When an acid catalyst is used, the initial step in the reaction mechanism involves acid-catalyzed tautomerization of the carbonyl compound to the enol. The acid also serves to activate the carbonyl group of another molecule by protonation, rendering it highly electrophilic. The enol is nucleophilic at the α-carbon, allowing it to attack the protonated carbonyl compound, leading to the aldol after deprotonation.
This under the right conditions can then dehydrate to give the unsaturated carbonyl compound, the aldol condensation product.
# Acid-catalyzed aldol addition
# Acid-catalyzed aldol  dehydration