Free radicals substituent stabilization

Like carbocations free radicals are stabilized by alkyl substituents The order of free radical stability parallels that of carbocation stability... 

A free radical is stabilized by an X substituent (Figure 13a) through a two-orbital, three-electron 7r-type interaction. The nucleophilicity of the radical is greatly increased. The X -substituted free radicals are more easily oxidized. The n effects of X substitution are somewhat augmented by the inductive effect of the electronegative X in stabilizing the radical. 

A carbon free radical is stabilized by a Z substituent (Figure 13b) through the -type interaction with the LUMO of the Z group. The SOMO is lowered in energy and the free radical is more electrophilic as a consequence. 

Some of the evidence indicating that alkyl substituents stabilize free radicals comes from bond energies The strength of a bond is measured by the energy required to break It A covalent bond can be broken m two ways In a homolytic cleavage a bond between two atoms is broken so that each of them retains one of the electrons m the bond... 

For most vinyl polymers, head-to-tail addition is the dominant mode of addition. Variations from this generalization become more common for polymerizations which are carried out at higher temperatures. Head-to-head addition is also somewhat more abundant in the case of halogenated monomers such as vinyl chloride. The preponderance of head-to-tail additions is understood to arise from a combination of resonance and steric effects. In many cases the ionic or free-radical reaction center occurs at the substituted carbon due to the possibility of resonance stabilization or electron delocalization through the substituent group. Head-to-tail attachment is also sterically favored, since the substituent groups on successive repeat units are separated by a methylene... 

Radical chlorination reactions show a substantial polar effect. Positions substituted by electron-withdrawing groups are relatively unreactive toward chlorination, even though the substituents may be potentially capable of stabilizing the free-radical intermediate " ... 

Like the 5/) -hybridized carbons of carbocations and free radicals, the sp -hybridized carbons of double bonds are electron attracting, and alkenes are stabilized by substituents that release electrons to these carbons. As we saw in the preceding section, alkyl groups are better electron-releasing substituents than hydrogen and aie, therefore, better able to stabilize an alkene. 

II), and its formation therefore is more probable. If the substituent X possesses unsaturation conjugated with the free radical carbon, as for example when X is phenyl, resonance stabilization may be fairly large. The addition product (I) in this case is a substituted benzyl radical. Comparison of the C—I bond strengths in methyl iodide and in benzyl iodide, and a similar comparison of the C—H bond strengths in methane and toluene, indicate that a benzyl radical of type (I) is favored by resonance stabilization in the amount of 20 to 25 kcal. 

Mutual conjugation of, as we describe it nowadays, + M and — M substituents is equivalent to an extra stabilization of the system. Thus, we can interpret this statement as the first formulation of the captodative effect even though the term was coined much later. The difficulty of organic chemists to comprehend the rather mathematically formulated theorem must have hindered its wider recognition and seems to be the reason that the phenomenon of interaction of -l-M and — M substituents has been reinvented several times since then. It is remarkable that these rediscoveries were always initiated by experimental studies in free radical chemistry. 

Balaban (1971 Balaban et aL, 1977) investigated radicals of type [1] by esr spectroscopy and noted their long lifetime, which he attributed to the push-pull character of the substituents involved and their mutual conjugation. Katritzky (Baldock et al., 1973, 1974 Katritzky and Soti, 1974) recognized in an analysis of merocyanines that there should be a related class of free radicals [2] which, in accordance with the stability of merocyanines. 

The familiarity with qualitative valence bond descriptions of substituent effects in combination with the known substituent effects in carbocations and carbanions led Viehe and his group to the postulate of a captodative effect for free radicals (Stella et ai, 1978 Viehe et al., 1979). They did not seem to be aware of the earlier work which was of a more physical organic character. The fact that carbocations [8] are stabilized by + M substituents, and carbanions [9] by - M substituents, raised the idea that free radicals, as... 

The first indication of the existence of a captodative substituent effect by Dewar (1952) was based on 7t-molecular orbital theory. The combined action of the n-electrons of a donor and a captor substituent on the total Jt-electron energy of a free radical was derived by perturbation theory. Besides the formulation of this special stabilizing situation and the quotation of a literature example [5] (Goldschmidt, 1920, 1929) as experimental evidence, the elaboration of the phenomenon was not pursued further, neither theoretically nor experimentally. 

As mentioned above, different approaches to the evaluation of the stabilization in free radicals do not necessarily lead to identical results. Pasto (Pasto et al., 1987 Pasto, 1988), also performing ab initio calculations, used the isodesmic reaction (4) for the evaluation of substituent effects. The heat... 

In addition to the stabilization by suitable substituents and the absence of other termination reactions than recombination, it is the strength of the bond formed in the dimerization which is a necessary cofactor for the observation of free radicals by esr spectroscopy. The stability of nitroxides [4] or hydrazyls [5] (Forrester et al., 1968) derives not only from their merostabilized or captodative character but also from a weak N-N bond in the dimer. The same should be the case for captodative-substituted aminyls... 

Spin-density distributions are inherent features of free radicals. Esr experiments take place when the radical is in its electronic ground state and the measurement of the spin distribution constitutes only a minute perturbation of the system. This feature and the fact that esr hyperfine splitting can be measured with high precision makes the esr method ideally suited for the study of substituent effects. Therefore, if spin delocalization is accepted as a measure of stabilization, the data in Table 6 provide quantitative information. However, these are percentage values and not energies of stabiliza-... 

Several attempts have been made to analyse the captodative effect through rotational barriers in free radicals. This approach seems to be well suited as it is concerned directly with the radical, i.e. peculiarities associated with bond-breaking processes do not apply. However, in these cases also one has to be aware that any influence of a substituent on the barrier height for rotation is the result of its action in the ground state of the molecule and in the transition structure for rotation. Stabilization as well as destabilization of the two states could be involved. Each case has to be looked at individually and it is clear that this will provide a trend analysis rather than an absolute determination of the magnitude of substituent effects. In this respect the analysis of rotational barriers bears similar drawbacks to all of the other methods. 

The study of substituted allyl radicals (Sustmann and Brandes, 1976 Sustmann and Trill, 1974 Sustmann et al., 1972, 1977), where pronounced substituent effects were found as compared to the barrier in the parent system (Korth et al., 1981), initiated a study of the rotational barrier in a captodative-substituted allyl radical [32]/[33] (Korth et al., 1984). The concept behind these studies is derived from the stabilization of free radicals by delocalization of the unpaired spin (see, for instance, Walton, 1984). The... 

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