enol

An enol is a functional group consisting of a vinyl alcohol, that is, a carbon–carbon double bond bearing a hydroxyl substituent (R2C=CHOH or R-CH=CHOH). In organic chemistry, enols are the tautomeric forms of carbonyl compounds such as aldehydes and ketones. The interconversion between the keto form and the enol form is keto–enol tautomerism, and the ratio of forms depends on structure, solvent, and conditions.

In many simple carbonyl compounds at room temperature, the enol form is present only in small amounts; however, certain structures stabilize the enol, increasing its population. Conjugation, intramolecular hydrogen bonding, and especially the presence of adjacent carbonyl groups in beta-dicarbonyl systems strongly stabilize the enol. For example, acetylacetone (pentane-2,4-dione) shows appreciable enol content due to stabilization by resonance and hydrogen bonding.

Enolization can occur via acid- or base-catalyzed pathways. Base-catalyzed enolization involves deprotonation at the alpha carbon to form an enolate ion, which can be protonated to yield the enol. Acid-catalyzed enolization proceeds through protonation of the carbonyl oxygen followed by deprotonation to give the enol form. Enolates, the deprotonated equivalents of enols, are highly reactive nucleophiles in many carbon–carbon bond-forming reactions.

Significant reactions involving enols or enolates include aldol condensations, Claisen condensations, Michael additions, and various cardio- or ring-forming strategies such as the Robinson annulation. The concept of enol and keto–enol tautomerism is fundamental to understanding the reactivity and acidity of carbonyl compounds in organic synthesis.