A reaction of aldehydes of the type

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):

This can be further dehydrated to 2-butenal (crotonaldehyde):