KINETICS OF THE REACTIONS OF ALKYL RADICALS (CH3, C2H5, I-C3H7, AND T-C4H9) WITH MOLECULAR BROMINE

The gas-phase kinetics of the reactions of four alkyl radicals (CH3, C2H5, i-C3H7, and t-C4H9) with molecular bromine have been studied over the temperature range 296-532 K. The reactions were isolated for quantitative study in a heatable tubular reactor coupled to a photoionization mass spectrometer. Radicals were homogeneously generated in the reactor by pulsed photolysis of suitable precursor molecules at 193 or 248 nm. The subsequent decays of the radical concentration in the presence of different Br2 concentrations were monitored in time-resolved experiments. Rate constants were obtained at five temperatures. The Arrhenius expressions (k = A exp(-E/RT)) that describe the temperature dependencies of the measured rate constants of the R + Br2 → RBr + Br reactions are as follows [numbers in brackets are log A/(cm3 molecule-1 s-1), E/(kcal mol-1)]: R = CH3 [-10.70 (±0.08), -0.39 (±0.25)]; R = C2H5 [-10.58 (±0.11), -0.82 (±0.41)]; R = i-C3H7 [-10.62 (±0.11), -1.07 (±0.45)]; R = t-C4H9 [-10.70 (±0.11), -0.97 (±0.45)]. The trend in the measured rate constants and the results of prior studies of the dynamics of the CH3 + Br2 reaction indicate that the reactivity of R + Br2 reactions is determined largely by long-range attractive forces and not by reaction thermochemistry. © 1990 American Chemical Society.