Substituent effects on the benzylic bond dissociation enthalpy in benzyl bromides (C–Br) and tert-butylbenzenes (C–CH3): a gas phase thermolysis and liquid phase photoacoustic study

Abstract

The bond dissociation enthalpies in a number of substituted benzyl bromides have been studied in the gas phase, using the toluene carrier technique, and in solution with photoacoustic calorimetry. Gas phase thermolysis with unsubstituted benzyl bromide gives an absolute C–Br bond dissociation enthalpy (E_d) at 298 K of 255 ± 4 kJ mol^–1. Competition experiments in the gas phase reveal no substituent effect on the value of E_d(C–Br). Gas phase thermolysis with substituted tert-butylbenzenes also shows no effect on the C–CH₃ bond dissociation enthalpy for p-CN, p-OH and p-Bu^t substituents, with a E_d(C–CH₃) value at 298 K of 299 ± 2 kJ mol^–1. In solution, photoacoustic experiments yield no detectable substituent effect for p-CN, p-Bu^t and m-CF₃ substitution of the benzyl bromide, in contrast with other reports. With photoacoustic calorimetry a bond dissociation enthalpy of 254 ± 4 kJ mol^–1 has been found for all benzyl bromides studied. A rationale for the absence of a substituent effect on the benzylic bond dissociation enthalpy is provided.