Reaction: 2 molecules of ester with alpha-H + strong base (NaOEt) → beta-keto ester
Example: 2CH3COOC2H5 + NaOEt → CH3COCH2COOC2H5 (ethyl acetoacetate) + C2H5OH
Mechanism:
- Base abstracts alpha-H from one ester → ester enolate
- Enolate attacks C=O of second ester → tetrahedral intermediate
- Loss of -OEt (leaving group) → beta-keto ester
- Final deprotonation of acidic alpha-H (between two C=O) by base drives equilibrium
Requirement: At least 2 alpha-H atoms (one for enolization, one for final deprotonation to drive equilibrium).
Crossed Claisen: One ester with no alpha-H (ethyl formate, ethyl benzoate, diethyl carbonate, diethyl oxalate) + one with alpha-H.