Polar addition of HBr to olefins proceeds in such a way, the carbon that is rich in hydrogen becomes richer. In mechanistic terms, we could restate the rules as follows: Polar addition of H+X¯ to olefins proceed in such a way that the negative component adds to the more stable carbonium ion intermediate (Fig 2.4.1).
Fig 2.4.1HBr and HI easily undergo free radical addition, promoted by light or heat. Free radical additions give the anti-Markovnikov product. Clean Markovnikov products are obtained when such reactions are carried out in polar solvents and care is taken to avoid light.