Isotope effects radical aromatic substitution

Flash photolysis of a substituted hexaarylbiimidazole and reactions of the imi-dazolyl radical. Coraor, G. R., Riem R. H. MacLachlan, A. Urban, E. J. (Exp. Stn., E. I. DuPont de Nemours, and Co., Wilmington, Del.). J. Org. Chem. 1971, 36 (16), 2272-5. (Eng). The rate of reaction of 2-(o-chlorophenyl)-4,5-diphenylimida-zolyl radicals (L with additives was studied in various solvents. Evidence based on measured rate consts., including kinetic D isotope effects, prove that the rate detg. step in the reaction L + aromatic amine is an electron change reaction at the amino N, while in the reaction L + hydroquinone the rate-detg. step is H abstraction. 

CT interaction 132). In the case of substituted benzenes with benzylic hydrogens, proton transfer yields radicals. The main evidence for this CT photoreduction mechanism is the lack of any isotope effect on kt when toluene- -d3 is substituted for toluene-hg. In the absence of any labile protons on the aromatic half of the CT complex, such as with benzene itself, the CT interaction is primarily a quenching one, unless an external proton source is present, in which case the complex is apparently protonated 183>. 

In solution of perchlorates and tosylates in acetic acid, the primary products of electro-oxidation of aromatic hydrocarbons are shown to be benzylic acetates, i.e., substitution occurs preferentially in the side chain. A large, primary isotope effect, ku/kjy = 2.6, provided strong evidence that in this type of substitution reaction the rate-determining step involved the loss of a proton.Further refinement of the mechanism correlated isomeric product ratios to the distribution of positive charge density in the intermediate cation radical. 

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