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 (HC=CHOH) how there is a substantial amount of electron density on the α carbon.
When an alkene reacts with an electrophile, such as HCl, initial addition of H gives an intermediate cation and subsequent reaction with Cl yields an addition product. When an enol reacts with an electrophile, however, only the initial addition step is the same. Instead of reacting with CI to give an addition product, the intermediate cation loses the OH proton to give an α-substituted carbonyl compound.