Q4(b). Claisen ester condensation: why R₂CHCO₂Et fails
Claisen-ester condensation is possible with substrates of type A but not possible with type B. Explain by writing the mechanism.
[structures given in the paper]
Answer in brief
The Claisen condensation is pulled to completion by irreversible deprotonation of the β-keto ester at C-2 . Type A (two α-H) leaves one H there and condenses; type B (one α-H) gives a product with no such H, which reverts under NaOEt.
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CoversClaisen condensationβ-keto esternucleophilic acyl additiondriving forceretro-Claisen
All parts of Question 4
- 4(a) Enamine bromination with ring contraction; Dakin reaction10 marks
- 4(b) Claisen ester condensation: why R₂CHCO₂Et fails10 marks
- 4(c) Fischer indole synthesis: 2-phenylindole10 marks
- 4(d) Stabilised and unstabilised ylids; E/Z selectivity in the Wittig reaction10 marks