A reaction of aldehydes of the type 2RCH2CHO⇌RCH2CH(OH)CHRCHO where R is a hydrocarbon group. The resulting compound is a hydroxy-aldehyde, i.e. an aldehyde-alcohol or aldol, containing alcohol (–OH) and aldehyde (–CHO) groups on adjacent carbon atoms. The reaction is base-catalysed, the first step being the formation of a carbanion of the type RHC−CHO, which adds to the carbonyl group of the other aldehyde molecule. For the carbanion to form, the aldehyde must have a hydrogen atom on the carbon next to the carbonyl group.
Aldols can be further converted to other products; in particular, they are a source of unsaturated aldehydes. For example, the reaction of ethanal gives 3-hydroxybutenal (acetaldol):2CH3CHO⇌CH3CH(OH)CH2CHO This can be further dehydrated to 2-butenal (crotonaldehyde):CH3CH(OH)CH2CHO → H2O+CH3CH:CHCHO
CH3CH(OH)CH2CHO → H2O+CH3CH:CHCHO