15.23 : קטונים עם אלדהידים ארומטיים שאינם ניתנים לשינוי: דחיסת Claisen–Schmidt

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.