Role of Cage Return and Solvent Viscosity in the Temperature-Dependent Kinetics of Benzylic Bromination

Article abstract of DOI:10.1021/jo00230a006

The effect of temperature on both the kinetic isotope effect in the homolytic abstraction of benzylic hydrogen by bromine and the competitive brominations of toluene and a ring-substituted toluene was interpreted as being due not only to the activation parameters involved in abstraction but upon the viscosity dependence of the kinetic results.Internal cage return was shown to be viscosity dependent, and the resultant kinetic isotope effect was corrected to account for cage reversal.The viscosity dependence in the relative rates of competitive bromination of toluene and p-chlorotoluene showed an inverse correlation to that obtained with temperature.The nonmonotonic Arrhenius plot previously reported could be explained on the basis of these two opposing effects, as well as the fact that over the range of temperature previously reported the reaction mixtures became nonhomogeneous.