Aldehydes And Ketones - Cross Aldol Condensation

A crossed aldol condensation works cleanly when one partner has no alpha-hydrogen and therefore cannot form an enolate, leaving only the other partner to act as the nucleophile. Benzaldehyde has no alpha-hydrogen, so it serves purely as the electrophilic carbonyl, while propanone supplies the enolate from its alpha-carbon. The enolate of propanone adds to the benzaldehyde carbonyl, and subsequent dehydration gives the conjugated ketone benzalacetone, 4-phenylbut-3-en-2-one, as the major product when reagents are in modest ratio. Cinnamaldehyde is wrong because it would require an acetaldehyde-derived enolate adding to benzaldehyde, not propanone. Dibenzalacetone forms only with excess benzaldehyde reacting at both alpha-positions of propanone, so it is not the predominant single-condensation product described. The non-dehydrated beta-hydroxy ketone is only an intermediate and is removed on heating. This selective crossed condensation is emphasised in advanced organic discussions and is a classic JEE Advanced question. The practical lesson is that a crossed aldol is only synthetically useful when one carbonyl cannot enolise, otherwise four different products form and the mixture is useless; pairing an aromatic aldehyde with a ketone is the standard way to exploit this, a transformation often called the Claisen-Schmidt condensation. Using excess benzaldehyde deliberately converts both alpha-positions to give the symmetric dibenzalacetone instead. As a check, the alpha-hydrogen-free aldehyde behaves as the electrophile while the ketone supplies the nucleophile, giving benzalacetone.