12.25:
Keto–Enol Tautomerism: Mechanism
Carbonyl compounds comprising α hydrogens tautomerize, interconverting between keto and enol forms in the presence of either a base or an acid.
In a base-catalyzed tautomerization, a base abstracts the α H from the keto tautomer to form a carbanion.
The negative charge on the carbon atom delocalizes over the oxygen atom, leading to the more stable enolate ion.
Finally, the conjugate acid of the base catalyst protonates the enolate oxygen to give the enol tautomer.
An acid-catalyzed tautomerization initiates when an acid transfers a proton to the oxygen atom of the keto form to give an oxonium ion.
The π electrons of the protonated carbonyl group move to the oxygen atom to give a resonance-stabilized cation.
Lastly, the conjugate base of the acid catalyst abstracts the α H to give the enol form.
Both tautomerizations are reversible and follow two steps: protonation of the carbonyl oxygen and deprotonation of the α H, although in the reverse order.
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.
