Kinetics of the photobromination of dichloro- and dibromoethane.: Estimate of the C-H bond dissociation energies and the heats of formation of the CH3CCl2 and CH3CBr2 radicals

The kinetics of the alpha-H abstraction reaction RH + Br-- > R + HBr, RH = 1, 1-dichloro (1) and 1,1-dibromoethane (2) was investigated in the temperature range 308-368 K using chloroethane and ethane as an external competitor, respectively. The relative rates were combined with the known rate parameters for chloroethane and ethane to obtain the absolute rate constants (cm(3) mol(-1) s(-1)): k(1) = [6.9+/-3.8(sigma)] x 10(11)exp [(-7300+/-360 cal mol(-1))/RT] and k(2) = [2.2+/-1.2(sigma)] x 10(11)exp [(-7110+/-360 cal mol(-1))/RT]. Using a justifiable approximation concerning the magnitude of the activation energies for the reverse reaction, the following thermochemical quantities (kcal mol(-1)) were derived: Delta H-f(o) (CH3CCl2) = 11.9+/-1.2, D-o(CH3CCl2-H) = 95.1+/-1.2, Delta H-f(o)(CH3CBr2) = 33.5+/-1.3, D-o(CH3CBr2-H) = 94.9+/-1.2. The relation between the reaction thermochemistry and the reactivity in chlorination and bromination of haloethanes is discussed.