Question

Why is a noncarbocation intermediate necessary for an antimarkovnikov halogenation to happen?

Answer 1

Anti-Markovnikov addition to a `pi` bond requires the addition of the non-hydrogen group to the less substituted carbon.

When a carbocation intermediate forms, it usually seeks to stabilize itself through rearrangements: which are accomplished through methyl or hydride shifts.

Hence, it will generally become more substituted, and Markovnikov addition will take place, as a result.

When we have a radical initiator, like `HOOH`, we can ensure that the radical intermediate (that has had the halogen added to the `pi` bond, already, picture below) becomes the most stable which will undergo hydrogen abstraction with `HBr`.

If you want a detailed mechanism, ask!