The reactive form in alpha halogenation is the enol; the π electrons of the double bond are approached by halogen in the second stage of the reaction. In fact, the double bond in an enol is significantly more nucleophilic than a typical alkene double bond because the enolic oxygen is electron donating by resonance. Electrophlic addition occurs readily, but unlike addition to a typical alkene, a cyclic halonium ion intermediate is not formed.