Aldol addition reactions of aldehydes or ketones can also be catalyzed by acids.

Recall that the aldol addition reaction is unfavorable for ketones under basic conditions. However, an acid shifts the equilibrium forward to exclusively form the unsaturated carbonyl product instead of an aldol.

Here, the carbonyl group is first activated by protonation to generate protonated ketones. The chloride ion then deprotonates the α carbon to give the enol tautomer as the nucleophile.

At this stage, the solution is a mixture of ketones, protonated ketones, and enol molecules.

To drive the reaction forward, the nucleophilic enol attacks the electrophilic carbon of the protonated ketone, yielding a positively charged intermediate.

The proton loss generates a neutral β-hydroxy ketone as the aldol addition product, which cannot be detected or isolated.

Under acidic conditions, the β-hydroxy ketone spontaneously undergoes dehydration to yield the condensation product.