mostly in the keto form, and enol forms are difficult to isolate. Keto-enol tautomerism is catalyzed by acids and bases. Acid catalysis causes protonation of the carbonyl oxygen atom, and the intermediate cation then loses a hydrogen from the alpha carbon to produce an enol. Base catalysis enol formation occurs because the carbonyl group makes the hydrogens of the alpha carbon weakly acidic so it can donate its alpha hydrogens to the base, resulting in a resonance-stabilized enolate ion, which is protonated to yield a neutral compound. Protonation on the oxygen atom yields an enol.
Because of their electron-rich double bonds, enols behave as nucleophiles, but are more reactive than alkenes because of the oxygen lone pair. When an enol reacts with an electrophile, the intermediate cation loses the -OH group proton to regenerate a carbonyl compound. The net result is a substitution reaction. Aldehydes and ketones are halogenated at their alpha positions by reactions with chlorine, bromine and iodine in acidic solution in a t
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