Free radicals hyperconjugation

As with carbocations, the stability order of free radicals is tertiary > secondary > primary, explainable by hyperconjugation, analogous to that in carbocations... 

Overberger, C. G., and A. Lebovits Azo-bis Nitriles. Decomposition of Azo Compounds. A Special Case of Carbon-Carbon Hyperconjugation in a Free Radical Reaction. J. Amer. chem. Soc. 76, 2722 (1954). 

Concerning steric factors, 43 is attacked in the most hindered position ( inverse effect of substitution ) likewise, 39 is attacked at the most hindered carbon. Obviously, the transition states for the formation of 44 or 50 show limited sensitivity to the degree of substitution, and the relief of ring strain is a more significant factor than the steric hindrance in the transition state. On the other hand, steric factors are important in systems such as P-phellandrene radical cation 40 which is attacked at the xo-methylene carbon (most easily accessible), or the tricyclane radical cation 56 which is attacked at the less hindered 3° carbon further removed from the dimethyl-substituted bridge (approach a). Both reactions also benefit Irom the formation of the most highly substituted, hyperconjugatively stabilized free radicals. 

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. 

CARBOCATIONS, CARBANIONS, FREE RADICALS, CARBENES, AND NITRENES primary ion has only two hyperconjugative forms while the tertiary has six ... 

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. 

On a resonance picture, we can consider structures such as 36a and 36b as contributing to the total structure. Stabilization of this type is referred to as hyperconjugation. It must be stressed that structures such as 36b only contribute to the structure to a minor extent, so there is no question of the hydrogen atom becoming free and leaving the radical. 

Comparison with the free vinoxy radical shows an -1% shift toward the enolate-like structures 19a and 20b (largely at the expense of intramolecular hyperconjugation structures such as 19c and 20c). This resonance shift increases the anionic (Lewis base) character at oxygen, thereby enhancing the donor-acceptor H-bonding interaction with water (which leads to a weak partial bond order of 0.007 between the monomers). Many other examples of such synergy between intra- and intermolecular resonance interactions have been... 

Table 7.18 shows details of the vinoxy NBOs of the relaxed A-state complex for comparison with free A-state vinoxy NBOs (Table 7.5). Niunerous changes are evident, corresponding to effective loss of keto-enol resonance and return to relatively normal C=C and C-0 bond lengths. Particularly noticeable is the reduced occupancy of the vacant Uq radical orbital (from 0.096e to 0.033e, reflecting loss of hyperconjugative stabilization of the radical center) and...

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