The aldol reaction is a typical example of a carbonyl condensation reaction. Under basic conditions, aldol reactions occur by nucleophilic addition of the enolate ion of the donor molecule to the carbonyl group of the acceptor molecule. A tetrahedral intermediate is formed that is easily protonated to give an ALDOL product. Notice the name 'aldol' is formed from'aldehyde' and 'alcohol'.
Aldol reactions occur in the presence of base or acid, via an enolate or an enol, respectively.
Overall reaction
For base mechanism please click here
Overall reaction
For acid mechanism please click here
Aldol reactions are more often than not intermolecular ie. reactions are between two molecules. If the two molecules are esters and a self-condensation takes place between them , this leads to a different set of reactions called Claisen Condensation reactions. However, aldol reactions within the same molecule can occur and these are called INTRAMOLECULAR aldol reactions.
If the carbonyl compounds involved are unsymmetrical ie. not identical, a mixture of products are formed. See Mixed Aldol Reactions.