An aldol is both an aldehyde and an alcohol.
Synthesis
Aldols can be formed in aldol reactions.
The term aldol is derived from ALDehyde and alcohOL. It can be used to refer to a product of aldol condensation or to describe any molecule containing alcohol (-OH) and carbonyl (-C=O) as main functional groups.
Aldol condensation is a useful C-C bond forming reaction. It occurs when enols or enolate ions attack the carbonyl functional group of an aldehyde or a ketone. This can happen under either base or acid catalysis. A base can abstract alpha-hydrogen from an aldehyde or ketone generating an enolate ion. Acid can protonate oxygen atom of carbonyl group (of aldehyde or ketone) creating an enol. Acid/base catalysis is reversible. The beta-hydroxy carbonyl products can revert back to starting materials.
A general example of base catalysis would be the addition of a dilute NaOH(aq) solution to acetaldehyde (CH3CHO). The product of this reaction would be 3-hydroxybutanal.
Ketones can undergo aldol condensation reactions with aldehydes or other ketones. The products of such reactions are still referred to as aldols.
Aldol condensation can occur intramolecularly.
Under more rigorous conditions (ex: higher temperatures and increased base concentrations) an aldol can lose a water molecule. This H2O comes from combining beta-hydroxyl group and alpha-hydrogen. Such a reaction is called beta-elimination. The product is (alpha)(beta)-unsaturated aldehyde or ketone. The resultant conjugated carbonyl system contributes to the stability of such molecules. Acids or bases can be employed for aldol dehydration reactions. This dehydration is also reversible. An intermolecular aldol reaction followed by dehydration is referred to as a Michael Reaction.