91Ó°ÊÓ

The keto and enol forms of carbonyl compounds are constitutional isomers.

In some carbonyl compounds, these constitutional isomers are interconvertible and are called tautomers.

Of these forms, generally, the keto form is more stable, and the equilibrium concentration of enol form is very low. Therefore, it is very difficult to isolate the enol form.

Enol form has a hydroxyl group attached to a double-bonded carbon.

Keto-enol tautomerism involves the movement of the alpha hydrogen followed by the shifting of bonding electrons.

Two changes are done at the double bond to convert enol form into keto form. They are as follows:

1. Change the to a at one end of the double bond.
2. At the other end of the double bond, add a proton.

(a) The keto-enol equilibrium of the compound is given as:

Keto form of the enol (a) with new bond labeled

(b) The keto-enol equilibrium of the compound is given as:

Keto form of the enol (b) with new bond labeled

(c) The keto-enol equilibrium of the compound is given as:

Keto form of the enol (c) with new bond labeled