12.25 : ケト-エノール互変異性: メカニズム

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.