UPSC Combined Geo-Scientist, Mains · 2026 · Mains Paper III
Q6(i). Woodward–Fieser rules for three dienes
Question
Using Woodward–Fieser rule, calculate λ_(max) for the following compounds and arrange them in the order of increasing λ_(max): A, B and C (angular methyl bicyclic dienes as printed).
Answer in brief
A=234 nm (heteroannular), B=273 nm (homoannular), C=249 nm (heteroannular). Increasing λ_(max): A < C < B.
Key points
- residues on C-a (ring CH₂), C-c (ring-fusion C, one residue) and C-d (ring CH₂); the double bond at the ring fusion is exocyclic to the other ring.
- the diene is in one ring, so the base is 253; the ring-fusion double bond is exocyclic to the left ring (+5); residues: two on the fusion carbon and one on the other terminal carbon.
- five alkyl groups on the four diene carbons (CH₃ and ring CH₂ on one terminal, ring residues on the three others); both double bonds are exocyclic to the other ring.
The full model answer
Worked out step by step, as it should go in the answer booklet. The solution sheet follows it.
| base | ring residues | exocyclic C=C | note | λ_(max) | ||
| A | 214 (heteroannular) | 3 × 5 = 15 | 1 × 5 | one C=C in each ring | 234 | |
| B | 253 (homoannular) | 3 × 5 = 15 | 1 × 5 | both C=C in the right ring | 273 | |
| C | 214 (heteroannular) | 5 × 5 = 25 | 2 × 5 | C(CH₃)=C and C=C in different rings | 249 |
- A: residues on C-a (ring CH₂), C-c (ring-fusion C, one residue) and C-d (ring CH₂); the double bond at the ring fusion is exocyclic to the other ring.
- B: the diene is in one ring, so the base is 253; the ring-fusion double bond is exocyclic to the left ring (+5); residues: two on the fusion carbon and one on the other terminal carbon.
- C: five alkyl groups on the four diene carbons (CH₃ and ring CH₂ on one terminal, ring residues on the three others); both double bonds are exocyclic to the other ring.
CoversWoodward–Fieser rulesWoodward–Fieser rules for three dienes
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