Determination of Some Carbon-Chlorine Bond Dissociation Energies

An investigation of the pyrolyses of benzyl chloride and benzoyl chloride has been carried out by means of the ``toluene carrier'' technique. It is shown that the course of these reactions is analogous to that of the previously investigated pyrolyses of benzyl bromide and benzoyl bromide, i.e., the rate determining steps are C6H5·CH2Cl→C6H5·CH2·+Clor C6H5·CO·Cl→C6H5·CO+Cl.From kinetic data we obtained the activation energies of the rate determining steps, and we identify these activation energies with the respective C–Cl bond dissociation energies. Thus we derive D(C6H5·CH2−Cl)=68 kcal/moleand D(C6H5·CO·−Cl)=73.2 kcal/mole.The consistency of these data with the respective C–Br bond dissociation energies is very satisfactory. For example, the heat of formation of the benzoyl radical, derived from the D(C6H5·CO–Cl), is 15.7 kcal/mole, and that calculated from the experimentally determined D(C6H5·CO–Br) is 15.6 kcal/mole.