22.2: Reactivity of Enols- α-Substitution Reactions

What kind of chemistry do enols have? Because their double bonds are electron-rich, enols behave as nucleophiles and react with electrophiles in much the same way that alkenes do. But because of resonance-electron donation of a lone pair of electrons on the neighboring oxygen, enols are more electron-rich and correspondingly more reactive than alkenes. Notice in the following electrostatic potential map of ethenol (\(\ce{H2C═CHOH}\)) how there is a substantial electron density (yellow-red) on the \(α\) carbon.

When an alkene reacts with an electrophile, E⁺, initial addition gives an intermediate cation and subsequent reaction with a nucleophile, such as a halide ion, yields an addition product (Section 7.7). When an enol reacts with an electrophile, however, only the initial addition step is the same (Figure \(\PageIndex{1}\)). Instead of reacting with a nucleophile to give an addition product, the intermediate cation loses the –OH proton to give an α-substituted carbonyl compound.

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