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

The bond dissociation enthalpies in a number of substituted benzyl bro mides have been studied in the gas phase, using the toluene carrier te chnique, and in solution with photoacoustic calorimetry. Gas phase the rmolysis with unsubstituted benzyl bromide gives an absolute C-Br bond dissociation enthalpy (E-d) at 298 K of 255 plusminus 4 kJ mol(-1). C ompetition experiments in the gas phase reveal no substituent effect o n the value of E-d(C-Br). Gas phase thermolysis with substituted tert- butylbenzenes also shows no effect on the C-CH, bond dissociation enth alpy for p-CN,p-OH and p-Bu-1 substituents, with a E-d(C-CH3) value at 298 K of 299 plusminus 2 kJ mol(-1). In solution, photoacoustic exper iments yield no detectable substituent effect for p-CN, p-Bu-t and m-C F3 substitution of the benzyl bromide, in contrast with other reports. With photoacoustic calorimetry a bond dissociation enthalpy of 254 pl usminus 4 kJ mol(-1) has been found for all benzyl bromides studied. A rationale for the absence of a substituent effect on the benzylic bon d dissociation enthalpy is provided.