Aldol reaction

A reaction in which two molecules of aldehyde combine to give an aldol – i.e. a compound containing both aldehyde and alcohol functional groups. The reaction is base-catalyzed; the reaction of ethanal refluxed with sodium hydroxide gives:

The mechanism is similar to that of the Claisen condensation: the first step is removal of a proton to give a carbanion, which subsequently attacks the carbon of the carbonyl group on the other molecule:
CH₃CHO + OH⁻ → –CH₂CHO + H₂O.