Indian Forest Service (Mains), Chemistry optional · 2025 · Paper II
Q1(h). Intramolecular Claisen condensation
Question
Complete the reaction of the displayed cyclopentane bearing adjacent acetyl/methyl and CH₂COOEt groups with one equivalent of base. [structure given in the paper]
Answer in brief
The acetyl-methyl enolate attacks the ester intramolecularly, giving a fused five/six β-diketone enolate; protonation gives the neutral bicyclic diketone.
Key points
- The ring carbon bearing the acetyl group is quaternary, so the ketone has α H only on its methyl side.
- Base forms that methyl enolate; its carbon attacks the tethered ester carbonyl through a six-membered ring closure.
- The tetrahedral intermediate collapses with loss of ethoxide, forming the new ring.
- The 1,3-diketone is more acidic than the starting ketone; deprotonation traps it as an enolate and explains consumption of one equivalent of base.
The full model answer
As written for the answer booklet: the diagram carries the working, the lines stay short. Read here only; it is not offered as a download.
Coversintramolecular Claisen condensationDieckmann-type cyclisationβ-diketoneenolateketone–ester condensation
All parts of Question 1
- 1(a) Aromaticity of small rings5 marks
- 1(b) Nitrene transfer to an alkene5 marks
- 1(c) Electrophilic substitution of heteroarenes5 marks
- 1(d) Elimination next to an existing alkene5 marks
- 1(e) Oxymercuration of norbornene5 marks
- 1(f) Medium-dependent pinacol rearrangement5 marks
- 1(g) Two thermal electrocyclic steps5 marks
- 1(h) Intramolecular Claisen condensation5 marks