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.

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.

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