Kinetics and thermochemistry of the CH₃CO radical: Study of the CH₃CO + HBr → CH₃CHO + Br reaction

The kinetics of the reaction between CH₃CO and HBr has been studied using a heatable tubular reactor coupled to a photoionization mass spectrometer. CH₃CO was produced homogeneously by laser photolysis in the presence and absence of HBr. Radical decays were monitored in time-resolved experiments. Rate constants were determined at five temperatures in the range 30-400 K and fitted to the Arrhenius expression, 6.4 (±3.6) × 10^-13 exp{4.45 (±1.50) kJ mol^-1/RT} cm³ molecule^-1 s^-1. This kinetic information was combined with known rate constants and Arrhenius parameters for the reverse reaction to obtain the heat of formation of CH₃CO. Both second law and third law procedures were used to obtain this thermochemical information from these rate constants. The two determinations of this heat of formation were in close agreement (differing by only 0.4 kJ mol^-1). These results, taken together, provide a CH₃CO heat of formation of-10.0 ± 1.2 kJ mol^-1 at 298 K which is 14 kJ mol^-1 higher than the value in common use. The current results imply a CH₃-CO bond enthalpy of 45.1 (±1.5) kJ mol^-1 which is 14 kJ mol^-1 lower than currently believed and a CH₃CO-H bond enthalpy of 373.8 (±1.5) kJ mol^-1 which is higher by this same figure. Former disparities between reported CH₃CO heats of formation associated with the equilibrium systems studied to obtain this thermochemical information are resolved.