Resonance stabilization of the benzyl radical

The absolute rate of dissociation of the radical anion of /i-nitrobenzyl chloride has been measured as 4 x 10 s . The w-nitro isomer does not undergo a corresponding reaction. This is because the meta nitro group provides no resonance stabilization of the benzylic radical. 

Halogenation of a larger alkyl side chain is highly regioselective, as illustrated by the halogenation of ethylbenzene. When treated with NBS, the only monobromo organic product formed is 1-bromo-l-phenylethane.This regioselectivity is dictated by the resonance stabilization of the benzylic radical intermediate. The mechanism of radical bro-mination at a benzylic position is identical to that for allylic bromination (Section 8.6A). 

Instability at the chain end could then arise via the benzylic hydrogen atoms due to the possible resonance stabilization of the resulting radical [Eq. (18)] ... 

When chlorination or bromination of alkenes is carried out in the gas phase at high temperature, addition to the double bond becomes less significant and substitution at the allylic position becomes the dominant reaction.153-155 In chlorination studied more thoroughly a small amount of oxygen and a liquid film enhance substitution, which is a radical process in the transformation of linear alkenes. Branched alkenes such as isobutylene behave exceptionally, since they yield allyl-substituted product even at low temperature. This reaction, however, is an ionic reaction.156 Despite the possibility of significant resonance stabilization of the allylic radical, the reactivity of different hydrogens in alkenes in allylic chlorination is very similar to that of alkanes. This is in accordance with the reactivity of benzylic hydrogens in chlorination. 

One-electron oxidation of toluene results in the formation of a cation radical in which the donor effect of the methyl group stabilizes the unit positive charge. Furthermore, the proton abstraction from this stabilized cation radical leads to the conjugate base, namely, the benzyl radical. This radical also belongs to the it type. Hence, there is resonance stabilization in the benzyl radical. This stabilization is greater in the benzyl radical than in the tt cation radical of toluene. As a result, the proton expulsion appears to be a favorable reaction, and the acid-base equilibrium is shifted to the right. This is the main cause of the acidylation effects that the one-electron oxidation brings. 

The Cs values for toluene, isopropylbenzene, and ethylbenzene are higher than benzene. This is due to the presence of the weaker benzylic hydrogens and can be abstracted easily because of the resonance stability of the resultant radical ... 

Figure 21.7 Stabilization of a benzylic radical by resonance with the unsaturated ring...

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). 

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. 

Contribution from the three structures, V-VII, stabilizes the radical in a way that is not possible for the molecule. Resonance thus lowers the energy content of the benzyl radical more than it lowers the energy content of toluene. This extra stabilization of the radical evidently amounts to 19 kcal/mole (Fig. 12.1). 

Benzylic resonance Resonance stabilization of the phenylinethyl (benzyl) radical, CaH.sCHs- (also cation and anion). 

The reaction is highly regioselective (bromination occurs primarily at the benzylic position) due to resonance stabilization of the intermediate benzylic radical. 

We saw in Section 6.9 that the stability order of alkyl carbocations is 3° > 2° > 1° > —CH3. To this list we must also add the resonance-stabilized allvl and benzyl cations. Just as allylic radicals are unusually stable because the... 

Reaction occurs exclusively at the benzylic position because the benzylic radical intermediate is stabilized by resonance. Figure 16.20 shows how the benzyl radical is stabilized by overlap of its p orbital with the ring 77 electron system. 

The amount of the resonance stabilization is similar to that for the benzyl radical. In radicals formed from monomers having C=0 or C=N groups conjugated with the carbon-carbon double bonds, the corresponding resonance structures... 

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. 

That resonance stabilization of intermediate biradicals is important in determining the efficiency of decarbonylation follows from the following examples yielding benzyl radicals upon loss of carbon monoxide(57) ... 

Benzyl chloride undergoes further chlorination to give di- and tri-chloro derivatives, though it is possible to control the extent of chlorination by restricting the amount of chlorine used. As indicated above, it is easier to mono-brominate than it is to mono-chlorinate. The particular stabilization conferred on the benzylic radical by resonance is underlined by the reaction of ethylbenzene with halogens. 

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