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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(g). Racemisation of an optically active 1,5-diene

  • 4 marks
  • Topic: Pericyclic reactions
  • Answered
Question

The optically active compound (X), a trans-fused bicycle with two C=C bonds in the large ring, racemizes on heating. Explain the observation.

Answer in brief

On heating, X undergoes a Cope ([3,3]-sigmatropic) rearrangement through its 1,5-diene unit; the transition state and the product allow ring closure to either face, so X is converted into its enantiomer: racemisation, with no bond permanently lost.

Key points

  • The molecule has no stereogenic centres that can invert by simple bond rotation: ring fusion hydrogens are trans and locked, so racemisation needs bond reorganisation.
  • A 1,5-diene reorganises by a chair-like six-centre TS ([σ2ₛ+π2ₛ+π2ₛ], thermally allowed): σ bond between C3 and C4 breaks as a new one forms between C1 and C6, and the double bonds shift.
  • The new σ bond can form on either face of the diene, so the rearranged skeleton leads back to X and to ent-X in equal amounts.

The full model answer

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

  • The molecule has no stereogenic centres that can invert by simple bond rotation: ring fusion hydrogens are trans and locked, so racemisation needs bond reorganisation.
  • A 1,5-diene reorganises by a chair-like six-centre TS ([σ2ₛ+π2ₛ+π2ₛ], thermally allowed): σ bond between C3 and C4 breaks as a new one forms between C1 and C6, and the double bonds shift.
  • The new σ bond can form on either face of the diene, so the rearranged skeleton leads back to X and to ent-X in equal amounts.
Model answer to Q6(g), 2026 Mains Paper III: solution page 19
Solution page 19 of 30
Read the whole 2026 solution2026 question paper

CoversCope rearrangementRacemisation of an optically active 15-diene

All parts of Question 6