UPSC Combined Geo-Scientist, Mains · 2026 · Mains Paper III
Q6(b). Cannizzaro; LiAlH₄ on a lactam; selective reduction of an ester
Question
Identify the major products in the given reactions: (i) 2-naphthaldehyde, KOH, heat → A+B; (ii) 5,5-dimethylpyrrolidin-2-one, LiAlH₄, THF → C; (iii) ethyl 3-cyanobenzoate, Ca(BH₄)₂ → D.
Answer in brief
(i) A = 2-naphthylmethanol, B = 2-naphthoic acid (as potassium salt): Cannizzaro. (ii) C = 2,2-dimethylpyrrolidine. (iii) D = 3-(hydroxymethyl)benzonitrile: the ester is reduced, the nitrile survives.
Key points
- (i) No α-H: hydroxide adds to one CHO; the adduct gives hydride to a second aldehyde (alcohol + carboxylate).
- (ii) LiAlH₄ reduces amides and lactams fully, C=O → CH₂ (amine; not the amino alcohol).
- (iii) Ca(BH₄)₂ is a milder, more Lewis-acidic borohydride than LiAlH₄: it reduces esters to alcohols but does not touch aryl nitriles.
The full model answer
Worked out step by step, as it should go in the answer booklet. The solution sheet follows it.
- (i) No α-H: hydroxide adds to one CHO; the adduct gives hydride to a second aldehyde (alcohol + carboxylate).
- (ii) LiAlH₄ reduces amides and lactams fully, C=O → CH₂ (amine; not the amino alcohol).
- (iii) Ca(BH₄)₂ is a milder, more Lewis-acidic borohydride than LiAlH₄: it reduces esters to alcohols but does not touch aryl nitriles.
CoversCannizzaroLiAlH₄ on a lactam
All parts of Question 6
- 6(a) Hofmann and Schmidt reactions4 marks
- 6(b) Cannizzaro; LiAlH₄ on a lactam; selective reduction of an ester4 marks
- 6(c) Gramine from indole and its displacement by cyanide4 marks
- 6(d) Benzoin condensation and Darzens reaction4 marks
- 6(e) Two epoxidations of carvone4 marks
- 6(f) Photochemistry of stilbene4 marks
- 6(g) Racemisation of an optically active 1,5-diene4 marks
- 6(h) Identify the amide from its IR bands4 marks
- 6(i) Woodward–Fieser rules for three dienes4 marks
- 6(j) Halogens from the M, M+2 and M+4 peaks4 marks