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UPSC Combined Geo-Scientist

Chemist

  • 3 descriptive papers
  • 200 marks each
  • 3 h per paper

Mains Paper III covers analytical and organic chemistry.

UPSC Combined Geo-Scientist, Mains · 2026 · Mains Paper III

Q6(d). Benzoin condensation and Darzens reaction

  • 4 marks
  • Topic: Named reactions
  • Answered
Question

Predict the reactants and identify the name of the reactions: (i) A+B with NaCN, EtOH gives 2-chlorobenzoin (Ph–CH(OH)–CO–C₆H₄Cl-2); (ii) C+D with t-BuOK, t-BuOH gives the spiro glycidic ester (cyclohexane-spiro-epoxide, CO₂Et).

Answer in brief

(i) A = 2-chlorobenzaldehyde, B = benzaldehyde: crossed benzoin condensation. (ii) C = cyclohexanone, D = ethyl chloroacetate: Darzens glycidic ester condensation.

Key points

  • (i) CN⁻ adds to 2-chlorobenzaldehyde (the more electrophilic aldehyde) and the cyanohydrin anion (an acyl-anion equivalent, umpolung) attacks benzaldehyde; loss of CN⁻ gives ArCO–CH(OH)Ph.
  • (ii) The base removes the acidic α-H of the chloroester; the enolate adds to the ketone (aldol), and the alkoxide displaces Cl⁻ by intramolecular S_N2 (3-exo-tet) to close the epoxide.

The full model answer

Worked out step by step, as it should go in the answer booklet. The solution sheet follows it.

  • (i) CN⁻ adds to 2-chlorobenzaldehyde (the more electrophilic aldehyde) and the cyanohydrin anion (an acyl-anion equivalent, umpolung) attacks benzaldehyde; loss of CN⁻ gives ArCO–CH(OH)Ph.
  • (ii) The base removes the acidic α-H of the chloroester; the enolate adds to the ketone (aldol), and the alkoxide displaces Cl⁻ by intramolecular S_N2 (3-exo-tet) to close the epoxide.
Model answer to Q6(d), 2026 Mains Paper III: solution page 17
Solution page 17 of 30
Read the whole 2026 solution2026 question paper

CoversC–C bond formationBenzoin condensation and Darzens reaction

All parts of Question 6