Organic esters, ketones, and aldehydes with an α-hydrogen ( bond adjacent to the carbonyl group) often form enols. The reaction involves migration of a proton () from carbon to oxygen:
In the case of ketones, the conversion is called a keto-enol tautomerism, although this name is often more generally applied to all such tautomerizations.  Usually the equilibrium constant is so small that the enol is undetectable spectroscopically.
In some compounds with two (or more) carbonyls, the enol form becomes dominant. The behavior of 2,4-pentanedione illustrates this effect:
Enols are derivatives of vinyl alcohol, with a  connectivity. Deprotonation of organic carbonyls gives the enolate anion, which are a strong nucleophile. A classic example for favoring the keto form can be seen in the equilibrium between vinyl alcohol and acetaldehyde (K = [enol]/[keto] ≈ 3). In 1,3-diketones, such as acetylacetone (2,4-pentanedione), the enol form is favored.
The acid-catalyzed conversion of an enol to the keto form proceeds by proton transfer from O to carbon.  The process does not occur intramolecularly, but requires participation of solvent or other mediators.