organic compounds with a hydroxyl group OH at a carbon-carbon double bond. The simplest enol is vinyl alcohol CH2=CHOH, which like most enols is unstable in the free state and changes into its isomer acetaldehyde:
The instability of enols is due to their being in tautomeric equilibrium with the corresponding carbonyl compounds:
Since the carbonyl form is energetically favored over the enol one by 54–71 kilojoules/mole (13–17 kcal/mole), equilibrium is usually displaced in the direction of formation of aldehydes and ketones. Thus acetone contains 2.5 x 10−4 percent enol and cyclohexanone 2.5 x 10−2 percent enol. However, in some cases the enol form is stable (for example, with β-diketones, β-ketoaldehydes, and cyclic α-diketones), because the energy difference is offset by energy gain owing to formation of a system of conjugated double bonds, as well as of a hydrogen bond owing to hydroxyl group formation. The enol content of an equilibrium mixture also depends on the solvent, temperature, and other factors.