Viscosity-dependent cage reactions. Multiple substitutions in radical-chain chlorinations

Article abstract of DOI:10.1021/ja00014a035

The mechanism by which the free-radical chlorination of alkanes produces polychlorides has been explained by the assumption that the geminate chlorine atom-chloroalkane pair react in their solvent cage at rates competitive with their diffusion from, and rotation in, the cage in which they are born. The major piece of evidence supporting this mechanism is that as the concentration of alkane is increased, polyhalogenation decreases. As the hydrocarbon that makes up the "cage walls" becomes more concentrated, the surrounding hydrocarbon scavenges the caged chlorine atoms before they react with their geminate chloroalkane. In the inert solvents in which these reactions take place, CCl₄ or various Freons, the viscosity of the solvent is decreased with increasing concentration of hydrocarbon, and the diffusion rates are therefore increased. The diffusion dependence (i.e., viscosity) of the amount of polychlorination is an important component of the observed concentration dependence. The influence of the changing viscosity upon the production of polychlorination becomes more important as the hydrocarbon (neopentane, 2,3-dimethylbutane, or cyclohexane) makin up the cage walls becomes less reactive. The viscosity dependence of cage rotation was observed in solvents of high viscosity for the least reactive hydrocarbon, neopentane.