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Organic Chemistry

22.2 Reactivity of Enols: α-Substitution Reactions

Organic Chemistry22.2 Reactivity of Enols: α-Substitution Reactions

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 (H2CCHOHH2CCHOH) how there is a substantial electron density (yellow-red) on the α carbon.

Two resonance forms of an enol tautomer as well as the electrostatic potential map with the ball-and-stick model of the second tautomer.

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 22.3). 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.

Figure 22.3 MECHANISM
General mechanism of a carbonyl α-substitution reaction. In step 3, the initially formed cation loses H+ to regenerate a carbonyl compound.
Acid-catalyzed mechanism of for alpha substitution of a carbonyl in three steps: enol  forms, this attacks electrophile from alpha carbon, and carbonyl oxygen is deprotonated to neutral product.
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