Resonance stabilization benzyl radical

Figure 16.20 A resonance-stabilized benzylic radical. The spin-density surface shows that the unpaired electron (blue) is shared by the ortho and para carbons of the ring.

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. 

Of this group only benzyl chloride is not an aryl halide its halogen is not attached to the aromatic ring but to an. v/r -hybridized carbon. Benzyl chloride has the weakest carbon-halogen bond, its measured carbon-chlorine bond dissociation energy being only 293 kJ/mol (70 kcal/mol). Homolytic cleavage of this bond produces a resonance-stabilized benzyl radical. 

In the industrial synthesis of benzyl chloride (Figure 1.21), only the H atoms in the benzyl position are replaced by Cl because the reaction takes place via resonance-stabilized benzyl radicals (cf. Table 1.1, bottom line) as intermediates. At a reaction temperature of 100 °C, the first H atom in the benzyl position is substituted a little less than 10 times faster (— benzyl chloride) than the second (—> benzal chloride) and this is again 10 times faster than the third (—> benzotrichloride). 

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. 

A phenyl radical adds to styrene to give a resonance-stabilized benzylic radical. This reaction starts the growth of the polymer chain. Each propagation step adds another molecule of styrene to the growing chain. This addition takes place with the orientation that gives another resonance-stabilized benzylic radical. 

Benzylic C - H bonds are weaker than most other sp hybridized C — H bonds, because homolysis forms a resonance-stabilized benzylic radical. 

Abstraction of a benzylic hydrogen by a Br- radical forms the resonance-stabilized benzylic radical in Step [2], which reacts with Br2 in Step [3] to form the bromination product. 

Figure 12.1. Molecular structure and rate of reaction. Resonance-stabilized benzyl radical formed faster than methyl radical. (Plots aligned with each other for easy comparison.)...

The bromine radical from NBS will abstract whichever hydrogen produces the most stable intermediate in this structure, that is a benzylic hydrogen, giving the resonance-stabilized benzylic radical. 

Head addition to styrene by benzoyloxy radicals is remarkable in that it results in the formation of a significant amount of primary alkyl radicals in preference to the resonance-stabilized benzylic radical. In constrast, the reaction of styrene with most other radicals yields almost exclusively tail addition. The key difference in selectivity between benzoyloxy and terf-butoxy radicals is likely due primarily to steric factors [153]. tert-Butoxy radicals are significantly larger than benzoyloxy radicals and therefore would be expected to add to the sterically less hindered tail position. Also, benzyloxy radicals are likely more electroi Uc than tert-butoxy radicals (due to the elo tron withdrawing effect of the carbonyl) making them more reactive and therefore less selective in their addition to the electron rich styrene molecule. 

Structure A s decomposition rate is second, because it forms a resonance stabilized benzyl radical. 

Phenylmethyl radical The resonance-stabilized benzylic radical C6H5CH2-... 

Under radical initiation conditions, typically peroxides, hypervalent iodine reagents can be homolytically cleaved to iodine-centered radicals. These iodine centered radicals abstract a hydrogen atom from a labile benzylic C—H bond to yield a resonance-stabilized benzylic radical. At this point in the mechanism, researchers seem divided on the next step. Some propose a second single electron transfer (SET) to form a benzylic carbocation, ° which undergoes ionic reactions to form product. Others suggest radical combination to form an alkyl halide or organic peroxide which reacts further under the reaction conditions to form product. 

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