Benzyl radicals from toluene

Alkyl radicals can be obtained by abstraction of a hydrogen atom from an alkyl group by another radical. This method was utilized for the generation of benzyl radicals from toluene with iert-butoxy radical obtained on heating di- er -butyl peroxide. BenzoyP and carboxymethyP radicals have also been obtained by this method. The reaction gives rise to a complex mixture of products and therefore is of rather limited use. 

How are we to account for the stability of the benzyl radical Bond dissociation energies indicate that 19 kcal/mole less energy (104 — 85) is needed to form the benzyl radical from toluene than to form the methyl radical from methane. 

Similar considerations suggest that the formation of the benzyl radical from toluene (Dixon and Norman, 1964b) may occur via addition of hydroxyl to the ring followed by the elimination of water, e.g. 

Mella, M., Fagnoni, M., and Albini, A., Benzyl radicals from toluene by photosensitization with naphthalene-l,4-dicarbonitrile. Benzylation and hydroxymethylation of unsaturated compounds, Eur. J. Org. Chem., 2137, 1999. 

The formation of benzyl radicals from toluene can be achieved by photosensitization in the presence of... 

Bamford, Jenkins and coworkers131157 concluded that many of the limitations of the Q-e scheme stemmed from its empirical nature and proposed a new scheme containing a radical reactivity term, based on experimentally measured values of the rate constant for abstraction of benzylic hydrogen from toluene (Ay i), a polar term (the Hammett o value) and two constants a and J which are specific for a given monomer or substrate (eq. 57) 146... 

Although it proved possible to conclude from the results of further experiments with the perester that succinimidyl radicals from this source could abstract benzylic hydrogen from toluene, the reaction system presented further difficulties which are still unresolved. For example, when solutions of NBS and MBN are mixed in the dark, a high concentration of [32] is immediately produced. Whilst this helped to establish the origin of the 27-line spectrum, it constitutes a fresh mechanistic puzzle. 

Recent studies conducted by the same group revealed that the radical cation of toluene generated by photoinduced electron transfer can be deprotonated in a protic cosolvent and thus efficient trapping by electrophilic alkenes is feasible, yielding benzylation products. Secondary hydrogen abstraction by the benzyl radical from methanol generates hydroxymethyl radicals, which can also be used for preparative hydroxymethylation of alkenes (Scheme 18) [24],... 

However, there are some exceptions. One of them is the possibility of (photo)-protonation or -deprotonation. If a matrix is doped with sufficient amounts of a proton donor or acceptor, chances are that the substrate will give up or accept a proton already on cocondensation or on subsequent photoexcitation. In fact, the higher noble gases (Ar, Kr, Xe) are themselves good proton acceptors, forming (NG H)+ complexes that can be identified by their characteristic IR vibrations. This feature allows occasionally to observe radicals formed by deprotonation of radical cations formed in noble gas matrices, for example, benzyl radical from ionized toluene. However, we know of no examples where a carbanion was formed by deprotonation in matrices. 

This reaction scheme is applicable to C H bond activation by strong electron acceptors. For example, the formation of benzyl acetate from toluene using Co111 as the oxidant has been shown to occur via the toluene cation radical [41] ... 

The bond dissociation energies in Table 1.2 (p. 21) show that only 85 kcal is needed for formation of benzyl radicals from a mole of toluene, as compared with 91 kcal for formation of /crr-butyl radicals and 88 kcal for formation of allyl radicals. Relative to the hydrocarbon from which each is formed, then, a benzyl radical contains less energy and is more stable than a terhbuiyl radical. 

Figure 12. 2k, vs. both Dp (bottom) and 2k, for benzyl radicals in toluene (top), for the self-termination of ( ) t-butyl radicals in various solvents, (O) t-propyl radical in n-heptane, O) i-propyl radical in n-hexadecane, ( ) i-propanolyl radical in acetone and tetraethoxysilane, ( ) (EtO)3SiOCHCH3 in acetone and tetraethoxysilane, (J hydroxymethyl radial in methanol from Table 10 (see text). The line is based on Eqs. 6 and 61 with [Pg.51]

We have performed laboratory measurements of rate constants of a few first steps of the tropospheric oxidation mechanism of some monocyclic aromatic hydrocarbons the following reactions have been studied OH radical with benzene or toluene (+M) benzyl radicals (from OH abstraction pathway) with O2, NO and NO2 on the other band, the branching ratios (abstraction/addition + abstraction) have been measured by a direct spectroscopic technique. All these measurements have been achieved thanks to the Discharge Flow (DF) technique with detection of OH by Resonance Fluorescence (RF) and other radicals by Laser Induced Fluorescence (LIF). 

We believe that initiator-derived phenyl radicals abstract benzylic hydrogen from toluene to produce benzyl radicals. The high yields of the imine suggest that benzyl radicals are efficiently trapped and oxidized by the imido complexes presumably via the intermediacy of M(V) amido complexes. Apparently the amide ligand of this intermediate is further oxidized to the Schiff base, again consistent with the proposed chemistry on the heterogeneous catalyst surface [15]. 

The increase of the exocyclic C—C bond stretching frequency from 1208 cm in toluene to 1264 cm in the benzyl radical and the simultaneous decrease of the C—C ring bond stretching frequencies (from 1494 and 1460cm to 1469 and 1446cm , respectively) result from electron density delocalization in the benzyl system. Furthermore, the force constant value for the C—C bond in the C6H5CH2 radical (5.5 X 10 N m ) is between the values for the ordinary C—C bond (4.5 x 10 N m ) and the double C=C bond (9.0 X 10 N m ) and is close to the corresponding force constant in the allyl radical (5.8 x 10 N m ). 

Direct esr evidence for the intermediacy of radical-cations was obtained on flowing solutions of Co(III) acetate and a variety of substituted benzenes and polynuclear aromatics together in glacial acetic acid or trifluoroacetic acid solution . A p value of —2.4 was reported for a series of toluenes but addition of chloride ions, which greatly accelerated the reaction rate, resulted in p falling to —1.35. Only trace quantities of -CH2OAC adducts were obtained and benzyl acetate is the chief product from toluene, in conformity with the equation given above. 

BzO is generated in the initiation. It abstracts H from toluene to give a benzyl radical. 

Benzyl radical abstracts C14 from S3 to give benzyl chloride and C13CS02 radical. This radical then fragments to give SO2 and CCI3, which then abstracts H- from toluene to complete the chain. 

Evident cases of abstraction/recombination mechanism are observed with phenylsubstituted carbenes. Diphenyl-diazomethane, which is photolyzed to give the triplet diphenyl-carbene, very readily abstracts a hydrogen atom from the benzyl group of toluene. The primarily formed radicals can now recombine to give a formal "insertion product — 1,1,2-triphenylethane — or they can recombine to form 1,1,2,2-tetraphenylethane and 1,2-diphenylethane... 

Addition of 0- to double bonds and to aromatic systems was found to be quite slow. Simic et al. (1973) found that O- reacts with unsaturated aliphatic alcohols, especially by H-atom abstraction. As compared to O, HO reacts more rapidly (by two to three times) with the same compounds. In the case of 1,4-benzoquinone, the reaction with O consists of the hydrogen double abstraction and leads to the 2,3-dehydrobenzoquinone anion-radical (Davico et al. 1999, references therein). Christensen et al. (1973) found that 0- reacts with toluene in aqueous solution to form benzyl radical through an H-atom transfer process from the methyl group. Generally, the O anion-radical is a very strong H-atom abstractor, which can withdraw a proton even from organic dianions (Vieira et al. 1997). 

The production of n-butylbenzene may be attributed to an inherent lack of complete selectivity in carbanion reactions, because the greater stability of an intermediate does not exclude the formation of the less stable product. This stability is only important when the step in forming intermediates is slow or when energy differences are large. On the other hand, the formation of n-butylbenzene from toluene and propylene may be due to a partial radical character of benzyl alkali metals. The latter would not seem to be the case because the potassium compounds should have greater ionic character, but they yield more n-butylbenzene. This agrees with the idea that lack of selectivity may be due to greater rate of reaction of potassium compounds with olefins. 

A benzylic radical is generated if a compound like toluene reacts with bromine or chlorine atoms. Hydrogen abstraction occurs from the side-chain methyl, producing a resonance-stabilized radical. The... 

Conversion of toluenes to the benzoic acid is also accomplished by anodic oxidation in acetic acid containing some nitric acid. It is not clear if this reaction involves the aromatic radical-cation or if the oxidising agents are nitrogen oxide radicals generated by electron transfer from nitrate ions [66, 67]. Oxidation of 4-fluorotoluene at a lead dioxide anode in dilute sulphuric acid gives 4-fluorobenzoic acid in a reaction which involves loss of a proton from the aromatic radical-cation and them in further oxidation of the benzyl radical formed [68]. 

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