15.23 : Ketones with Nonenolizable Aromatic Aldehydes: Claisen–Schmidt Condensation

Benzaldehyde, like formaldehyde, lacks an α hydrogen and cannot enolize to form an enolate. Hence, the reaction of benzaldehyde with a ketone in the presence of an aqueous base forms a single crossed product. This reaction is referred to as Claisen–Schmidt condensation.

As the self-condensation of ketones is generally not favored in basic conditions, the self-condensed products do not form in the reaction between ketones and benzaldehyde. The general reaction of Claisen–Schmidt condensation is shown in Figure 1.

Figure 1. The Claisen–Schmidt condensation reaction

The mechanism of the Claisen–Schmidt condensation reaction is similar to the other aldol reactions. The base deprotonates the α carbon of the ketone to form the ketone enolate. A ketone enolate then attacks the benzaldehyde to form an unsaturated carbonyl product as the final product instead of an aldol. This is due to the extended conjugation in the unsaturated carbonyl product stabilizing the molecule.

A similar reaction is observed with aromatic ketones and benzaldehyde in basic conditions.