Low-Pressure Study of the Reactions of Br Atoms with Alkenes. 1. Reaction with Propene

Abstract

The reaction of Br atoms with propene was studied by the mass spectrometry discharge-flow method at low pressure (0.5−2.0 Torr of He) between 233 and 320 K. Under these experimental conditions, both the hydrogen atom abstraction Br + C3H6 → C3H5 + HBr (1a) and the addition Br + C3H6 + M → C3H6Br + M (1b) channels were observed. The Arrhenius expressions k1a = (8.15 ± 4.5) × 10-13exp{−(1250 ± 160/T)} cm3 molecule-1 s-1 and k1b = (3.8 ± 0.8) × 10-33 exp {(1200 ± 50)/T} cm6 molecule-2 s-1 were obtained for these channels, respectively. The expression k1bk10/k-1b = (8.9 ± 3.1) × 10-36 exp{(4140 ± 200)/T} cm6 molecule-2 s-1 was obtained for the combination of the rate constants for the adduct formation (k1b) and adduct decomposition (k-1b) reactions and for the reaction C3H6Br + Br2 → C3H6Br2 + Br (10). From these kinetic data, the enthalpy of reaction 1b and the heat of formation of the C3H6Br radical were calculated: ΔH°1b = −(7.7 ± 1.4) kcal mol-1 and ΔH°f(C3H6Br) = 23.8 ± 1.4 kcal mol-1. In addition, the rate constant of the reaction Br + C3H5 → C3H5Br (6) was measured: k6 = (2.0 ± 0.5) × 10-10 cm3 molecule-1 s-1, at T = 298 K and 1 Torr total pressure.