[aysha (OP)]

Why does a Br• radical add to the alkene with characteristic regioselectivity, giving a primary alkyl bromide when the polar addition of HBr to an alkene would give a tertiary alkyl bromide? (1) attack at the unsubstituted end of the alkene is less sterically hindered; and (2) the tertiary radical thus formed is more stable than a primary radical.

In fact, of all the hydrogen halides, only HBr will add to alkenes in this fashion: HCl and HI will undergo only polar addition to give the tertiary alkyl halide.

But why?

[Bored chemist]

Good question. Do you have any thoughts on the matter?