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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(f). Photochemistry of stilbene

  • 4 marks
  • Topic: Photochemistry
  • Answered
Question

Predict the major products (A–C) formed (with appropriate stereochemistry, if any) in the following reaction sequence: trans-stilbene hνAhνBI₂C.

Answer in brief

A = cis-stilbene (photoisomerisation); B = trans-4a,4b-dihydrophenanthrene (photochemical 6π conrotatory cyclisation); C = phenanthrene (iodine oxidises B).

Key points

  • E→ Z via the excited state with free rotation about the C=C bond; the photostationary mixture is rich in cis.
  • cis-Stilbene has the hexatriene unit (ring C=C, vinyl C=C, ring C=C); photochemical electrocyclisation of 6π electrons is conrotatory (Woodward–Hoffmann), giving the trans-4a,4b-H dihydrophenanthrene.
  • I₂ removes the two H atoms (as HI), restoring aromaticity: phenanthrene. Without an oxidant B reverts to cis-stilbene.

The full model answer

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

  • hν: E→ Z via the excited state with free rotation about the C=C bond; the photostationary mixture is rich in cis.
  • cis-Stilbene has the hexatriene unit (ring C=C, vinyl C=C, ring C=C); photochemical electrocyclisation of 6π electrons is conrotatory (Woodward–Hoffmann), giving the trans-4a,4b-H dihydrophenanthrene.
  • I₂ removes the two H atoms (as HI), restoring aromaticity: phenanthrene. Without an oxidant B reverts to cis-stilbene.
Model answer to Q6(f), 2026 Mains Paper III: solution page 19
Solution page 19 of 30
Read the whole 2026 solution2026 question paper

Coversstilbene cyclisation (Mallory)Photochemistry of stilbene

All parts of Question 6