12.25 : Keto–Enol Tautomerism: Mechanism

The keto and enol forms are known as tautomers and they constantly interconvert (or tautomerize) between the two forms under acid or base catalyzed conditions. Both the reactions involve the same steps—protonation and deprotonation— although in the reverse order.

The base-catalyzed reaction is initiated by the abstraction of the α hydrogen from the carbonyl compound. This abstraction forms a carbanion that is stabilized by delocalization of the negative charge across the carbonyl bond.

The resonance results in a more stable enolate ion with negative change concentrated on the oxygen atom. The enolate ion thus formed is protonated to give the enol tautomer.

In acid-catalyzed keto–enol tautomerism, the acid protonates the carbonyl oxygen to form an oxonium ion intermediate. The oxonium ion undergoes resonance stabilization by moving the π electrons from the C=O bond towards the protonated oxygen. The α hydrogen is then deprotonated to form the enol tautomer.