Electron delocalization benzylic radicals

The unpaired electron in benzyl radical is shared by the benzylic carbon and by the nng carbons that are ortho and para to it as shown by the spin density surface in Figure 119 Delocalization of the unpaired electron from the benzylic carbon to the ortho and para positions can be explained on the basis of resonance contributions from the fol lowing structures... 

Below 1000 K, benzyl will be consumed largely through radical-radical processes as discussed earlier. However, at higher temperatures, particularly at low O2 concentration, some form of homolysis should be important. Bearing in mind the stability of the radical (electron delocalization), benzyl should be consumed relatively slowly. However, it is clear from Just s [118] experiments that it reacts very rapidly which rules out a number of the more obvious homolyses such as... 

We attributed the decreased bond dissociation energy in propene to stabilization of allyl radical by electron delocalization Similarly electron delocalization stabilizes benzyl rad ical and weakens the benzylic C—H bond... 

In orbital terms, as represented in Figure 11.10, benzyl radical is stabilized by delocalization of electrons throughout the extended tt system formed by overlap of the p orbital of the benzylic carbon with the tt system of the ring. 

The chain propagation step consists of a reaction of allylic radical 3 with a bromine molecule to give the allylic bromide 2 and a bromine radical. The intermediate allylic radical 3 is stabilized by delocalization of the unpaired electron due to resonance (see below). A similar stabilizing effect due to resonance is also possible for benzylic radicals a benzylic bromination of appropriately substituted aromatic substrates is therefore possible, and proceeds in good yields. 

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 ). 

The propagation steps involve removal of a hydrogen atom from one of the methyl substituents on the benzene ring. Abstraction from the methyl group is favourable because it generates a resonance-stabilized benzylic radical, in which the unpaired electron can be delocalized into the aromatic ring system. 

Delocalization of the odd electron into extended n systems results in considerable radical stabilization. The C—H BDE at C3 of propene is reduced by 13 kcal/mol relative to that of ethane. That the stabilization effect in the allyl radical is due primarily to delocalization in the n system is shown by the fact that the rotational barrier for allyl is 9 kcal/mol greater than that for ethyl. Extending the conjugated system has a nearly additive effect, and the C—H BDE at C3 of 1,4-pentadiene is 10 kcal/mol smaller than that of propene. Delocalization of the odd electron in the benzyl radical results in about one-half of the electron density residing at the benzylic carbon, and the C—H BDE of the methyl group in toluene is the same as that in propene. 

The relative stabilities of radicals follow the same trend as for carhoca-tions. Like carbocations, radicals are electron deficient, and are stabilized by hyperconjugation. Therefore, the most substituted radical is most stable. For example, a 3° alkyl radical is more stable than a 2° alkyl radical, which in turn is more stable than a 1° alkyl radical. Allyl and benzyl radicals are more stable than alkyl radicals, because their unpaired electrons are delocalized. Electron delocalization increases the stability of a molecule. The more stable a radical, the faster it can be formed. Therefore, a hydrogen atom, bonded to either an allylic carbon or a benzylic carbon, is substituted more selectively in the halogenation reaction. The percentage substitution at allylic and benzyhc carbons is greater in the case of bromination than in the case of chlorination, because a bromine radical is more selective. 

Calculate CH bond dissociation energies in propene and in toluene, leading to allyl and benzyl radicals, respectively. (The energy of hydrogen atom is given at right.) Is bond dissociation easier or more difficult in these systems relative to bond dissociation in 3-ethylpentane (methyl CH) Examine spin density surfaces for allyl and benzyl radicals. Draw Lewis structures that account for the electron distribution in each radical. Does spin delocalization appear to stabilize radicals in the same way charge delocalization stabilizes ions ... 

The photochemically generated cyclopentane-1,3-diyl diradials (87) were part of a study of spin delocalization through the EPR Z)-paramctcr. These biradicals were a model system for cumyl and benzyl radicals and experimental data were combined with MO calculations to map the electronic effects on D by varying the aromatic substituent (Ar = heterocycle).218 This parameter was also measured for a related series of... 

Examples are benzyl (( )), phenoxy (( )) and a-tetralyl < 6 > radicals. A very simple scheme, based on Herdon s resonance theory (26), for estimation of the stability of benzylic hydrocarbon radicals has recently been proposed (5) and partly verified (llg). This scheme estimates the extra stability from odd-electron delocalization from the simple formula,... 

In fact, the stability of the triphenylmethyl radical we know to be due mainly to steric, rather than electronic, factors. X-ray crystallography shows that the three phenyl rings in this compound are not coplanar but are twisted out of a plane by about 30°, like a propeller. This means that the delocalization in this radical is less than ideal (we know that there is some delocalization from the ESR spectrum) and, in fact, it is little more delocalized than the diphenylmethyl or even the benzyl radical. 

Radical polymerization of CH2=CHZ is favored by Z substituents that stabilize a radical by electron delocalization. Each addition step occurs to put the intermediate radical on the carbon bearing the Z substituent. With styrene as the starting material, the intermediate radical is benzylic and highly resonance stabilized. Figure 30.3 shows several monomers used in radical polymerization reactions. 

In the toluene radical cation the electron hole is delocalized over the re-system and the deprotonation reaction is coupled with intramolecular electron transfer from the scissible bond to the aromatic ring, leading to the benzyl radical. In the 4-nitrobenzyl chloride radical anion, the unpaired electron resides in a re orbital that does not belong to the leaving group and C-Cl bond cleavage occurs simultaneously with an intramolecular electron transfer from this orbital to the a orbital of the scissible bond. 

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. 

Use SpartanVicw to examine spin surfaces for the allyl radical ahd the benzyl radical (CfiHsCHj ). Draw resonance structures that describe how the unpaired electron is delocalized in each. 

In a second similar experiment 1Z2) the benzyl radical, PhCHs could be detected after the photodissociation of PhCHjCl, adsorbed on silica gel, under experimental conditions similar to the previous case. The EPR spectrum of this radical in the adsorbed state is more complicated (Fig. 20b) consisting of a triplet, with components split additionally into quadruplets. The spectrum could be satisfactorily interpreted by a delocalization of the lone electron of the methylene group, which is partly injected into the benzene ring. 

Radicals can be stable without being persistent the benzyl radical 33 is stabilized by delocalization of the electron onto the ortho and para... 

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