Hydrogens alpha of ketones are slightly acidic due to the oxygen's electron withdrawing effect. So in water, those hydrogens are in an acid/base equilibrium, leaving the carbon and coming back on. If there is enough chlorine around, sometimes a chlorine will jump on when a hydrogen leaves. Same with D2O, in excess D2O eventually all the alpha hydrogens will be replaced by D. I think this reaction has a slight Sn1 character, but I could be wrong.