Halogenation, radical, allylic benzylic hydrogen

The relative stabilities of radicals follow the same trend as for carhoca-tions. Like carbocations, radicals are electron deficient, and are stabilized by hyperconjugation. Therefore, the most substituted radical is most stable. For example, a 3° alkyl radical is more stable than a 2° alkyl radical, which in turn is more stable than a 1° alkyl radical. Allyl and benzyl radicals are more stable than alkyl radicals, because their unpaired electrons are delocalized. Electron delocalization increases the stability of a molecule. The more stable a radical, the faster it can be formed. Therefore, a hydrogen atom, bonded to either an allylic carbon or a benzylic carbon, is substituted more selectively in the halogenation reaction. The percentage substitution at allylic and benzyhc carbons is greater in the case of bromination than in the case of chlorination, because a bromine radical is more selective. 

Examination of reactions that involve attack not only by halogen atoms but by other free radicals as well has shown that this is a general rule benzylic hydrogens are extremely easy to abstract and thus resemble allylic hydrogens. We can now expand the reactivity sequence of Sec. 6.22 ... 

We know that the more stable the radical, the faster it can be formed. This means that a hydrogen bonded to either a benzylic carbon or an allylic carbon will be preferentially substituted in a halogenation reaction. As we saw in Section 9.4, bromination is more highly regioselective than chlorination, so the percent of substitution at the benzylic or allylic carbon is greater for bromination. 

Halogen can be introduced into a molecule is several ways. An important method involves replacement of the OH unit of an alcohol with halogen. The hydrogen atom of an aUtyl fragment, particularly an allylic or benzylic fragment can be replaced with halogen by a free-radical mechanism. Both of these transformations will be examined in this section. 

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