Aldol condensation takes place between aldehydes or ketones to give α,ꞵ-unsaturated carbonyl compounds, whereas Claisen condensation occurs between two ester molecules to produce a ꞵ-ketoester.

Aldol condensation takes place under acidic and basic conditions, while Claisen condensation occurs only under basic conditions, and the types of bases employed are different in both reactions.

Under basic conditions, the respective reactants undergo deprotonation of the α hydrogen to generate corresponding nucleophilic enolates.

The nucleophiles then attack their unenolized carbonyl forms to give a tetrahedral intermediate. After this step, the fate of the intermediate varies in each reaction.

In aldol condensation, the intermediate is protonated by the solvent to give the addition product—aldol, whereas, in Claisen condensation, the intermediate eliminates the alkoxide ion to give the nucleophilic acyl substituted 1,3-dicarbonyl product.

The addition product further undergoes dehydration at high temperatures to produce α,ꞵ-unsaturated carbonyls. 

The nucleophilic acyl substituted product undergoes irreversible deprotonation and subsequent acidification to give ꞵ-ketoesters.