Methyl affinity

Szwarc and his co-workers have measured the methyl affinities (that is, reactivities toward methyl radicals compared with that of benzene) of a number of heterocyclic compounds. These, together with the methyl affinities of some homocyclic compounds, are set out in Table IX. 

It is of interest that both the methyl affinities and the reactivities of aromatic compounds toward the pheny radical are correlated both by Froax and by atom localization energies. Dewar has shown that the energy required to remove one atom from conjugation (in a hydrocarbon containing an even number of carbon atoms) is greater... 

Kawabata, Tsuruta, and Furukawa (121) have reported a linear relationship between the logarithms of their Q values and the logarithms of the methyl affinities of Szwarc and co-workers (111, 123, 124). James and MacCallum (125) have found a linear relationship between the logarithms of the Qo values calculated from the definition of Zutty and Burkhart (122) and the logarithms of the rates of addition of ethyl radicals to various substituted ethylenes. Similar... 

A very early indication that C-nitroso-compounds are particularly susceptible to radical attack was the methyl affinity value of 105 determined for nitrosobenzene (Heilman el al., 1957). This figure reflects the reactivity of nitrosobenzene, relative to that of benzene, towards attack by methyl radicals. It was apparently several years before this reactivity was linked with nitroxide formation... 

These calculations show that (i) the methyl affinities (MeAs) of C=S compounds are consistently much higher (for X = H, 15 to 25 kcalmol-1 over the range Y = F to Y = NH2) than those of the C=0 homologs, and (ii) the sensitivity to substituent effects of the thiocarbonyls is ca 72% that of the carbonyls. Protonation and methylation therefore display the same pattern of structural effects (there is also a nearly perfect correlation between the PAs and MeAs for each family, although in all cases the PA exceeds the MeA by some 100 kcalmol-1). 

Such relative lack of kinetic impact of olefinic fluorine substituents on alkyl radical addition reactions is consistent with Tedder s early studies on methyl affinities, the results of which are shown in Table 16, where the range of reactivities observed for the addition of methyl radical to ethylenes with varying fluorine content is seen to be relatively small [93,163],... 

Goldschmidt and Beer.372 The 2-alkylated product is the main one formed. This orientation is not unexpected since, compared with the phenyl radical, an alkyl radical should have some nucleophilic character. The j8-/y- ratio is also lower than in the phenylation, as expected for a more nucleophilic radical.371 Pyridine has a methyl affinity of 3 compared with benzene.373 This, however, does not represent the relative amount of picolines and toluene formed with acetyl peroxide.371... 

When methyl radicals were used as initiator, no detectable amount of VAc radicals was obtained under any experimental conditions. The reactivity of VAc is apparently very low compared with the results for acrylic monomers (14). These findings are, however, in agreement with the data from methyl affinity studies by Szwarc (32), who reported the reactivity of VAc monomer towards methyl radicals to be about l/40th that of acrylonitrile (AN) and methyl methacrylate. 

The results obtained are reported in Table 8 and show that the methyl affinities (k4/k2) of MB, NB and TDE are comparable with those known for acyclic and cydic olefins, respectively, having approximately the same degree of substitution at the double bond. Conversely, the k4/k2 ratio for (III,a) is 5 times greater than that expected for two separated double bonds with comparable degree of substitution, but 1—2 orders of magnitude sillier than those observed for other dienic systems, e. g. butadiene, cydopoitadiene, etc. This result illustrates the opposite effects due to resonance stabilization and steric hindrance. 

Matsuoka and Szwarc (1961) photolyzed diazomethane in the presence of isooctane and styrene in one case, and in the presence of isooctane and styrene-a,j8,j8-d3 in the other case, and thus determined the methyl affinities of styrene and of trideuteriostyrene with respect to isooctane by the method of competing reactions. The isotope effect ( d/ h) was 1-07-1 ll. Matsuoka and Szwarc used these data to justify the conclusion that in radical addition reactions the initial structure of a reactive center is preserved in the transition state. In styrene and in trideuteriostyrene the reactive centers are both terminal carbon atoms doubly bonded to another carhon. In the product the reactive carbon atom becomes tetrahedral, and thus the transition state could conceivably resemble either reactant (trigonal) or product (tetrahedral). It can be calculated, however, that a change of configuration from trigonal to tetrahedral in the transition state should exhibit an isotope effect kj)jk ) of about 1-8 in the reactions studied by Matsuoka and Szwarc the tetrahedral configuration was therefore excluded by these authors. 

Table 3.7 Affinity of methyl radical for olefins [56, 57, 67] Monraner Structure Methyl affinity...

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