Aldol condensation is an acidor base-catalyzed condensation between aldehydes or ketones to give an α,ꞵ-unsaturated carbonyl compound. A base-promoted condensation between ester molecules to produce a ꞵ-ketoester is known as the Claisen condensation. In the presence of a base, both reactions involve deprotonation of the acidic α hydrogen to produce the corresponding enolates. The nucleophilic enolates attack their respective nonenolized carbonyl compound forming a tetrahedral intermediate.

The resulting intermediates of aldol and Claisen condensation undergo different pathways. In aldol condensation, the alkoxide intermediate gets protonated to give an addition product, the ꞵ-hydroxy carbonyl compound. In Claisen condensation, the intermediate expels the alkoxide group, thereby restoring C=O and producing a nucleophilic acyl substituted 1,3-dicarbonyl compound.

In the final step, the addition product at an elevated temperature undergoes dehydration to form α,ꞵ-unsaturated carbonyl compound. While in Claisen condensation, the acyl substituted molecule undergoes irreversible deprotonation followed by acidification yielding a ꞵ-ketoester.