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:
2CH3CHO → CH3CH(OH)CH2CHO
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.
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