UPSC Combined Geo-Scientist, Mains · 2026 · Mains Paper III
Q6(a). Hofmann and Schmidt reactions
Question
Predict the major product in the following reactions, and name the reactions: (i) benzamide with Br₂/KOH; (ii) 1-tetralone with NaN₃, H₂SO₄ (solvent CHCl₃, as printed).
Answer in brief
(i) Hofmann rearrangement: aniline (PhNH₂, one carbon fewer). (ii) Schmidt reaction: the cyclic lactam 2,3,4,5-tetrahydro-1H-1-benzazepin-2-one (aryl migrates to N).
Key points
- (i) PhCONH₂ →PhCONHBr → PhCON⁻Br → PhN=C=O →PhNH₂ + CO₂. The aryl group migrates from C to N with loss of Br⁻; the isocyanate is hydrolysed.
- (ii) The ketone is protonated, adds HN₃; loss of N₂ with migration of the group anti to the leaving N₂: for aryl alkyl ketones aryl migrates, so N is bonded to the benzene ring and the ring expands by one atom (6 to 7).
The full model answer
Worked out step by step, as it should go in the answer booklet. The solution sheet follows it.
- (i) PhCONH₂ →PhCONHBr → PhCON⁻Br → PhN=C=O →PhNH₂ + CO₂. The aryl group migrates from C to N with loss of Br⁻; the isocyanate is hydrolysed.
- (ii) The ketone is protonated, adds HN₃; loss of N₂ with migration of the group anti to the leaving N₂: for aryl alkyl ketones aryl migrates, so N is bonded to the benzene ring and the ring expands by one atom (6 to 7).
Coversmigration to electron-deficient nitrogenHofmann and Schmidt reactions
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