1 Compound A has the molecular formula C8H10. When A is treated with chlorine in the presence of ultraviolet light, compound B having the molecular formula C8H9Cl is formed.

A also reacts with chlorine under a different condition to produce compound E which is isomeric with B. When B is heated with aqueous NaOH, compound C with molecular formula C8H10O is formed. However, when E is subjected to heating with aqueous NaOH, no new compound is formed.

When C is heated with acidified potassium manganate (VII), compound D is formed, which gives an orange precipitate when heated with 2,4-dinitrophenylhydrazine. When E is heated with acidified potassium manganate (VII), effervescence is observed and compound F with molecular formula C7H5O2Cl is formed.

Deduce the structural formulae of compounds A to F, explaining your reasoning.

2 A certain organic solvent was distilled to produce a single compound A. When A was reacted with 2,4-dinitrophenylhydrazine, an orange precipitate was produced. With aqueous alkaline iodine, A gave a pale yellow precipitate. A did not react with either hot acidified potassium dichromate (VI) nor with aqueous bromine. Reduction of A with LiAlH4

in dry ether yielded an equimolar mixture of two isomers, B and C, with the molecular formula C8H16O.

Suggest the structural formulae of A, B and C, explaining the reactions involved.

3 Alcohol A has the molecular formula C6H14O. Reaction of A with acidified potassium manganate (VII) produces compound B. B forms a yellow precipitate when warmed with aqueous alkaline iodine. Heating A with concentrated H2SO4 at 170oC produces C, C6H12. Vigorous oxidation of C with hot concentrated acidified potassium manganate (VII) forms 3-methylbutanoic acid as one of the products.

Deduce the structural formulae of compounds A to C, explaining your reasoning.

4 Compound A, C6H7ON, is sparingly soluble in water but dissolves in hydrochloric acid. It also dissolves in aqueous NaOH but not in aqueous Na2CO3.

On reaction with 1 mol ethanoyl chloride, A forms compound B, C8H9O2N. B is no longer soluble in hydrochloric acid but is still soluble in aqueous NaOH. On reacting B with aqueous bromine, compound C with molecular formula C8H6O2NBr3 is formed.

When A is treated with 2 mol ethanoyl chloride, compound D with molecular formula C10H11O3N is formed. D is neither soluble in hydrochloric acid nor in aqueous NaOH.

Deduce the structural formulae of compounds A to D, explaining your reasoning.

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Chemistry H2 9746Tutor TuteeRevision Exercise 21: Structural Elucidation

5 A neutral compound A has the molecular formula, C14H18O3NBr. When A is refluxed with aqueous NaOH, three compounds B, C and D are obtained.

Upon analysis, B is found to be a straight-chain molecular with molecular formula C3H9N. When B is treated with excess chloroethane, E is formed. E has a relative molecular mass of 179.5 and gives a white precipitate immediately when treated with aqueous silver nitrate.

C has the molecular formula C3H5O3Na. Upon acidification, it yields F. F is also obtained when A is refluxed with dilute H2SO4. It was found that 2 mol of aqueous NaOH is required for complete neutralisation of 1 mol of F.

D (C8H7O3Na), upon addition of acidified potassium dichromate(VI) and heating with immediate distillation, yields G. Treating G with 2,4-dinitrophenylhydrazine gives an orange precipitate with the following structure:

Deduce the structural formulae of compounds A to G, explaining your reasoning.

6 P, C10H16O, is an insect sex attractant. When P is gently oxidised with warm acidified potassium dichromate (VI) it gives Q, C10H14O, which gives an orange precipitate with 2,4-dinitrophenylhydrazine reagent, but does not give an orange precipitate with Fehling’s reagent.

Treatment of Q with cold dilute acidified potassium manganate (VII) gives R, C10H20O7, but Q with hot concentrated acidified potassium manganate (VII) gives S, T and carbon dioxide in the ratio 1:1:2.

S, C3H6O, gives a yellow precipitate with alkaline aqueous iodine.

T can be synthesised from the following reaction sequence.

HCN H+/K2Cr2O7 boilingCH2(CHO)2 U V T, C5H4O6

NaCN (aq) warm H2SO4 (aq)

Identify compounds S, T, U and V. Suggest a possible structure for Q and thus for R and P. Explain your reasoning.

7 An optically active compound F, C9H12O, reacts with acidified K2Cr2O7 to give G, C9H10O. F reacts with iodine in aqueous sodium hydroxide to give a yellow precipitate and the sodium salt of the acid H, C8H8O2. H is oxidised by acidified KMnO4 to form I, C8H6O4.

Suggest the structures of F, G, H and I, giving relevant equations to explain your reasoning.

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8 J, C9H11I, on boiling with aqueous potassium hydroxide gave a compound K, C9H12O. With acidified sodium dichromate(VI), compound K yielded L, C9H10O. With hot acidified potassium manganate(VII), compound K yielded M, C7H6O2. K when warmed with potassium hydroxide and iodine is able to produce yellow precipitate.

Suggest the structures of J, K, L and M, giving relevant equations to explain your reasoning.

9 Compound C, C6H10O, is an insect pheromone. C reacts with Fehling’s solution, decolourises Br2(aq) and gives an orange precipitate with 2,4-dinitrophenylhydrazine reagent. Treatment of C with H2 over a nickel catalyst produces D, C6H14O, which reacts with hot acidified K2Cr2O7 to give E, C6H12O2. Neither D nor E reacts with 2,4-dinitrophenylhydrazine, but both react with sodium metal.

Compound E can be obtained from 3-bromopentane by the following route.

Identify the lettered compounds D to F, explaining your reasoning and suggest a possible structure for C.

10 The molecules of compound K, C7H15Br, are chiral. On treatment with NaOH(aq), K produces alcohol L, C7H15OH, which does not react with hot acidified Na2Cr2O7(aq). Treating compound K with ethanolic NaOH produces a mixture of four different isomeric alkenes with the formula C7H14, only two of which are cis-trans isomers of each other.

Suggest the structural formulae of compound K and the four alkenes.

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