REACTION OF ATOMIC BROMINE WITH DIFLUOROCHLOROMETHANE - THE HEAT OF FORMATION OF THE CCLF2 RADICAL AND THE D-DEGREES(CCLF2-H) BOND-DISSOCIATION ENERGY

The gas-phase photobromination of CHClF2 (1) in the presence of CH3Cl (2) as competitor has been studied in the temperature range 80-150-degrees-C at halomethane pressures of approximately 35 Torr and a Br2 pressure of approximately 2.3 Torr. The temperature dependence of the rate constant ratio is found to obey the Arrhenius expression ln (k1/k2) = (-0.0885 +/- 0.0580) - (1144 +/- 20)/T. This result is combined with an earlier relative study of CH3Cl (2) vs C2H6 (3) and a recent direct determination by kinetic spectroscopy of the rate constant for the bromination of ethane (k3) to obtain absolute rate parameters for the reaction CHClF2 + Br --> CClF2 + HBr. Using a justifiable approximation concerning the magnitude of the activation energy difference for the reverse reactions between any two competitors of similar complexity, and other thermochemical data from the literature, the following quantities have been derived: DELTA-H(f)-degrees 298(CClF2) = -66.7 +/- 2 kcal mol-1 and D-degrees (CClF2-H) = 100.7 +/- 2 kcal mol-1 where the uncertainties are conservative estimates. On the basis of the new value of k3, activation energies for the bromination of CH4 and other halomethanes are reported.