Kinetics and thermochemistry of the C3H5 + HBr ? C3H6 + Br equilibrium

Article abstract of DOI:10.1039/a905347f

As an accurate heat of formation of the allyl radical is very important for modeling high-temperature combustion, the kinetics of the reaction of the allyl radical with HBr were studied in a tubular reactor coupled to a photoionization MS. The decay of the allyl radical was monitored as a function of HBr concentration under pseudo-first-order conditions. The reaction was studied at 400-523 K and the determined rate constants determined were fitted to an Arrhenius expression, i.e., k = (4.6 ± 3.2) x 10^-13 exp(-(12.6 ± 4.1) kJ/mol/RT). The obtained rate constants were combined with those for the reverse reaction taken from the literature to obtain the enthalpy of formation of the allyl radical as 166.1 ± 4.3 kJ/mol at 298 K, which agreed well with previous measurements. The entropy was 248 ± 15 J/K/mol. The allylic C-H bond energy of propylene derived from the enthalpy of reaction value was 363.9 ± 4.3 kJ/mol, which led to a value of 59.4 ± 4.9 kJ/mol for the resonance stabilization energy of the allyl radical. Ab initio calculations showed that the allylic bond of propene is the weakest bond of the molecule.