclic β-amino acids have attracted attention especially following their recognition as an important class of compounds in the design and synthesis of potential pharmaceutical drugs such as Taxol? (paclitaxel) and its analogue Taxotere? (docetaxel), currently considered to be among the most important drugs in cancer chemotheraphy. The only source of Taxol is the bark of the pacific yew tree, Taxus brevifolia, but its Taxol content is relatively low. In efforts to overcome this supply problem, chemists have been working on semi-syntheses. These methods involve synthetic side-chain coupling to C13-OH of the more readily available Baccatin III derivatives as given below, which can be isolated in higher yield from the needles of various Taxus species (e.g. Taxus baccata).

a) The racemic synthesis of side chain is started by refluxing benzaldehyde with p-anisidine in toluene to afford compound A. Subsequent reaction of compound A with acetoxyacetyl chloride in the presence of triethylamine gives the racemic mixture of cyclic compounds B1 and B2 with the molecular formula (C18H17NO4). The resultant racemic mixture of B1 and B2 is subjected to enzymatic resolution by using Lipase (Amano PS). Lipases are well known biocatalysts to hydrolyze the ester units. They selectively hydrolyze only one enantiomer of a racemic mixture. As a result of this hydrolysis, B1 (as the unreacted enantiomer) and C (hydrolysis product) are isolated. B1 has absolute configuration 3R,4S whereas, C has 3S,4R.

45

Preparatory Problems, Theoretical
O H + OCH 3 p-anisidine Lipase (Amano PS) pH 7 NH 2 toluene reflux O AcO Cl

A
N(Et)3 , CH2 Cl2

B1

+

B2

racemic mixture

B1
where applicable.

+

C

Draw the structures of compounds A, B1, B2, and C with the correct stereochemistry b) Hydroxy unit of compound C is protected with triethylsilyl chloride in the presence of pyridine and 4-dimethylaminopyridine (DMAP) to give the compound D. Treatment of D with cerium(IV) ammonium nitrate (CAN), a feasible reagent for the oxidative cleavage of N-aryl bond, followed by neutralization with NaHCO3 solution yields the desired compound E. The target β-lactam N-unit is protected with bis(tert-butyl)dicarbonate (Boc)2O to afford the compound F. In the final step, Baccatin III derivative is coupled with the compound F in the presence of NaH in THF.

Cl

Si

C

Pyridine, DMAP CH2 Cl2

D

i) (NH4) 2Ce(NO 3) 6 (CAN) ii) NaHCO3

E
O O O O O

((Boc) 2O)

Baccatin-III-O G

Baccatin III-OH NaH, THF

F

Draw the structures of compound D, E, F and G with the correct stereochemistry.

46

Preparatory Problems, Theoretical

Problem 30

Life of Ladybug

In nature, there are many known species of the ladybug family (Coccinellidae). Besides their cuteness, they play a beneficial ecological role in controlling populations of some harmful insects. When molested or disturbed, they emit droplets of a fluid from their joints. This process known as “reflex bleeding”, serves as an efficient deterrent. This fluid was isolated, characterized and termed as coccinelline. The structure of coccinelline is given below.

H

O N CH3

Coccinelline
a) In laboratory synthesis of coccinelline, very common and readily available starting compounds are chosen. The reaction between dimethyl malonate and acrolein in the presence of sodium methoxide yields compound A. Subsequently, it is heated in basic solution followed by esterification with methanol under acidic condition to afford compound B. The
13

C-NMR spectrum of compound B shows two characteristic signals at

around 170 and 200 ppm. Then, the compound B is treated with ethane-1,2-diol under slightly acidic condition to give compound C. The signal of compound B at around 200 ppm disappears in the
13

C-NMR spectrum of compound C. Compound C undergoes a

self condensation reaction in the presence of NaH to yield compound D. Decarboxylation reaction of compound D affords compound E. In the next step, compound E is first reacted with ammonium acetate followed by reduction with sodium cyanoborohydride. In this step, compound F is formed via the reductive amination of the compound E.

47

Preparatory Problems, Theoretical
1. OH -, H 2O heat 2. H+, CH 3OH

O H3CO

O OCH3

+ O

H

NaOCH 3

A

B
H +, HO

OH

E
1. NH4OAc 2. NaCNBH 3

OH -, H 2O heat

D
(C 15H 24 O7 )

NaH

C

F
(C 13H 25NO 4)
Draw the structures of the compounds A, B, C, D, E and F. b) In the next part of synthesis, compound F is first deprotected at pH 1 and, then, the pH is adjusted to 5 followed by the addition of acetone dicarboxylic ester which can easily be enolizable. As a result of this step, the tricyclic compound G is isolated as the sole product. Draw the structure of compound G. Propose a plausible mechanism for the formation of compound G. (Hint: After deprotection step, iminium ion is formed and reacted with enolizable acetone dicarboxylic ester.) c) The final part of synthesis involves the decarboxylation of compound G under basic condition to afford compound H. The
13

C-NMR spectrum of compound H shows the

characteristic signal at around 200 ppm. The reaction of compound H with methylene triphenylphosphorane yields compound I which is subsequently hydrogenated to afford precoccinelline. In the final step, precoccinelline is oxidized with m-chloroperbenzoic acid (m-CPBA) to get coccinelline.

48

Preparatory Problems, Theoretical
H 3CO

O O O

pH 1 to pH 5

F

+

G

OH -, H 2O heat

H

H 3CO

Ph3 P=CH 2

N CH 3

H 2, Pd

I

Precoccinelline

Draw the structures of H and I.

49

Preparatory Problems, Practical

Practical Problems
Safety
The participants of the Olympiad must be prepared to work in a chemical laboratory and be aware of the necessary rules and safety procedures. The organizers will enforce the safety rules given in Appendix A of the IChO Regulations during the Olympiad. The Preparatory Problems are designed to be carried out only in a properly equipped chemistry laboratory under competent supervision. Since each country has own regulations for safety and disposables, detailed instructions are not included herein. Mentors must carefully adapt the problems accordingly. The safety (S) and risk (R) phrases associated with the materials used are indicated in the problems. See the Appendix A and B of the Regulations for the meaning of the phrases. The Regulations are available on the website http://www.icho43.metu.edu.tr Safety cautions for the practical questions must be provided by the Mentors. Major cautions are:
? ? ? ?

Use fume hoods if indicated. Safety goggles and laboratory coat should be worn at all times in the laboratory. Never pipette solutions using mouth suction. Dispose reagents into the appropriate waste containers in the laboratory.

50

Preparatory Problems, Practical

Problem 31 Preparation of trans-dichlorobis(ethylenediamine)cobalt(III)chloride and kinetics of its acid hydrolysis
Geometrical isomers differ in the spatial arrangement of atoms or groups around a central atom. Geometrical isomers usually have distinctive physical and chemical properties. The cis- and trans-isomers of octahedral cobalt complexes are among the most well known examples. The complexes of Co(III) ion are sufficiently stable and occur in separable isomeric forms. For example, dichlorobis(ethylenediamine)cobalt(III) ion, [Co(en)2Cl2]+ can exist in either cis- or trans-form as shown below. Ethylenediamine (en) is a bidentate ligand, which coordinates to the metal through both of its nitrogen atoms. The cis complex is dark purple, whereas the trans complex is green.
Cl N Co N Cl N N

+
N

N Cl Co N N Cl

+

trans

cis

At low concentrations the green trans-[Co(en)2Cl2]+ ion undergoes a first order acid hydrolysis reaction, and a red mixture of the cis and trans isomers of [Co(en)2(H2O)Cl]2+ complex ion are formed as shown below.

trans-[Co(en)2Cl 2]+ + H2O → cis- and trans-[Co(en)2(H2O)Cl]2+ green red mixture

In this experiment, the chloride salt of trans-dichlorobis(ethylenediamine)cobalt(III) complex will be prepared and the rate constant of its acid hydrolysis reaction will be determined. Chemicals and reagents ? Ethylenediamine, H2NC2H4NH2, 10% (v/v) ? Cobalt(II)chloride hexahydrate, CoCl2·6H2O ? Hydrochloric acid, HCl(aq), concentrated solution ? Sulfuric acid, H2SO4(aq), 1.0 mol·L-1

51

Preparatory Problems, Practical
Substance H2NC2H4NH2 CoCl2?6H2O HCl (aq) H2SO4 (aq) Phase Liquid, 10% (v/v) Solid Concentrated sol. 1.0 mol·L
-1

R Phrase 11 20 21 22 35 22 42 43 49 50 53 23 24 25 34 36 37 38 23 24 25 35 36 37 38 49

S Phrase 3 16 26 29 36 37 39 45 22 45 53 60 61 26 36 37 39 45 23 30 36 37 39 45

Apparatus and glassware ? Evaporating dish ? Steam bath ? Beakers, 25 mL (2) ? Pipette, 10 mL ? Stirring rod ? Graduated cyclinder, 25 mL ? Colorimeter or UV-Vis spectrometer ? Cell for colorimeter A. Preparation of trans-dichlorobis(ethylenediamine)cobalt(III) chloride 1. In a fume hood, dissolve 1.6 g of cobalt(II) chloride hexahydrate in 5.0 mL of water in an evaporating dish. 2. Add 9.0 mL of 10% v/v solution of ethylenediamine into the evaporating dish. 3. Place the dish on a steam bath and stir for 40 min maintaining the volume of the solution by adding small portions of hot water. During this process, Co(II) is oxidized to Co(III) by the oxygen of the air, therefore good agitation is necessary to promote the dissolution of oxygen in the solution. 4. Add 10 mL of concentrated HCl solution and continue heating and stirring (without adding water) until a thin slurry of crystals forms. 5. Dry the crystals on the steam bath to eliminate HCl and H2O. 6. Weigh and calculate the yield. B. Kinetics of acid hydrolysis of trans-dichlorobis(ethylenediamine)cobalt(III) chloride 1. Dissolve 0.10 g of trans-[Co(en)2Cl2]Cl in 5.0 mL of water in a test tube. 2. Transfer about 2 mL of the solution into the absorbance cell using a pipette and record the absorbance of the solution at 620 nm using a colorimeter or spectrometer,

52

Preparatory Problems, Practical
with the help of your assistant or technician. Record absorbance value as Ao at to, 0.0 min into the Table given below. 3. Place 0.20 g of trans-[Co(en)2Cl2]Cl in a beaker and add 10.0 mL of 1.0 M H2SO4 solution into the beaker.. 4. Transfer about 2 mL of the solution into the absorbance cell using a pipette and with the help of your assistant or technician record the absorbance of the solution at 620 nm at intervals of about 10 min as ti for 90 minutes into the table given below. Time to t1 t2 t3 t4 t5 t6 t7 t8 t9 t10 Treatment of Data A. 1. Calculate the percent yield of trans-dichlorobis(ethylenediamine)cobalt(III) chloride. B. 1. Plot ln versus time. Min 0.0 Absorbance A

2. Estimate the first order rate constant from the plot. 3. Explain why absorbance values can directly be used for concentrations.

Problem 32 Analysis of calcium salts
Several different salts of calcium are available in nature. Carbonate, chloride and sulfate salts are among the most common ones. Unlike the carbonate and sulfate salts, calcium chloride is soluble in water. Calcium carbonate reacts with Br?nsted acids releasing carbon dioxide: CaCO3(s) + 2 H+(aq) ? Ca2+(aq) + H2O(l) + CO2(g)

53

Preparatory Problems, Practical
In this experiment, the composition of a mixture of calcium salts will be determined based on solubility, and reaction of CaCO3 with a strong acid. Chemicals and reagents ? A mixture of calcium salts: 40.0% wt CaCO3, 5.0% wt CaCl2 and 55.0% wt CaSO4 (Any mixture can be used. The amount of sample and the hydrochloric acid can be adjusted for a desired volume of carbon dioxide gas.) ? ? ? Hydrochloric acid, HCl(aq) 3.0 mol·L-1 Sodium chloride, NaCl Acetone, CH3COCH3 Phase Solid Solid Solid 3.0 mol·L Solid Liquid
-1

Substance CaCO3 CaCl2 CaSO4 HCl(aq) NaCl CH3COCH3

R Phrase 36 37 38 36 23 24 25 34 36 37 38 36 11 36 66 67

S Phrase 37 38 41 22 24 22 24 25 26 36 37 39 45 26 36 2 9 16 26

Apparatus and glassware ? Apparatus shown in Figure 32-1 ? Ice bath ? Pipette, 10 mL ? Graduated cyclinder, 25 mL ? Funnel ? Filter paper (Whatman 42) ? Stirring bar ? Stirrer ? Septum A. Preparation of ice-bath 1. Add sufficient amount of NaCl in an ice-bath containing about 300 mL of ice-water mixture to obtain a solution at a temperature below -5 ?C. Use ice at a temperature below -10 ?C.

54

Preparatory Problems, Practical
B. Reaction with hydrochloric acid
1. Assemble the experimental setup as shown in Figure 32-1 in a hood. Check that the experimental set up is held on a support and the graduated tube is connected to the Schlenk tube by a Tygon tubing.

Figure 32-1 Apparatus for measuring the volume of gas evolved from the reaction.

2. 3.

Fill the graduated glass tube with water by pouring water through the bulb opening. Transfer exactly 1.00 g of the salt mixture from the glass vial to the Schlenk tube through the funnel. Put a magnetic stirring bar into the Schlenk tube.

4. 5. 6.

Rinse the funnel with about 3-4 mL water to get all the sample down to Schlenk tube. Freeze the water by immersing the bottom of the Schlenk tube into the ice-bath. Add 10.0 mL of 3.00 mol·L-1 HCl solution into the Schlenk tube and close the Schlenk tube with a septum.

7. 8.

By changing the bulb height adjust the water level in the graduated tube to zero. Open the stopcock connecting the Schlenk tube to the graduated tube and remove the ice bath.

9.

When the ice in the Schlenk tube melts, the acid rapidly reacts with CaCO3 liberating carbon dioxide gas. Stir the solution vigorously.

10. Wait until no more change in the level of water inside the graduated tube takes place. When no more gas evolution is observed, record the volume of the gas evolved.

55

Preparatory Problems, Practical
11. Open the Schlenk tube and filter the solution. Wash the solid with distilled water and then rinse with acetone. Decant the filtrate into the waste acid container. 12. Record the mass of calcium sulfate dried. Treatement of data 1. Calculate the number of moles of carbon dioxide gas liberated. (Vapor pressure of the acid solution at the temperature of experiment has to be considered). 2. 3. 4. 5. Calculate the amount of CaCO3 reacted. Calculate the weight percentage of CaSO4 present in the sample. Calculate the weight percentages of CaCO3 and CaCl 2 present in the salt mixture. Discuss the possible sources of experimental error.

Problem 33 Potassium bisoxalatocuprate(II) dihydrate: Preparation and analysis
The high natural abundance and high concentrations of copper and copper ores make copper an economical choice for many industrial applications. Copper may exist in three oxidation states, +1, +2, and +3. However, most copper compounds are commonly encountered as salts of Cu2+, while Cu3+ is the least stable form. Copper, being a transition metal, also forms coordination compounds. In this experiment, potassium bisoxalatocuprate(II) dihydrate will be prepared by reaction of copper(II) sulfate pentahydrate with potassium oxalate. CuSO4·5H2O(aq) + 2 K2C2O4(aq) → K 2Cu(C2O4)2·2H2O(s) + K2SO4(aq) + 3 H2O(l) The number of oxalato ligands in the complex ion will be determined by titration with standard permanganate solution. The copper content of the complex ion will be determined by iodine-thiosulfate titration. Chemicals and reagents ? ? ? ? ? Copper(II) sulfate pentahydrate, CuSO4.5H2O Potassium oxalate monohydrate, K2C2O4.H2O Sulfuric acid, H2SO4(aq), 2.5 mol.L-1 Potassium permanganate, KMnO4, 0.020 mol.L-1 Sodium carbonate, Na2CO3

56

Preparatory Problems, Practical
? ? ? ? ? ? ? Acetic acid, CH3COOH, dilute Potassium iodide, KI Sodium thiosulfate, Na2S2O3, 0.020 mol·L-1 Starch indicator (freshly prepared, 5% w/v) Potassium thiocyanate, KSCN Ethanol, C2H5OH Acetone, CH3COCH3 Substance CuSO4.5H2O K2C2O4.H2O H2SO4 KMnO4 Na2CO3 CH3COOH KI KSCN C2H5OH CH3COCH3 State Solid Solid 2.5 mol·L-1 0.020 mol·L-1 Solid dilute solution Solid Solid Liquid Liquid R Phrase 22 36 38 50 53 21 22 35 8 22 50 53 36 37 38 10 35 36 38 42 43 61 21 22 11 11 36 66 67 S Phrase 22 60 61 24 25 26-30-45 60 61 2 22 26 24 25 26 36 37 39 45 51 60 26 36 37 39 45 24 25 7 9 16 33 2 9 16 26

Apparatus and glassware ? ? ? ? ? ? ? ? ? ? Erlenmeyers, 250 mL (2) Beakers, 50 mL (2) Funnel Pipettes, 5 mL, 10 mL Graduated cylinder, 50 mL Burettes (2) Stirring rod Stirring bar Ice-bath Washing bottle

57

Preparatory Problems, Practical
? ? Filter paper Heater-stirrer

A. Preparation of potassium bisoxalatocuprate(lI) dihydrate 1. Dissolve 4.1 g copper(II) sulfate hexahydrate, CuS04.5H20 in 8.0 mL of water and heat the solution to 90 ° C. 2. Gradually add the hot solution, while stirring, to a solution (also at 90 °C) of 12.3 g K2C2O4.H2O in 35 mL of water. 3. Allow the solution to cool down to room temperature and then cool in an ice-bath to 10° Filter off the solid, wash with ice cold water followed by ethanol and then C. acetone. Dry at 40 °C in air for 1 h. 4. Weigh the dried sample. B. Determination of oxalate in the prepared compound 1. Transfer an accurately known amount (0.16-0.18 g) of potassium bisoxalatocuprate(lI) dihydrate prepared in Part A into a 250-mL erlenmeyer flask, and dissolve the complex by adding about 25 mL of water. 2. Add 20 mL of 2.5 M sulfuric acid and heat the solution to about 80 °C.

H2SO4(aq) + K2Cu(C2O4)2·2H2O(s)(aq) → K2C2O4(aq) + CuSO4(aq) + H2C2O4(aq) + 2 H2O(l)

3. Titrate the solution with standardized 0.020 mol.L-1 potassium permanganate solution, until the color of the solution becomes pink persistent for 1-2 minutes (end-point). Record the volume of standard KMnO4 solution used.

16 H+(aq) + 2 MnO4-(aq) + 5 C2O42-(aq) → 10 CO2(g) + 2 Mn2+(aq) + 8 H2O(l)

C. Determination of copper in the prepared compound 1. Add solid Na2CO3 to the solution obtained in Part B-2 until a precipitate first appears. Then add dilute acetic acid (10% w/v) until the pH is about 5. Finally, add about 1 g of solid potassium iodide into the solution.

58

Preparatory Problems, Practical
Na2CO3(aq) + CuSO4(aq) → Na2SO4(aq) + CuCO3(s) 2 Cu2+(aq) + 5 I-(aq) → 2CuI(s) + I3-(aq) 2. Titrate the liberated iodine which forms the I3- ion with standardized 0.020 M sodium thiosulfate solution using freshly prepared 5% (w/v) starch solution as indicator. A sharper end-point is obtained by the addition of 1-2 g potassium thiocyanate as the end-point is approached.

Treatment of data
1. Calculate the % yield of potassium bisoxalatocuprate(lI) dihydrate prepared. 2. Calculate the weight % of oxalate present in potassium bisoxalatocuprate(lI) dihydrate complex. Compare the experimental value with the theoretical value. 3. Write the oxidation reduction reactions involved for the determination of copper content. 4. Calculate the weight % of copper present in potassium bisoxalatocuprate(lI) dihydrate. Compare the experimental value with the theoretical value.

Problem 34 Synthesis and analysis of Aspirin
Aspirin, acetylsalicylic acid is both an organic ester and an organic acid. It is used extensively in medicine as an analgesic, pain releiver and as a fever-reducing drug. It is generally prepared by reaction of salicylic acid with acetic anhydride according to the following reaction.
O O OH H + O CH3 O O O CH 3 O CH3 O O H + CH 3 O OH

salicylic acid

acetic anhydride

acetylsalicylic acid

acetic acid

The amount of acetylsalicylic acid can be determined by titrating with a strong base such as sodium hydroxide.

CH3CO2C6H4CO2H(aq) + OH-(aq) → CH3COOC6H4COO-(aq) + H2O(l)

59

Preparatory Problems, Practical
However, being an ester acetylsalicylic acid is easily hydrolyzed, hence, during a normal titration with a strong base the alkaline conditions break it down leading to errors in analysis. Thus, a back titration method is applied, in which in the first step, all the acid present in solution is completely hydrolyzed by excess strong base such as NaOH. The aspirin/NaOH acid-base reaction consumes one mole of hydroxide per mole of aspirin. The slow aspirin/NaOH hydrolysis reaction also consumes one mole of hydroxide per mole of aspirin, Thus, the number of moles of NaOH added should be more than twice that of aspirin. Then, the amount of excess hydroxide is determined by titration with standard acid solution. In this experiment, acetylsalicylic acid will be prepared. The total amount of acid present will be determined by using a back titration method. Chemicals and reagents ? ? ? ? ? ? ? Salicylic acid, CH3CO2C6H4CO2H Acetic anhydride, CH3C2O3CH3 Phosphoric acid, H3PO4 or sulfuric acid, H2SO4, concentrated Ethanol, C2H5OH Sodium hydroxide, NaOH 0.50 mol·L-1 Hydrochloric acid, HCl 0.30 mol·L-1 Phenolphthalein indicator Substance CH3CO2C6H4CO2H CH3C2O3CH3 H3PO4 H2SO4 C2H5OH NaOH(aq) HCl(aq) Phase Solid Liquid Concentrated Concentrated R Phrase 22 36 37 38 41 61 10 20 22 34 23 24 25 35 36 37 38 49 23 24 25 35 36 37 38 49 11 20 21 22 36 37 38 40 35 23 25 34 38
-1

S Phrase 22 26 36 37 39 26 36 37 39 45 23 30 36 37 39 45 23 30 36 37 39 45 7 16 24 25 36 37 39 45 26 37 39 45 26 36 37 39 45

Liquid
0.50 mol·L-1 0.30 mol·L

Apparatus and glassware ? ? ? ? Beakers, 100 mL, Erlenmeyer, 250 mL (2) Pipettes, 5 mL and 10 mL Graduated cylinder, 50 mL

60

Preparatory Problems, Practical
? ? ? ? ? ? ? ? ? ? Burette, 50 mL Stirring rod Watch glass Buchner funnel Filter paper Vacuum filtration flask Melting point capillary tube Thermometer, 110° C Melting point apparatus Washing bottle

A. Synthesis of Aspirin, acetylsalicylic acid 1. Place accurately weighed 3.00 g of salicylic acid in a 100 mL Erlenmeyer flask. 2. Add 6.0 mL of acetic anhydride and 4 to 8 drops phosphoric acid to the flask and swirl to mix everything thoroughly. 3. Heat the solution to about 80-100° by placing the flask in hot water for about 15 C minutes. 4. Add 2 mL of cold water dropwise until the decomposition of acetic anhydride is completed and then 40 mL of water and cool the solution in ice bath. If crystals do not appear, scratch the walls of the flask with a stirring rod to induce crystallization. 5. Weigh the filter paper that will be used in filtration. Filter the solid by suction filtration through a Buchner funnel and wash the crystals with a few milliliters of ice cold water at about -5 ° C. 6. For recrystallization, transfer by dissolving the crystals into a beaker and add 10 mL ethanol and then add 25 mL warm water. 7. Cover the beaker with a watch glass and once crystallization has started place the beaker in an ice bath to complete the recrystallization. 8. Apply suction filtration as described in step 5. 9. Place the filter paper with the product onto a watch glass and dry in oven at 100 ° C for about 1 h and weigh the product. 10. Determine the melting point (135 ° to verify purity. C)

61

Preparatory Problems, Practical
Determination of amount of acetylsalicylic acid 1. Dissolve 0.5 g of aspirin in 15 mL of ethanol in a 250 mL Erlenmeyer flask. 2. 3. Add 20 mL of 0.50 mol·L-1 NaOH solution. In order to speed up the hydrolysis reaction, heat the sample in a water bath about 15 min after addition of two or three boiling chips to the flask swirling the flask occasionally. Caution: Avoid boiling, because the sample may decompose. 4. Cool the sample to room temperature and add 2-4 drops of phenolphthalein indicator to the flask. The color of the solution should be faint pink. If the solution is colorless add 5 mL of 0.50 mol·L-1 NaOH solution and repeat the steps 3 and 4. 5. 6. Record the total volume of 0.50 mol·L-1 NaOH solution added. Titrate the excess base in the solution with 0.30 mol·L-1 HCl solution until the pink color just dissappears and the solution becomes cloudy. 7. 8. Record the volume of 0.30 mol·L-1 HCl solution added. Repeat the titration two more times using two new samples.

Treatment of data 1. 2. 3. Calculate the yield of aspirin prepared. Calculate the amount of acetylsalicylic acid present in the Aspirin sample. Calculate the purity of aspirin and express in weight percentage.

Problem 35 Determination of iron and copper by iodometric titration
Master alloys are formed by mixing a base metal such as Al, Ni or Cu with a high percentage of one or two other metals. Master alloys are widely used in industry as semi-finished products. In metallurgical plants, master alloys are added to other molten metal mixtures for some purposes such as alteration of the composition to achieve certain chemical, electrical or mechanical properties in the final product. In this experiment, a sample solution of master alloy containing iron and copper ions will be analyzed by a two stage titrimetric method. First, the amount of Fe(III) ions will be determined by precipitation with pyrophosphate in acidic solution and then, the amount of Cu(II) will be calculated from the total amount of ions present in solution determined by an indirect iodometric titration with standard sodium thiosulfate solution.

62

Preparatory Problems, Practical
Chemicals and reagents: ? Test solution 0.10 M (simulating a digested sample of alloy containing both Fe3+ and Cu2+ ions in 4-6 g·L-1) ? ? ? ? ? Sodium thiosulfate standard solution, Na2S2O3, 0.050 mol·L-1 Sodium pyrophosphate solution, Na4P2O7, 5.0% (w/v) Hydrochloric acid solution, HCl(aq), 4.5 mol·L-1 Potassium iodide solution, KI, 10 %(w/v) Starch solution, 5.0% (w/v). Phase 4.5 mol·L
-1

Substance HCl solution KI solution Apparatus and glassware ? ? ? ? ? Burette, 50 mL

R Phrase 23 25 34 38 36 38 42 43 61

S Phrase 26 36 37 39 45

10% (w/v)

Graduated cylinder, 50 mL Erlenmeyer flask, 250 mL (2) Pipettes, 5 mL and 10 mL Watch glasses (2)

A. Determination of copper(II) ion 1. Transfer 10.0 mL of the test solution into a 250 mL Erlenmeyer flask, add 50 mL water and mix thoroughly. 2. To the same flask add 20 mL of 5.0% (w/v) pyrophosphate, 5.0 mL of 4.5 mol·L-1 HCl and 40 mL of 10% (w/v) KI. When pyrophosphate is added a precipitate may form. 3. Close the flask with a watch glass and leave in dark for 3-5 min for the formation of white precipitate. 4. Titrate the content of the flask with standard 0.020 mol·L-1 Na2S2O3 until a pale yellow color is obtained. 5. At this point, add 5 mL of starch indicator (5%w/v) and titrate until the color of solution changes from dark blue to milky white. 6. Record the volume of sodium thiosulfate solution added.

63

Preparatory Problems, Practical
B. Determination of total amount of copper(II) and iron(III) ions 1. Transfer 10.0 mL of the test solution into 250 mL Erlenmeyer flask, add 50 mL water and mix thoroughly. 2. To the same flask add 2 mL of 4.5 mol·L-1 HCl and 40 mL of 10% (w/v) KI into the solution and mix thoroughly. 3. Close the flask with a watch glass and leave in dark for 3-5 min. A small amount of white precipitate may be observed. 4. Titrate the solution with a standard 0.050 mol·L-1 Na2S2O3 until a pale yellow color is obtained. 5. At this point, add 5.0 mL of 5.0 % (w/v) starch indicator and titrate until the color of the solution changes from dark blue to milky white. 6. Record the volume of sodium thiosulfate solution added. Treatment of data and questions 1. Write the equations for the titration processes. 2. Explain why the solution is acidified. 3. Explain why starch is added close to the end of titration. 4. Calculate the number of moles of Cu2+ and Fe3+ ions present in the test solution. 5. Calculate the mass ratio of Cu2+ and Fe3+ ions.

Problem 36 Phenol propargylation: Synthesis of 1-nitro-4-(prop-2ynyloxy)benzene and (prop-2-ynyloxy)benzene
Propargyl unit can be anchored to phenolic substances via a SN2 type reaction under slightly basic condition. The resultant products can be feasible candidates as substrates for the following Huisgen dipolar cycloaddition reaction.

64

Preparatory Problems, Practical
In this experiment, two parallel experiments, using phenol as a reactant in one of them, and 4-nitrophenol as a reactant in the other, will be performed under the same conditions. Both experiments will be stopped after 3 h as indicated in the following procedure. Chemicals and reagents ? ? ? ? ? ? ? ? ? ? ? Phenol, C6H5OH 4-Nitrophenol, NO2C6H4OH Propargyl bromide, CH≡CCH2Br Toluene, C6H5CH3 DMF, dimethylformamide, (CH3)2NCHO Potassium carbonate, K2CO3 Ethyl acetate, CH3COOC2H5 Heptane, C7H16 Ether, C2H5OC2H5 Brine, saturated NaCl solution Anhydrous sodium sulfate Na2SO4 Phase liquid solid liquid liquid liquid solid liquid liquid liquid R Phrase 24 25 34 R36/37/38. 23 24 25 34 11 20 25 36/37/38 63 67 11 20 48 63 65 67 20 21 36 61 22 36 37 38 11 36 66 67 9 11 20 22 12 19 22 66 67 S Phrase 28 45 28 16 26 28A 37/39 16 25 29 33 45 53 16 23 29 33 9 16 23 29 33 9 16 29 33

Substance C6H5OH NO2C6H4OH CH≡CCH2Br C6H5CH3 (CH3)2NCO K2CO3 CH3COOC2H5 C7H16 C2H5OC2H5

Apparatus and glassware ? ? ? Round bottom flask, 50 mL Pipettes Magnetic stirrer

65

Preparatory Problems, Practical
? ? ? TLC precoated silica gel plates (Silica Gel PF-254) UV-lamp Flash column chromatography, thick-walled glass column filled with a flash grade Silica Gel 60. Prelaboratory work. Before starting the experiment, estimate the phenolic substrate that would undergo a faster reaction. Explain the reason. 1. Place 1.0 mmol 4-nitrophenol (or phenol) to a 50 mL round bottom flask containing 1.0 mL DMF. Stir the mixture at room temperature for 5 min and then add 1.2 mmol propargyl bromide (80% weight solution in toluene) and 1.2 mmol potassium carbonate. 2. Stir the resulting mixture at room temperature for 3 hrs until TLC analysis indicates the completion of the reaction. For TLC, use precoated silica gel plates (Silica Gel PF-254) and visualize the spots by UV-light. Use ethyl acetate: heptane 1:3 mixture as an eluent. 3. 4. 5. Dilute the reaction mixture with 1.0 mL of water and extract with 10.0 mL of ether. Wash the organic phase 3 times with 1.5 mL of brine, then dry over anhydrous sodium sulfate. Evaporate the solvent to afford the crude corresponding propargyl ether and weigh the product. Purify the crude product of (prop-2-ynyloxy)benzene by flash column chromatography which is performed by using thick-walled glass column with a flash grade Silica Gel 60.

Treatment of data 1. 2. 3. Calculate the Rf values of 4-nitrophenol and 1-nitro-4-(prop-2-ynyloxy)benzene. Repeat the same calculations for phenol and (prop-2-ynyloxy)benzene. Calculate the chemical yield of 1-nitro-4-(prop-2-ynyloxy)benzene isolated as a solid substance. Measure the melting point of this substance. Calculate the chemical yield of (prop-2-ynyloxy)benzene.

Problem 37 Huisgen dipolar cycloaddition: Copper(I)-catalyzed triazole formation
One of the most popular reactions within the “click chemistry” concept is the azide-alkyne Huisgen dipolar cycloaddition using a copper(I) catalyst. The procedure given below is an example for copper(I) catalyzed triazole formation considered as a “click chemistry” concept.

66

Preparatory Problems, Practical
CuSO4.5H2O (5 mol %), sodium ascorbate (10 mol %)
N N N N

N

+

N

t-BuOH:H2O (1:1) 60 0C

Chemicals and reagents ? ? ? ? ? ? Benzyl azide, C6H5CH2N3 Phenyl acetylene, C6H5C≡CH Copper(II) sulfate pentahydrate, CuSO4·5H2O Sodium ascorbate, NaC6H7O6 Aqueous ammonia solution, NH3(aq), 10% wt tertiary-Butylalcohol, (CH3)3COH

Substance C6H5CH2N3 C6H5C≡CH CuSO4 5H2O NaC6H7O6 NH3 (CH3)3COH

Phase Liquid Liquid 1.0 mol·L-1 Solid 10% wt solution liquid

R Phrase 10 20 21 22 10 36 37 38 22 36 38 50 53 10 23 24 34 50 11 20

S Phrase 26 36 37 39 16 33 60 22 60 61 24 25 9 16 26 33 36 37 39 45 61 9 16

Apparatus and glassware ? ? ? ? ? ? ? ? Screw-top vial 20 mL Pasteur pipettes Stirring bar Plastic syringes (1 and 2 mL) Graduated cylinder Thermometer TLC precoated silica gel plates (Silica Gel PF-254) Heater and stirrer

67

Preparatory Problems, Practical
? 1. 2. Buchner funnel Dissolve 133 mg (1 mmol) of benzyl azide in 1mL of t BuOH:water (1:1) solution and add via syringe in a 20 mL screw-top vial having a stirring bar. Close the cap and add followings to the reaction vial via a syringe through PTFE (cap liner) of the cap. a) 1 mmol of phenyl acetylene dissolved in 1.0 mL of tBuOH:water (1:1) solution. b) 9.8 mg (10 mol %) of sodium ascorbate in 0.5 mL of t BuOH:water (1:1) solution. c) 2-3 Drops (~5 mol %) of 1.0 mol/L aqeous copper(II) sulfate pentahydrate. 3. Stir the mixture at 60 oC for 1-2 h until completion by TLC (use ethyl acetate: heptane 1:2 mixture as an eluent). 4. Dilute the reaction mixture with 10 mL of ice water and add 2.0 mL of 10% wt aqueous ammonia solution. Stir for another 5 min and collect the solid precipitate with a Buchner filter and air-dry overnight. Treatment of data 1. Calculate the yield of the product.

68


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