Reactivity of Enols:a-Substitution Reactions Common a-substitution reaction in the laboratory is halogenation of aldehydes and ketones at their a positions by reaction with Cl2,Br2,or 12 in acidic solution Br2 Acetic acid Acetophenone q-Bromoacetophenone(72%) Common a-substitution reaction in the laboratory is halogenation of aldehydes and ketones at their a positions by reaction with Cl2 , Br2 , or I2 in acidic solution Reactivity of Enols: α-Substitution Reactions