Benzyl free radical resonance stabilization

We have said that benzyl and allyl free radicals are stabilized by resonance but we must realize, of course, that they are stable only in comparison with simple alkyl radicals like methyl or ethyl. Benzyl and allyl free radicals are extremely reactive, unstable particles, whose fleeting existence (a few thousandths of a second) has been proposed simply because it is the best way to account for certain experimental observations. We do not find bottles on the laboratory shelf labeled benzyl radicals or allyl radicals. Js there, then, any direct evidence for the existence of free radicals ... 

Consideration of bond dissociation energies has already show n us that a benzyl free radical is an extremely stable one. We have accounted for this stability on the basis of resonance involving the benzene ring (Sec. 12.14). 

In addition to cation intermediates, radical intermediates can be used to introduce bromine or chlorine into a molecule. Both allylic and benzylic moieties form resonance stabilized free radicals that react with bromine or chlorine to give the corresponding halide. Allylic radicals are easily accessible from the corresponding allylic halides, particularly allyl iodides (secs. 13.3-13.5). Benzylic radicals are available from benzylic halides and also directly from the hydrocarbon, if it bears a benzylic hydrogen. Addition of bromine to 167 (in the presence benzoyl peroxide and photochemical initiation) gave benzylic bromide 168 in high... 

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. 

With resonance possibilities, the stability of free radicals increases 149 some can be kept indefinitely.150 Benzylic and allylic151 radicals for which canonical forms can be drawn similar to those shown for the corresponding cations (pp. 168, 169) and anions (p. 177) are more stable than simple alkyl radicals but still have only a transient existence under ordinary conditions. However, the triphenylmethyl and similar radicals152 are stable enough to exist in solution at room temperature, though in equilibrium with a dimeric form. The concen-... 

The energy balance of photodissociation the importance of stabilization of the free radicals. When chlorobenzene or chloro-Np loses the halogen atom, a phenyl or a naphthyl radical is formed with the odd electron localized in an sp2 orbital which is orthogonal to the aromatic zr orbitals such a radical is not stabilized through resonance, unlike the benzyl- or the methyl-Np radicals for which several resonance structures can be drawn (Figure 4.32). 

These resonance stabilization energies of free radicals can be quite large, e.g. 50 kj mol 1 for benzyl-, and 70kjmol 1 for methyl-Np. These must be included in the overall energy balance of the reaction, and can make all the difference between a fast, highly exergonic process, and an endergonic process which in practice does no take place at all. 

Alkylbenzenes undergo free-radical halogenation much more easily than alkanes because abstraction of a hydrogen atom at a benzylic position gives a resonance-stabilized benzylic radical. For example, ethylbenzene reacts with chlorine in the presence of light to give cr-chlon >e thy I benzene. Further chlorination can occur to give a dichlorinated product. 

The stability of the radicals depends on the nature of the atom that is the radical centre and on the electronic properties of the groups attached to the radical. As in the case of carbocations, the order of stability of the free radicals is tertiary > secondary > primary > methyl. This can be explained on the basis of hyperconjugation as in the case of carbocations. The stability of the free radicals also increases by resonance possibilities. Thus, benzylic and allylic free radicals are more stable and less reactive than the simple alkyl radicals. This is due to the delocalization of the unpaired electron over the Tr-orbital system in each case. 

In the absence of a Lewis acid, halogenation of toluene at its boiling point with bromine or chlorine and under UV irradiation e.g. sunlight) occurs in the side chain. The reaction proceeds by a free-radical mechanism that is initiated by the photolytic dissociation of a chlorine molecule (Scheme 9.10). The benzyl radical is stabilized by resonance (see Chapter 3). 

The remarkable dissociation to form free radicals is the result of two factors. First, triphenylmethyl radicals are unusually stable because of resonance of the sort we have proposed for the benzyl radical. Here, of course, there are an even larger number of structures (36 of them) that stabilize the radical but not the hydrocarbon the odd electron is highly delocalized, being distributed over three aromatic rings. 

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