The aldol reaction is one of the most important carbon-carbon bond forming reactions in organic chemistry. It involves the reaction between two carbonyl compounds - aldehydes or ketones - to form a new product called an aldol, which contains both an aldehyde or ketone group and a hydroxyl group.
The aldol reaction begins with enolate formation. A strong base, such as hydroxide ion, removes an alpha-hydrogen from the carbonyl compound. This alpha-hydrogen is acidic due to the electron-withdrawing effect of the carbonyl group. The resulting enolate ion is stabilized by resonance between the carbanion and the carbonyl oxygen.