Benzyhc halogenation

It was observed that in the case of bromo- or iodo-substituted benzyhc tellurides the lithium-Br exchange and/or the halogen displacement by the n-butylhthinm competes with the Te-Li exchange, resulting in lower yields of the desired products. This drawback is avoided by using ether as solvent. 

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. 

The greater stability of benzylic radicals accounts for the fact that when ethylbenzene is halogenated, the major product is the 1-halo-l-phenylethane. The benzyhc radical is formed much faster than the 1° radical ... 

Primary and secondary aUyhc and benzyhc halides can react either by an 8 2 mechanism or by an SnI mechanism in ordinary nonacidic solvents. We would expect these hahdes to react by an Sn2 mechanism because they are structurally similar to primary and secondary alkyl hahdes. (Having only one or two groups attached to the carbon bearing the halogen does not prevent Sn2 attack.) But primary and secondary allylic and benzyhc hahdes can also react by an SnI mechanism because they can form relatively stable allylic carbocations and benzylic carbocations, and in this regard they differ from primary and secondary alkyl hahdes. ... 

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