Alkenes, radical halogenation reactivity with bromine

Simple alkyl halides can be prepared by radical halogenation of alkanes, but mixtures of products usually result. The reactivity order of alkanes toward halogenation is identical to the stability order of radicals R3C- > R2CH- > RCH2-. Alkyl halides can also be prepared from alkenes by reaction with /V-bromo-succinimide (NBS) to give the product of allylic bromination. The NBS bromi-nation of alkenes takes place through an intermediate allylic radical, which is stabilized by resonance. 

The reaction of an alkene with HBr and peroxide gives the bromide product with the bromine on the less substituted carbon in an anti-Markovnikov addition reaction. This reaction work wells with HBr, but not with HCl or HI because the bromine radical reacts in a selective manner. Differences in the reactivity of halogen radicals are addressed in Chapter 11, Section 11.9. 

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