Enolates are formed by deprotonation of a carbonyl compound by a base. This is base catalysed enolization.
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The proton that is removed by the base is perpendicular to the plane of the carbonyl group so that the breaking σ-bond is lined up with the π-bond.
The same mechanism applies to aldehydes, ketones, acids, esters, amides, nitriles etc.
Back to Conjugate addition summary
D. Seebach, Angew. Chemie Int. Ed. English, 1988, 27, 1624–1654.