Toluene phenyl radical

Bridger and Russell10 generated phenyl radicals by the thermolysis of phenyla-zotriphenylmethane at 60 °C. They did this in the presence of various hydrocarbons— toluene, cyclohexane, etc. The following reactions occur ... 

Competition experiments for the partitioning of phenyl radical between a hydrocarbon and the reference compound, carbon tetrachloride, from results given in Ref. 10. The lines show that the product ratio is directly proportional to the ratio of the concentrations of the competing reagents. The plots depict data for toluene (circles.) and cyclohexane (squares). 

Some radicals (e.g., tert-butyl, benzyl, and cyclopropyl), are nucleophilic (they tend to abstract electron-poor hydrogen atoms). The phenyl radical appears to have a very small degree of nucleophilic character. " For longer chains, the field effect continues, and the P position is also deactivated to attack by halogen, though much less so than the a position. We have already mentioned (p. 896) that abstraction of an a hydrogen atom from ring-substituted toluenes can be correlated by the Hammett equation. 

The partial combustion of toluene, with the generation of the intermediate 2-methyl-phenyl radical (MP, in Scheme 2.15) leading to the prototype quinone methide, has recently been investigated by high-level post-HF and DFT theoretical studies.22... 

This process competes favorably with benzylic hydrogen abstraction in toluene, less in ethylbenzene, and least in cumene (31). Such reactions do not seem significant in the oxidation of benzene derivatives. However, naphthalene reacts about 20 times as rapidly with phenyl radical as does benzene (16), and radical addition to the naphthalene nucleus may at least partly account for the slow oxidation rate in the methylnapthalenes. Among the minor products from both methylnaphthalene oxidations were compounds of molecular weight 296 ... 

Few mechanisms of plasma reactions have been studied in any detail. In the reaction of toluene the bimolecular dimerization to bibenzyl competes with the monomolecular generation of methyl and phenyl radicals which are... 

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... 

It has been suggested that the ketone enolate ions 27 initiate the photostimulated cycle. The coupling reaction of the phenyl radical with 31 is faster than with 27, forming a radical anion that fragments to give benzyl radicals, which are reduced to toluene or react with 31 to afford a new radical anion (161) , responsible for forming the substitution product 161 (equation 105). 

Fig. 6-6. The reaction scheme of the singlet-sensitized decomposition of dibenzolyperoxide (DBP) in toluene. In RPb, the g-value of the benzoyloxy radical (2.0123) is much larger than that of the phenyl radical (2.0024). (Reproduced from Ref. [34] by permission from The Chinese Chemical Society)...

Z)(G-H) in benzene has not been determined directly, but has been inserted here for purposes of comparison. It may be derived from the activation energy of the pyrolysis of bromobenzene, which has been shown by Szwarc and Williams to produce phenyl radicals and bromine atoms when the reaction is carried out using the toluene carrier gas technique. Z)(C6H5 -Br) is deduced to be 70 9 kcal, whence Z)(G6H5 -H) = 101 8 kcal, if the heat of formation of bromobenzene quoted by Szwarc and Williams is known to the required accuracy. 

Reaction (147) is the dominant means of oxidizing benzyl radicals. It is a slow step, so the oxidation of toluene is overall slower than that of benzene, even though the induction period for toluene is shorter. The oxidation of the phenyl radical has been discussed, so one can complete the mechanism of the oxidation of toluene by referring to that section. Figure 12 from Ref. [66] is an appropriate summary of the reactions. 

In its relative reactivity toward toluene, ethylbenzene and cumene the more highly substituted 1-methyl-2,2-diphenylcyclopropyl radicaP , derived from the decomposition of the precursor diacyl peroxide, resembles the chlorine radical more than it does the phenyl radical (Table 3). Similarly, comparison of the relative reactivities of primary, secondary and tertiary aliphatic hydrogens toward chlorine atoms (1.0 3.6 4.2) and phenyl radicals (1.0 9.3 44) with the relative reactivities of the C-H bond in the methanol/ethanol/2-propanol series toward the 1-methyl-2,2-diphenylcyclopropyl radical (1.0 2.4 15) further confirms the low selectivity of the cyclopropyl radical. Again, this radical resembles the chlorine atom in its reactivity more than it does the phenyl radical. 

Hey and Grieve [125] found in 1934 that the nitro group activates the aromatic ring towards homolytic substitution. For example, the competitive phenylation of toluene and nitrobenzene by phenyl radicals showed that the yield of nitrodiphenyls was about four times greater than the yield of methyldiphenyls. 

Radical 12 is rather stable under polymerization conditions, but radical 11 decays into a triazole and the phenyl radical, which initiates new chains. Hence, the rate of polymerization is higher with 11 than with 12, because the decay prevents retarding of the buildup of large persistent radical concentrations such as an additional radical generation. This effect of the radical decay is equivalent to the rate enhancement by partial removal of nitroxides by appropriate additives, which was first applied by Georges et al.31 Interestingly, at 95 °C and in toluene solution, the lifetime of 11 is only about 15 min, whereas a reasonable control was found in polymerizations of styrene that lasted many hours at 120— 140 °C.120 Obviously, the radical moiety 11 is stable while it is coupled to the polymer chain. However, the different time scales raise the question of the upper limit of the conversion rate of the persistent radical to a transient one that can be tolerated in living radical polymerization processes (see section IV. C). 

Hydrogen can be removed from aromatic compounds to make radicals. If one carbon is removed from benzene, the radical is called phenyl. If one carbon is removed from toluene, the radical is called benzyl. The stmctures and molecular formulas for the phenyl and benzyl radicals are shown in Figure 5.46. 

The use and importance of aromatic compounds in fuels sharply contrasts the limited kinetic data available in the literature, regarding their combustion kinetics and reaction pathways. A number of experimental and modelling studies on benzene [153, 154, 155, 156, 157, 158], toluene [159, 160] and phenol [161] oxidation exist in the literature, but it would still be helpful to have more data on initial product and species concentration profiles to understand or evaluate important reaction paths and to validate detailed mechanisms. The above studies show that phenyl and phenoxy radicals are key intermediates in the gas phase thermal oxidation of aromatics. The formation of the phenyl radical usually involves abstraction of a strong (111 to 114 kcal mof ) aromatic—H bond by the radical pool. These abstraction reactions are often endothermic and usually involve a 6 - 8 kcal mol barrier above the endothermicity but they still occur readily under moderate or high temperature combustion or pyrolysis conditions. The phenoxy radical in aromatic oxidation can result from an exothermic process involving several steps, (i) formation of phenol by OH addition to the aromatic ring with subsequent H or R elimination from the addition site [162] (ii) the phenoxy radical is then easily formed via abstraction of the weak (ca. 86 kcal moT ) phenolic hydrogen atom. 

The dissociation of BPO during the polymerization reactions yields both benzoyloxy and phenyl radicals, both of which initiate radical polymerization. Furthermore, chain transfer from polymeric radicals to solvent (toluene) can... 

The mass spectrum obtained at 300°C involves the lines at miz 91, 92, and 122 caused by desorption of phenylethanol (this temperature corresponds to its maximal desorption). The main lines miz 51, 78, 92, 91, and 104) in the spectrum obtained at 495°C attest that phenylethylene and toluene desorb. The formation of phenylethylene is due to unimolecular decomposition of bound phenylethanol with H transfer from the CH2 group (nearest to the aromatic ring) to O from a =SiOR group. The last mass spectrum, obtained at 606°C, includes the lines corresponding to benzene (m/z 78) and biphenyl (m/z 154). In as much as the existence of these molecules in the surface layer at such temperature is improbable, they can be formed due to migration of phenyl radicals along the... 

Methyl radicals may be present due to breakdown of the benzene rings. These probably react with phenyl radicals in competition with hydrogen to form toluene. The mechanism can be postulated as follows ... 

Phenyltrimethylsilane is also formed in small amounts. It is probably formed by combination of a trimethylsilyl radical with a phenyl radical formed from the degradation of a toluene molecule. This is offered as an explanation for the abscence of benzene in the products. 

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