enolisation

Enolisation is a chemical reaction in which a ketone or aldehyde containing an alpha-hydrogen atom is converted into its enol tautomer. This process involves the migration of a proton from the alpha-carbon to the carbonyl oxygen, resulting in the formation of a carbon-carbon double bond adjacent to the oxygen atom. Enols are thus isomers of carbonyl compounds that differ in the position of a double bond and a hydrogen atom. The equilibrium between the carbonyl form and the enol form is known as keto-enol tautomerism.

The reaction is typically catalyzed by either acids or bases. In acid-catalyzed enolisation, a proton is first added to the carbonyl oxygen, making the carbonyl carbon more electrophilic. This facilitates the removal of an alpha-hydrogen by a base, leading to the formation of the enol. In base-catalyzed enolisation, a base abstracts an alpha-hydrogen, forming a carbanion (enolate ion). This enolate ion then abstracts a proton from water or another proton source to yield the enol. The enol form is generally less stable than the keto form, meaning the equilibrium usually lies heavily towards the keto side. However, the presence of certain structural features, such as conjugation or intramolecular hydrogen bonding, can significantly stabilize the enol form. Enolisation is a fundamental reaction in organic chemistry, playing a crucial role in various reactions like the aldol condensation and the haloform reaction.