Kinetics of the CH2OH + HBr and CH2OH + HI Reactions and Determination of the Heat of Formation of CH2OH

Article abstract of DOI:10.1021/j100192a040

The reaction between CH2OH and HBr (1) and CH2OH and HI (2) have been studied using excimer laser flash photolysis coupled with time-resolved photoionization mass spectrometry over the temperature range 298-538 K giving k₁ = 8.7 (+/- 3.2) * 10^-13 exp<3.7 (+/- 1.3) kJ mol^-1/RT> and k₂ = 2.7 (+/- 0.5) * 10^-12 exp<4.8 (+/- 0.5) kJ mol^-1/RT> cm³ molecule^-1 s^-1.This kinetic information was combined with rate constants for the reverse reactions taken from the literature to obtain the heat of formation of CH2OH.Both second law and third law procedures were used to obtain this thermochemical information from these rate constants.Three independent determinations of this heat of formation were in close agreement (differing at most by 1.0 kJ mol^-1).The results taken together indicate a CH2OH heat of formation of -8.9 +/- 1.8 kJ mol^-1 at 298 K.The kinetic behavior of reactions 1 and 2 is comparable to that of other polyatomic free radicals with HBr and HI, radicals which have ionization potentials similar to that of CH2OH.The significantly higher CH2OH heat of formation obtained in the current investigation implies a stronger C-H bond energy in CH3OH than currently believed and has implications in the modeling of the kinetics of methanol pyrolysis and oxidation, both of which are discussed.