Toluene radical attack

Radical attack on methylbenzene (toluene, 60) results in preferential hydrogen abstraction by Cl leading to overall substitution in the CH3 group, rather than addition to the nucleus. This reflects the greater stability of the first formed (delocalised) benzyl radical, PhCH2 (61), rather than the hexadienyl radical (62), in which the aromatic stabilisation of the starting material has been lost ... 

The polar effect was at first invoked to explain various directive effects observed in aliphatic systems. Methyl radicals attack propionic acid preferentially at the a-position, ka/kp = 7.8 (per hydrogen), whereas chlorine " prefers to attack at the /3-position, ka/kp = 0.03 (per hydrogen). In an investigation of f-butyl derivatives, a semiquanti-tative relationship was observed between the relative reactivity and the polar effect of the substituents, as evidenced by the pK, of the corresponding acid. In the case of meta- and / ara-substituted toluenes, it has been observed that a very small directive effect exists for some atoms or radicals. When treated by the Hammett relation it is observed that p = —0.1 for H , CeHs , P-CH3C6H4 and CHs . On the contrary, numerous radicals with an appreciable electron affinity show a pronounced polar effect in the reaction with the toluenes. Compilation of Hammett reaction constants and the type of substituent... 

Assuming the usual initial distribution of the three reactive species, it is obvious that OH radical plays the major role in the decomposition of these solutes. Toluene is also significantly attacked by hydrogen atoms. These data may help to explain the effects shown in Table 4. Benzene decomposition was most severely affected by the changing water quality. This may be due to almost complete reliance on OH radical attack, and the presence of a sixfold higher DOC concentration competing for OH in wastewater. 

Reaction of monocyclic aromatics with O3 and NO3 radicals is generally very slow and unimportant. Atmospheric degradation of aromatics is initiated by OH radical attack. The kinetics of the reaction of OH radicals with aromatic compounds are well established [65]. As seen from Table 2 the fife-time of aromatics with respect to reaction with OH is typically a few days or less. Reaction proceeds by addition to the ring and H-atom abstraction from either the substituent groups or possibly from the ring C - H sites. In all cases the addition channel is dominant. For benzene, toluene, 0, m, p-xylene, and ethyl benzene the H-atom abstraction pathway accounts for 5-10% of the overall reaction [15], the remaining 90-95% proceeds via addition. For toluene ka/(ka + h,) = 0.07 [15,65]. 

When considering the rate constants for deprotonation reported in Table 2 one should, however, take into account that in methylbenzene radical cations with < 3 methyl groups, nucleophilic attack of water on the aromatic ring can compete efficiently with side-chain deprotonation. An elegant explanation which accounts for this competition has been provided for the toluene radical cation on the basis of the three-electron three-orbital three-configuration approach [135, 136]. Three electrons are involved in the deprotonation reaction—the unpaired electron delocalized over... 

When a radical attacks a C—H or C-halogen bond, the interactions are with a and a orbitals. The latter orbitals are usually high in energy, and we can expect that the major interaction is therefore with the HOMO (Fig. 5-4a), namely the a orbital. Radicals abstracting hydrogen atoms are generally regarded as electrophilic. Reactions of various radicals with p-substituted toluenes have been studied and Hammett plots made (Table 5-1). The p-values are small,... 

The methylene unit attached to the ethereal oxygen is particularly reactive towards radical attack and thus chain-transfer processes would be expected to substantially reduce the molecular weight of the final polymer. Some reduction is noticed but both this solvent and toluene can be used for free-radical polymerization (and note the successful polymerization of benzyl acrylate in Protocol 1). 

Free radicals such as the hydroxyl or hydroperoxyl radicals are expected to react with PAH by analogy to their reactivity with simple aromatics in the gas phase e.g. toluene is converted into cresols (Atkinson et al. (69)). The rate constant of hydroxyl radical attack on anthracene, pyrene and benzo(a)pyrene has been measured recently in aquous solution (Chekulaev and Shevchuk (90)). 

The CTC and neutral-monomer reactivity ratios may change both by dilution and with the nature of the solvent. For example, in the p-dioxene-MA-acrylonitrile system in benzene or toluene, the acrylonitrile content in the polymer increased to a maximum and then decreased as the amount of solvent increased.The observed solvent effects support the concept that when a CTC becomes solvated with a 7r-electron-rich solvent the CTC becomes stable to free-radical attack. 

The cation—radical intermediate loses a proton to become, in this case, a benzyl radical. The relative rate of attack (via electron transfer) on an aromatic aldehyde with respect to a corresponding methylarene is a function of the ionization potentials (8.8 eV for toluene, 9.5 eV for benzaldehyde) it is much... 

The reaction of benzyl radicals wdth several heterocyclic compounds W as more extensively studied by Waters and Watson, " - who generated benzyl radicals by decomposing di-tert-butyl peroxide in boiling toluene. The products of the reaction with acridine, 5-phenyl-acridine, 1 2- and 3 4-benzacridine, and phenazine were studied. Acridine gives a mixture of 9-benzylacridine (17%) (28) and 5,10-dibenzylacridan (18%) (29) but ho biacridan, w hereas anthracene gives a mixture of 9,10-dibenzyl-9,10-dihydroanthracene and 9,9 -dibenzyl-9,9, 10,10 -tetrahydrobianthryl. This indicates that initial addition must occur at the meso-carbon and not at the nitrogen atom. (Similar conclusions were reached on the basis of methylations discussed in Section III,C.) That this is the position of attack is further supported by the fact that the reaction of benzyl radicals with 5-... 

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 first step of a free radical aromatic substitution, the formation of the a-com-plex, is also an addition step. The o,m,p-product ratio therefore also responds to steric effects. This is shown for the free radical phenylation and dimethylamination of toluene and r.-butylbenzene in Table 8. The larger the substituent on the aromatic system and the bulkier the attacking radical, the more p-substitution product is obtained at the expense of o-substitution. In the phenylation reaction the yield of m-product also increases in contrast to the dimethylamination reaction. The substitution pattern of this latter reaction is, in addition to the steric effect, governed heavily by polar effects because a radical cation is the attacking species113. ... 

Abramovitch, Roy, and Uma 51> disagreed with this, pointing out a number of inconsistencies with that conclusion. Thus, while the total rate ratios are not much different from unity, as expected for a homolytic substitution, the values of °h A = 1.0, °HeA = 0.96, and °hA = 0.80 do not support this mechanism since such electron-donating substituents should facilitate attack by an electrophilic free radical 59-60> and lead to total rate ratios greater than unity. Also, the partial rate factor calculated for attack at the meta position of toluene was unusually low, and it is not clear why this position should be deactivated towards attack either by a free radical or by an electrophilic species. 

The H2—02 radical pool that then develops begins the reactions that cause the fuel concentration to decay. The most effective attackers of the methyl side chain of toluene are OH and H. OH does not add to the ring, but rather abstracts a H from the methyl side chain. This side-chain H is called a benzylic H. The attacking H has been found not only to abstract the benzylic H, but also to displace the methyl group to form benzene and a methyl radical [69], The reactions are then... 

To elevate p-selectivity in nitration of toluene is another important task. Commercial production of p-nitrotoluene up to now leads with twofold amount to the unwanted o-isomer. This stems from the statistical percentage of o m p nitration (63 3 34). Delaude et al. (1993) enumerate such a relative distribution of the unpaired electron densities in the toluene cation-radical—ipso 1/3, ortho 1/12, meta 1/12, and para 1/3. As seen, the para position is the one favored for nitration by the attack of NO (or NO2 ) radical. A procednre was described (Delande et al. 1993) that used montmorillonite clay supported copper (cupric) nitrate (claycop) in the presence of acetic anhydride (to remove excess humidity) and with carbon tetrachloride as a medinm, at room temperature. Nitrotoluene was isolated almost quantitatively with 23 1 76 ratio of ortho/meta/para mononitrotoluene. 

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