(a) (i) An organic compound (X) has the molecular formula $C_5H_{10}O$. Draw structures for (X) if it: (3 + 2)

(I) does not give Tollen's test but gives a positive iodoform test.

(II) does not give Tollen's test and iodoform test but undergoes Aldol condensation.

(III) undergoes Cannizzaro's reaction.

(ii) Show how each of the following compounds can be converted to benzoic acid:

(I) Acetophenone (II) Ethyl benzene

OR

(b) Answer the following questions: $(5 \times 1)$

(i) Draw structure of the 2, 4-dinitrophenyl hydrazone derivative of benzaldehyde.

(ii) Arrange the following in increasing order of their reactivity towards HCN: Di-tert. butyl ketone, Acetaldehyde, Acetone

(iii) Give a simple chemical test to distinguish between benzoic acid and ethyl benzoate.

(iv) Write the name of the reagent to convert Ethanenitrile to Ethanal.

(v) Draw the structure of 'X' in the following reaction: Cyclohexanol —($CrO_3$)→ 'X'

Primary answer — part (a)

(a)(i) Structures of $C_5H_{10}O$ (one degree of unsaturation = one $C=O$ group)

• (I) No Tollens', positive iodoform → a methyl ketone:

Pentan-2-one, CH₃–CO–CH₂–CH₂–CH₃ (also acceptable: 3-methylbutan-2-one, CH₃–CO–CH(CH₃)₂). It has the CH₃CO– unit needed for iodoform but, being a ketone, gives no Tollens' test.

• (II) No Tollens', no iodoform, undergoes Aldol → a non-methyl ketone with α-hydrogen:

Pentan-3-one, CH₃CH₂–CO–CH₂CH₃. Not a methyl ketone (no iodoform), a ketone (no Tollens'), but has α-H for aldol.

• (III) Undergoes Cannizzaro → an aldehyde with NO α-hydrogen:

2,2-Dimethylpropanal (pivaldehyde), (CH₃)₃C–CHO.

(a)(ii) Conversions to benzoic acid

• (I) Acetophenone: $C_6H_5COCH_3$ —(alkaline $KMnO_4$, Δ; then $H_3O^+$)→ $C_6H_5COOH$. The oxidant degrades the –COCH₃ side chain to –COOH.
• (II) Ethylbenzene: $C_6H_5CH_2CH_3$ —(alkaline $KMnO_4$, Δ; then $H_3O^+$)→ $C_6H_5COOH$. Any alkyl side chain with a benzylic H is oxidised fully to –COOH.

OR — part (b) (brief)

1. (i) 2,4-DNP of benzaldehyde: C₆H₅–CH=N–NH–C₆H₃(NO₂)₂ (the N is attached to a 2,4-dinitrophenyl ring).
2. (ii) Increasing reactivity towards $HCN$: Di-tert-butyl ketone < Acetone < Acetaldehyde (steric + electronic effects; aldehyde most reactive).
3. (iii) Add $NaHCO_3$: benzoic acid gives brisk effervescence ($CO_2$); ethyl benzoate does not.
4. (iv) Reagent: DIBAL-H (di-isobutylaluminium hydride), then $H_2O$ — (Stephen reduction, $SnCl_2$/HCl, also acceptable).
5. (v) Cyclohexanol —($CrO_3$)→ Cyclohexanone (X).