Reactivity of Enols

Enols, the tautomers of carbonyl compounds, are nucleophilic in nature owing to their electron-rich double bonds.

In addition, the electron-donating resonance effect of the adjacent hydroxyl group leads to substantially greater electron density on the α carbon, making it more reactive than the corresponding alkene.

Enols are key reactive intermediates in reactions at the α carbon, like α-halogenation.

Typically, the nucleophilic α-carbon atom of an enol attacks an electrophile, creating a resonance-stabilized cation intermediate with a new bond on the α carbon. Subsequent deprotonation yields a neutral α-substituted product.

This reaction pathway is distinct from that of alkenes, where cation intermediates react with nucleophiles to form addition products.

In addition, enolization is characterized by reversible proton transfer at the α carbon, as seen in deuterium exchange experiments.

As enols are achiral at the α carbon, this leads to slow racemization of keto compounds with chiral α carbon centers.