Chlorine Atom/Benzene System. 1. The Role of the 6-Chlorocyclohexadienyl Radical

Article abstract of DOI:10.1021/ja00280a030

The concept of radical reactivity mediated by solvation has rested mainly on the alteration of Cl^. properties by aromatic solvents.For this reason, the full scope of the benzene/Cl^. system has been reexamined to evaluate the discription of that system based largely on a ?-complex (solvation).At the present time, the ?-complex description rests narrowly on the assignment of a 490-nm absorption, which, even if correct, could not provide an unambiguous structure assignment.Results are presenteed which described the selectivities in alkane substitutions as a function of the concentrations of both benzene and the alkane.Selectivities increase with decreasing alkane concentrations, reaching a plateau below 0.1 M alkane.The change in selectivity is the result of variable contributions of both a low- and a high-selectivity intermediate, LSI and HSI, respectively.The observed selectivity at a given and is the consequence of a unique / ratio.A range of substrates and their effect of DMB selectivity were studied, and from these results details regarding the chemistry of the HSI were extracted.Several features of the LSI/HSI equilibrating system are realized. (1) Reaction of alkyl radicals with Cl2 in benzene produces the LSI, (2) the LSI does not exhibit the characteristics of free chlorine atom, and (3) at alkane concentrations and , added reagents (T) which react with CCH, such as maleic anhydride (MA) or Cl2, bring about an increase in the LSI/HSI ratio.Low-selectivity hydrogen abstractions (LSI function) are best ascribed to a mixture of chlorine atom and chlorine atom/benzene ?-complex.The chemistry of CCH is as follows: (1) loss of the ipso H to O2 yielding PhCl and HO2^., (2) reactions of Cl2 or (3) maleic anhydride with the aromatic nucleus of CCH resulting in additions to the ring, (4) the transfer of Cl to alkenes, and (5) the highly selective retardation of rates of reaction with alkanes producing alkyl radicals, HCl, and benzene.The results of a kinetic analysis, accounting for the effect of , , and CCH trapping agents (T), are presented.For CCH, the following reactivity order is established: maleic anhydride (6) > trans-dichloroethene(5) > 2,3-dimethylbutane (2) > pentane (1) > Cl2 <1.00> > neopentane (<0.1).The selectivity of CCH (per H) was found to be 3 (130) > 2 (27) > 1 <1.00>.These properties can be rationalized with canonical structures for CCH wherein spin density at carbon, chlorine, and the ipso hydrogen makes contributions to the hybrid.