Enolizable ketones chlorination

Ketones containing an enolizable hydrogen can be halogenated at the a position (the carbon adjacent to the carbonyl) with bromine, chlorine, NBS, or NCS. The reaction probably proceeds via addition of X2 to the enol form of the carbonyl (secs. 9.2.A, 9.8.A). Elimination of HX from the addition product generates an other enol, which tautomerizes to the a-haloketone. Reaction of cyclohexanone with bromine, for example, would give 2-bromocyclohexanone (174) and a similar reaction with NCS (173) would give 2-chloro-cyclohexanone (172). 

Formally, it involves an electrophilic addition to the a-carbon of an enolizable carbonyl compound. Introduction of a chlorine atom at this position helps to accelerate enolization by virtue of the increased electron-withdrawing character of Cl relative to H, so further addition of chlorine atoms is highly favored. Although simple ketones such as acetone are theoretically capable of enolization, only a vanishingly small fraction of the compound is present in the form which is reactive... 

---