UPSC Combined Geo-Scientist, Mains · 2026 · Mains Paper III
Q6(e). Two epoxidations of carvone
Question
Identify the major products A and B formed in the following reactions of carvone: mCPBA gives A; H₂O₂/NaOH gives B.
Answer in brief
A: epoxide on the isopropenyl group (electrophilic epoxidation, mCPBA, the more electron-rich alkene). B: epoxide on the ring C=C of the enone (nucleophilic, Weitz–Scheffer epoxidation).
Key points
- it attacks the nucleophilic, electron-rich, 1,1-disubstituted C=C (isopropenyl), not the electron-poor enone C=C.
- it adds to the β-carbon of the enone (Michael) and the enolate closes the epoxide by displacing HO⁻; the isolated alkene is untouched.
The full model answer
Worked out step by step, as it should go in the answer booklet. The solution sheet follows it.
- mCPBA is an electrophilic oxidant: it attacks the nucleophilic, electron-rich, 1,1-disubstituted C=C (isopropenyl), not the electron-poor enone C=C.
- HOO⁻ is a nucleophile: it adds to the β-carbon of the enone (Michael) and the enolate closes the epoxide by displacing HO⁻; the isolated alkene is untouched.
Coverschemoselective epoxidationTwo epoxidations of carvone
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