Free Radical Addition of HX to Alkenes

In the presence of a peroxide initiator, hydrogen halide adds to alkene via an anti-Markovnikov, free-radical mechanism. The carbanion radical product of the first propagation step will be more stable if the carbon with the lone electron, the radical carbon, is highly substituted. For this reason, the halogen atom binds to the less substituted carbon, in other words, anti-Markovnikov addition.