15.23:
Ketones with Nonenolizable Aromatic Aldehydes: Claisen–Schmidt Condensation
Recall that the self-condensation of ketones in basic conditions is not favored.
Accordingly, in an aqueous base and external heating, the ketone reacts with benzaldehyde that lacks an α hydrogen to form a single unsaturated carbonyl product. This reaction is called Claisen–Schmidt condensation.
As for the mechanism, the ketone enolate attacks the benzaldehyde to exclusively form the unsaturated carbonyl product, instead of an aldol. This is attributed to the extended conjugation that stabilizes the product.
Between the cis and trans stereoisomers of the product, the trans isomer is obtained in high yield because it has fewer steric interactions.
Aromatic ketones undergo a similar reaction with benzaldehyde to generate a trans isomer as the major product.
Claisen–Schmidt condensation can also be performed in acidic conditions.
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.