8/12/2019 Compilation of Structure Elucidation Questions

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A Compilation of Structure Elucidation Questions

Chuah Wei Bin

1. (N2002/P3/Q8(either))

This question is about compound F, C8H8O2, which is formed when phenyl ethanoate, CH3CO2C6H5,is warmed with aluminium chloride.

Compound F is insoluble in water, but dissolves in NaOH (aq).

It reacts with 2,4-dinitrophenylhydrazine, 2,4-DNPH, but not with Fehlings solution.

With bromine water, F gives G, C8H6O2Br2.

With alkaline aqueous iodine followed by acidification, F gives H, C7H6O3.

Compound H dissolves in both NaOH(aq) and Na2CO3(aq). Compound H can also be obtained by

reacting oil of wintergreen, J, with hot NaOH(aq), followed by H2SO4(aq).

CO2CH3

OH

J

Deduce the structures of compounds F, G and H. Explain the chemistry of the reactions described,writing equations where appropriate. [There is no need to comment on the chemistry of the formationofF from phenyl ethanoate.]

2. (N2002/P3/8(or))

This question is about compoundK, C6H7ON, which is formed when phenylhydroxylamine, C6H5NHOH,is warmed with dilute sulfuric acid.

Compound K is not very soluble in water, but dissolves in HCl(aq).

It also dissolves in NaOH(aq), but not in Na2CO3(aq).

On reaction with 1 mol of ethanoyl chloride, CH3COCl, K forms compound L, C8H9O2N.

L is no longer soluble in HCl(aq), but is still soluble in NaOH (aq). On reaction with Br2(aq), L produces

compoundM, C8H7O2NBr2. WhenK is reacted with 2 mol of ethanoyl chloride, it produces compoundN, which is not soluble in either HCl (aq) or NaOH(aq).

Compound K can be synthesised by treating phenol with dilute nitric acid, followed by reaction withzinc metal and hydrochloric acid.

Deduce the structures of compounds K, L, M and N. Explain the chemistry of the reactions described,writing equations where appropriate. [There is no need to comment on the chemistry of the formationofK from phenylhydroxylamine.]

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8/12/2019 Compilation of Structure Elucidation Questions

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3. (CJC2011/P3/Q4(c))

A hydrocarbon A has the molecular formula of C10H14. When A is treated with hot aqueous acidifiedKMnO4(aq), benzoic acid is formed. When reacted with bromine in the presence of ultraviolet light,

A produced four isomeric monobromo compounds with the formula C10H13Br. Upon heating withalcoholic NaOH, two of the monobromo compounds B and C gave the same compound which can existas a pair of geometric isomers, D and E. Heating B with aqueous NaOH produces compound F whichdoes not react with acidified potassium dichromate(VI). Suggest the structures ofAto F, explainingyour reasoning.

4. (TPJC2011/P3/Q5(c))

Compound M (C10H13O3N) reacts fully with Br2(aq) in the ratio 1 : 2, but it does not decolourise

cold alkaline KMnO4. M also shows no reaction with PCl5. When warmed in aqueous alkali andsubsequently neutralised, M produces P (C8H11O2N) and Q (C2H4O2). P changes acidified K2Cr2O7from orange to green, and forms R (C8H9O3N) in the process. Both Q and R react with Na2CO3solution to give effervescence. R also reacts fully with Br2(aq) in the ratio 1 : 2. Incidentally, R can

be found naturally as a building block for proteins. It was noted that R does not have intramolecularhydrogen bonding.

Suggest structures for M, P, Q and R, explaining how the identities you suggest fit in with the

experimental observations described above.

5. (VJC2011/P3/Q4(b))

D (C10H8O2) is a neutral compound. On heating with aqueous NaOH followed by acidification, Dyields only one organic product, E (C10H10O3), which is soluble in aqueous Na2CO3. When E istreated with H2 and Ni, F(C10H12O3) is formed. Unlike E, F does not display cis-trans isomerism.

1 mol of F is found to decolorize exactly 2 mol of aqueous Br2. Upon reaction with LiAlH4 in dryether, F forms the compound G. G is no longer soluble in Na2CO3. When G is passed over hot Al2O3,H is produced.

H reacts with gaseous HCl to give the major product I, which exists as optical isomers. On warmingI with aqueous AgNO3, aw white precipitate is formed together with an organic product which doesnot change the colour of acidified K

2Cr

2O7

solution.

Deduce the structures of all the lettered compounds (Dto I) and explain the reactions involved.

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