If the catalyst is a moderate base such as hydroxide ion or an alkoxide, the aldol reaction occurs via nucleophilic attack by the resonance-stabilized enolate on the carbonyl group of another molecule. The product is the alkoxide salt of the aldol product. Then aldol, the aldol addition product itself is then formed.
After which it may undergo dehydration to give a unsaturated carbonyl compound, the aldol condensation product. The scheme shows a simple mechanism for the base-catalyzed aldol reaction of an aldehyde with itself.
Base-catalyzed aldol reaction
Base-catalyzed dehydration
Although only a catalytic amount of base is required in some cases, the more usual procedure is to use a stoichiometric amount of a strong base such as LDA or NaHMDS. In this case, enolate formation is irreversible, and the aldol product is not formed until the metal alkoxide of the aldol product is protonated in a separate workup step.