AUSTRALIAN CHEMISTRY OLYMPIAD

FINAL PAPER

PART B

1992

Instruction to candidates

(1) You are allowed 10 minutes to read this paper, and 3 hours to complete the questions.

(2) You are not permitted to refer to books or notes but you may use a non-programable electroniccalculator and molecular models.

(3) You must attempt all questions.

(4) Answers must be written in the blank space provided immediately below each question in the exambooklet. Rough working must be on the backs of pages. Only material presented in the answer boxeswill be assessed.

(5) Answers must provide clearly laid out working and sufficient explanation to show how youreached your conclusions.

(6) Your name must be written in the appropriate place on each page of your answers.

(7) Use only black or blue ball point pen for your written answers, pencil or other coloured pens are notacceptable.

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11

1 2

3 4 5 6 7 8 9 10

12 13 14 15 16 17 18

19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36

37 38 39 40 41 42 43 44 45 46 46 48 49 50 51 52 53 54

55 56 57 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86

87 88 89 104 105 106

58 59 60 61 62 63 64 65 66 67 68 69 70 71

90 91 92 93 94 95 96 97 98 99 100 101 102 103

1

107 108 109

Na

HH He

Li Be B C N O F Ne

Mg Al Si P S Cl Ar

K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr

Rb Sr Y Zr Nb Mo Tc Ru Pd Ag CdRh In Sn Sb Te I Xe

Cs Ba La Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi Po At Rn

Fr Ra Ac Unq Unp Unh

Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

Th Pa U Np Pu Am Cm Bk Cf Es Fm Md No Lr

Uns Uno Une

22.99

1.008 4.003

20.18

39.95

83.80

131.3

(222)

6.941 9.012 10.81 12.01 14.01 16.00

24.31 26.98 28.09 30.97 32.06 35.45

19.00

79.90

126.9

(210)(209)

78.96

127.6

74.92

121.8

209.0

72.59

118.7

207.2

69.72

114.8

204.4

65.38

112.4

200.6

63.55

107.9

197.0

58.69

106.4

195.1

58.93

102.9

192.2

55.85

101.1

190.2

54.94

(98)

186.2

52.00

95.94

183.9

50.94

92.91

180.9

47.88

91.22

178.5

44.96

88.91

138.9

(227)

40.08

87.62

137.3

226.0

39.10

85.47

132.9

(223)

1.008

140.1 140.9 144.2 (145) 150.4 152.0 157.3 158.9 162.5 164.9 167.3 168.9 173.0 175.0

(260)(259)(258)(257)(252)(251)(247)(247)(243)(244)(237)238.0(231)232.0

Atomic number

Atomic mass

Periodic Table of the Elements

Atomic mass values given here are to four significant figures. A value given in parentheses denotes the mass of the longest-lived isotope.

8B4B 5B 6B 7B3B

2A

1A

1B 2B

3A 4A 5A 6A 7A

8A

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Question 1

(a) The following gas phase reactions are known to occur at varying temperatures.

n-C4H10 → (CH3)2C = CH2 + H2

C2H4 + Cl2 → CH2ClCH2Cl

CH4 + 12O2 → CH3OH

Given the following bond energies

Bond Bond energy(kJ)

C H 413

C C 347

C C 614

H H 432

C Cl 339

C O 358

O H 467

Cl C 239

O O 146

O O 495

(i) Predict the approximate values of the heats of reaction and specify which are exothermic andwhich are endothermic.

(ii) For each reaction specify whether the entropy change is negative or positive.(iii) Indicate if a reaction would be reversible, and what role pressure would play in the equilibrium.

(b) In an investigation of the kinetics of the catalytic decomposition of NH3 into its elements at1100°C, the following half lives of NH3 samples (with no N2 or H2 initially present) andrespective initial partial pressures of NH3 were found.

Half life(minutes) 7.6 3.7 1.7

Initial partial pressure of NH3(atmospheres) 0.349 0.171 0.076

(i) Determine the order of the reaction(ii) Determine the rate constant of decomposition.

Question 2Charged species such as hydrated metal ions do not generally have an appreciable solubility in

non-polar liquids. However, if the metal ion forms a neutral complex with an organic ligand,the hydrophobic metal-ion complex is often much more soluble in organic solvents. When anaqueous solution of metal ion is shaken with an immiscible organic solvent containing suchan organic ligand, the metal-ion complex is distributed between the two phases.

This equilibrium process is characterised by the Distribution Ratio, D where

D = [total concentration of all forms of metal ion in the organic phase][total concentration of all forms of metal ion in the aqueous phase]

The experimental procedure is known as solvent extraction. One widespread application ofsolvent extraction is to use a ligand which forms a coloured complex with the metal ion. Inthis way, a metal ion in a very dilute solution (say Ni2+(aq) in tap water ) can be concentratedby solvent extraction (or separated from other metal ions) as a coloured complex into a much

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smaller volume of an organic solvent. This more concentrated, coloured organic solutionmight then be analysed using visible spectrophotometry for example.

Often, the organic complexing agent is a weak acid (HL), whose conjugate base is the ligand(L-). Protons and metal ions compete for L-, so the fraction of metal ion which is extractedinto the organic solvent is pH-dependent.

The equilibria involved in this process are shown schematically below, for a specific example.Nickel(II) forms a very thermodynamically stable complex (log β2 = 17.72) in water with theconjugate base of dimethylglyoxime. Dimethylglyoxime is a very weak monoprotic acid (pKa= 10.66) in water. The intensely red bis(dimethylglyoximato)nickel(II) is quite soluble inorganic solvents so very low levels of Ni(II) can be can be spectrophotometrically determinedafter complexation with DMG and extraction into chloroform.

H+

+

KMLKHL

Ka

β2

aqueous phase

Ni2+ + 2 DMG- Ni(DMG)2

H-DMG

Ni(DMG)2H-DMGorganic phase

In this example, the red nickel complex is much more soluble in the chloroform phase than inwater:

KML = [Ni(DMG)2]org[Ni(DMG)2 ]aq

= 300

while dimethylglyoxime is only slightly more soluble in the organic phase than in water:

KHL = [H-DMG]org[H-DMG]aq

= 3

Note that the charged species, H+, Ni2+ and DMG-(aq) are not at all soluble in chloroform (asexpected).

(a) After writing down expressions for all the equilibrium constants in this system, express[Ni(DMG)2]org in terms only of [Ni2+] and [DMG-] and relevant equilibrium constants.

(b) Express [DMG-] in terms only of [H+] and [H-DMG]org and relevant equilibrium constants.

(c) Express D in terms only of [H+] and [H-DMG]org and relevant equilibrium constants. You mayassume that [Ni2+] >> [Ni(DMG)2 ]aq.

(d) Express the fraction of Ni(II) which remains in the water phase in terms only of D and thevolumes of the two phases.

(e) Calculate the concentration of nickel which remains in the aqueous phase when 1L of 10-4 MNi2+ at pH 5.0 is extracted with 50 mL chloroform containing 0.01M dimethylglyoxime.

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Question 3Alcohol dehydrogenases are enzymes which catalyse the transfer of a hydrogen from a

substrate alcohol to a coenzyme (NAD+) (1). In the process the NAD+ is reduced to NADH(2) the hydrogen being added stereospecifically to one face of (1). The face chosen dependsupon the exact nature of the enzyme. Using suitably deuterated alcohols as substrates it ispossible to probe the stereochemistry of the hydrogen transfer. The resulting coenzyme (2)can chemically degrade as shown to afford a stereospecifically deuterated succinic acid (3).This is illustrated in Scheme 1 for yeast alcohol dehydrogenase.

A newly isolated alcohol dehydrogenase has been probed by this method, and a specificallydeuterated succinic acid of unknown stereochemistry isolated from the chemicaldegradation. Further analysis shows that this is chemically and optically identical with areference sample of monodeuteration succinic acid obtained by the enzymatic anti additionof D2O to (E)-but-2-ene-1,4-dioic acid (5) followed by chemical removal of the alcohol groupfrom the intermediate (2S,3X-deuterio 2-hydroxybutan-1,4-dioic acid).

Scheme (1)

N N

N

HDH

R

CONH2 CONH2

R

DHCONH2

R

CH3OCH2

CO2H

DH

CO2H

HO2C

H

H

CO2 H

(1) (2)

yeast alcoholdehydrogenase

RCD2OH

CH3CO2H/CH3OH

(3)

(i) O3

(ii) oxid.

(4)

44

(5)

(E)-but-2-endioic acid

Note: When D2O is added to olefins under biological conditions the deuterium bond to oxygenis labile and easily lost only C-bonded deuterium remains.

a) Using Fischer formulae, write down the structure of 2S,3X-deuteriomalic acid obtained andindicate if X corresponds to a R or S configuration.

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b) Indicate the 4R or 4S stereochemistry of the reduced deuterated coenzyme.

c) Indicate if the hydrogen transfer occurs from the Si or Re face of the coenzyme.

Question 4X is a metal. Some of the reactions to prepare its compounds are given in the following scheme:

G

A X C H

B

E

F

D I

(CH3CO)2O

KOHHNO3dil.

NH4F HCl dil. H2SO4 dil. H2S

I2 Br2

The selenide of X, prepared by heating X in a current of H2Se and subliming the product inhydrogen at a dull red heat, contains 41.26% of Se.

a) Determine the relative atomic mass (atomic weight) of X, and identify the element.

b) Give the formulae of compounds A to I and write balanced equations for their formation.

c) To a 0.020 mol/dm3 solution of X2+ ions, 0.200 moles of NH3 were added with formation of[X(NH3)4]2+ ions. Calculate the amount of KI to be added in order to keep the concentrationof [X(NH3)4]2+ equal to 3.5 x 10-6 mol/dm3 and the molar concentrations of all species at theequilibrium. (Kstab for [X(NH3)4]2+ and [Xl4]2- are 4.0 x 106 and 1.25 x 106, respectively).

Question 5Ambucaine (C17H28N2O3) is a local anaesthetic derived from benzoic acid. Its infra-red

spectrum shows an absorption near 1730 cm-1 and ambucaine dissolves in dilute mineralacid.

The synthesis starts with the nitration of methyl benzoate using a sulfuric-nitric acid mixture.Subsequent saponification (NaOH/H2O) of the nitrated product followed by neutralisation ofthe resulting sodium salt, yields A (C7H5NO4). Catalytic reduction of A with hydrogen in thepresence of palladium affords B which unlike A is soluble in dilute mineral acid. Reaction ofB with nitrous acid followed by heating in water gives C (C7H6O3) which gives a violetcolouration with ferric chloride solution. Further nitration of C gives D which reacts withethanol in the presence of a trace of concentrated acid to give E (C9H9NO5), which issoluble in cold aqueous sodium hydroxide but not sodium bicarbonate. E reacts withpotassium hydroxide and 1-bromobutane to yield F (C13H17NO5) which is no longer solublein cold dilute sodium hydroxide. Saponification of F with aqueous KOH, yields G which isreacted firstly with thionyl chloride (SOCl2) to give H . It in turn is reacted with 2-(N,N-diethylamino)ethanol to afford I (C17H26N2O5) which upon catalytic hydrogenation (H2/Pd)affords Ambucaine.

(a) Deduce the structures of compounds A to I and hence the structure of Ambucaine.

(b) Proposed a reasonable mechanisms to account for the conversion of E to F and G to I.