Claisen condensation, self-condensation of an ester to form a β-keto ester and an alcohol, is analogous to aldol condensation of aldehydes and ketones.

The proton attached to the α carbon of a β-keto ester is even more acidic (Ka ~ 10-11) than the α carbon proton of an aldehyde or ketone. The two adjacent carbonyl oxygens are electron withdrawing by resonance and stabilize the conjugate base.

From the perspective of organic synthesis, the resonance stabilized β-keto ester anion is a source of nucleophilic carbon. Acetoacetic ester synthesis uses the conjugate base of the β keto ester, ethyl acetoacetate, as a nucleophile for SN2 substitution on a primary alkyl halide (or with lower yield on a secondary alkyl halide). This is typically followed by saponification and decarboxylization to yield the final ketone product.

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