The kinetics of the gas-phase thermal decomposition of bromodifluoromethane

Abstract

The pyrolysis of bromodifluoromethane has been examined over the temperature range 437–515 °C with initial pressures of reactant between 20 and 250 Torr. The predominant mode of decomposition involves an α-elimination of hydrogen bromide to give difluoromethylene as the initial step (i) but there is also a free-radical chain component CF₂HBr = CF₂+ HBr (i) to the decomposition which is reduced to negligible importance by the presence of propene. The α-elimination component of the reaction is homogeneous and of the first order in the initial stages, but inhibition by hydrogen bromide becomes important as the reaction proceeds. The first-order rate constant obtained from the initial rates is independent of pressure and is given by log k₁/s^–1= 14·33 ± 0·32 –(55,600 ± 1100)/4·576T. The rate constant (k₂) for the reverse reaction has also been evaluated as log k₂/l mol^–1 s^–1= 8·33 ± 0·37 –(9600 ± 1300)/4·576T and combination of these Arrhenius parametes with the relevant heats of formation and standard entropies give ΔH°_f(CF₂)=–42 kcal mol^–1 and S°(CF₂)= 66·2 cal mol^–1 K^–1 at 750 K, which are in good agreement with literature values.