STABILITIES OF ALKYL RADICALS

We assess the relative stability of alkyl radicals by measuring the enthalpy change (AH°) for the homolytic cleavage of a C—H bond m an alkane... 

The order of stability of alkyl radicals is the same as for carbocations. 

The influence of fluorine substituents on the stability of alkyl radicals derives from the same complex interplay of inductive and resonance effects that affects their structure. Simple orbital interaction theory predicts that substituents of the -X type (that is, electronegative substituents bearing lone pairs) should destabilize inductively by virtue of their group electronegativities, and stabilize by resonance to the extent of their ability to delocalize the odd electron. 

Both alkyl radicals and carbocations are electron-deficient species, and the structural features that stabilize carbocations also stabilize radicals. Alkyl radicals are stabilized by adjacent lone-pair-bearing heteroatoms and it bonds, just as carbocations are, and the order of stability of alkyl radicals is 3° > 2° > 1°. However, there are two major differences between the energy trends in carbocations and alkyl radicals. 

Evidence for a radical pathway includes the observation that the reaction is accelerated by radical initiators (such as oxygen or peroxides) and the presence of UV light. Moreover, the order of reactivity for the R group is IIP > II0 > 1°, which is inconsistent with a direct displacement mechanism, but is in accord with the stability of alkyl radicals. Radical inhibitors (such as steri-cally hindered phenols) retard the rate of reaction with sterically-hindered alkyl halides, but not when R = methyl, allyl, and benzyl. When stereoisomerically pure alkyl halides are used, OA results in the formation of a 1 1 mixture of stereoisomeric alkyl iridium complexes, consistent with the formation of an intermediate radical R-. 

The yield of tcrt-butylperacetate formed by decomposition of 14 in cumene varied greatly with R (Me Et < iPr). This difference can be explained on the basis of the relative stability of alkyl radicals. Bailey [166] has shown that the rates of scission reactions of radicals is dependent upon the stability of the expelled radicals. For example, copolymerization of cyclic ketal 16 with styrene results in simple vinyl polymerization while copolymerization of 17 with styrene results in 100% scission to form the ring-opened copolymer. 

Decomposition of 14 in styrene produced very similar products. About the same amount of fert-butylperacetate was formed in both styrene and cumene indicating that the scission reaction of 15 is much faster than its rate of addition to the styrene double bond. This is surprising since the analogous addition of tert-butoxy radical to styrene is very fest relative to the analogous scission reaction to form acetone and methyl radicals. Again, this can be explained by the relative stability of alkyl radicals. 

The structure and geometry of carbon radicals are similar to those of alkyl carboca-tions. They are planar or nearly so, with bond angles of approximately 120° about the carbon with the unpaired electron. The relative stabilities of alkyl radicals are similar to those of alkyl carbocations because they both possess electron-deficient carbons. 

What is the reason for the ordering in stability of alkyl radicals To answer this question, we need to inspect the alkyl radical structure more closely. Consider the structure of the methyl radical, formed by removal of a hydrogen atom from methane. Spectroscopic measurements have shown that the methyl radical, and probably other alkyl radicals, adopts a nearly planar configuration, best described by sp hybridization (Figure 3-2). The unpaired electron occupies the remaining p orbital perpendicular to the molecular plane. 

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