Keto–Enol Tautomerism: Overview

The alpha hydrogens in aldehydes and ketones are acidic in nature. Abstraction of the α proton generates the enolate ion, which is further stabilized by resonance.                    

The presence of the carbonyl group makes the α hydrogens in aldehydes and ketones more acidic than acetylenic, vinylic, and alkane hydrogens.          

This enhanced acidity of alpha hydrogens and the raised stability of enolate anion leads to the interconversion between keto and enol form giving rise to keto-enol tautomerism.

The keto and enol forms are constitutional isomers meaning that they have the same molecular formula but with different arrangement of atoms.

Since the C=O double bond is much stronger than the C=C double bond, the keto form predominates in simple aldehydes and ketones.

However, in the case of β-diketones, where the α hydrogen lies between two carbonyl groups, the enol form is predominantly favored at equilibrium. This is due to the unique positioning of α hydrogen, which stabilizes the enol formed by resonance and hydrogen bonding.