Enolizable aldehydes and enolizable ketones, in the presence of an acid or base catalyst in aqueous medium at high temperature, undergo a reaction, giving an α, β-unsaturated aldehyde or an α, β-unsaturated ketone, respectively, as the product. This reaction is known as aldol condensation. The base-catalyzed aldol condensation, in which the catalyst is usually the hydroxide ion, is more common.
Step 1: The hydroxide ion deprotanates the aldehyde reversibly.
Step 2: Enolate ion 1 adds to the unreacted aldehyde.
Step 3: Alkoxide ion 2 is protonated by water.
Step 4: Aldol 3 is an enolizable aldehyde. A small amount of it is converted to the corresponding enolate ion (4) by the hydroxide ion.
Step 5: Enolate ion 4 loses a hydroxide ion.
Step 1 through 3 are an aldol reaction, steps 4 and 5 a 1,2-elimination via E1cB mechanism. Thus, aldol condensation is aldol reaction