Keto-enol tautomerism.

The enol form is not a resonance form of an aldehyde or ketone. An aldehyde or ketone and its enol are structural isomers that exist in equilibrium. Structural isomers that interchange through an equilibrium process are known as tautomers. Equilibrium generally favors the carbonyl form, but some enols are especially stable due to a lowering of the free energy of the enol form by resonance, which occurs with phenol and β-diketones. In order for an aldehyde or ketone to have an enol tautomer, it must possess an α-proton, which migrates, accompanied by a switch of a single and an adjacent double bond.

Keto-enol tautomerism opens aldehydes and ketones up to an entire class of reactions involving electrophilic attack at the α carbon.

A tricky curveball with keto-enol tautomerism might arise from the fact that if the α carbon is a chiral center, enolization leads to racemization of the aldehyde or ketone.