Alkyl free radicals hyperconjugation

Although the results of electron-spin resonance studies of alkyl substituted radicals and radical-ions are of great significance to the study of hyperconjugation (Symons, 1962), it seems that little attention has been paid to the results by workers primarily concerned with hyperconjugation rather than with free radicals. Thus in a recent conference on hyperconjugation (Tetrahedron, 1959) references to electron-spin resonance results were notable by their complete absence. 

The stability of the radicals depends on the nature of the atom that is the radical centre and on the electronic properties of the groups attached to the radical. As in the case of carbocations, the order of stability of the free radicals is tertiary > secondary > primary > methyl. This can be explained on the basis of hyperconjugation as in the case of carbocations. The stability of the free radicals also increases by resonance possibilities. Thus, benzylic and allylic free radicals are more stable and less reactive than the simple alkyl radicals. This is due to the delocalization of the unpaired electron over the Tr-orbital system in each case. 

Free radicals, like carbocations, have an unfilled 2p orbital and are stabilized by substituents such as alkyl groups that can donate electrons by hyperconjugation. According to the resonance description of hyperconjugation, the unpaired electron, plus those in radical site, are delocalized. 

A methyl group releases electrons to an attached double bond in much the same way that it releases electrons to an 5p -hybridized carbon of a carbocation or free radical—by an inductive effect and by hyperconjugation. The resonance description of hyperconjugation in an alkene is consistent with a flow of electrons from the alkyl group to the carbons of the double bond. 

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