Low-pressure study of the reactions of Br atoms with alkenes. 1. Reaction with propene

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 k(1a) = (8.15 +/- 4.5) x 10(-13) exp{-(1250 +/- 160/T)} cm(3) molecule(-1) s(-1) and k(1b) = (3.8 +/- 0.8) x 10(-33) exp{(1200 +/- 50)/T} cm(6) molecule(-2) s(-1) were obtained for these channels, respectively. The expression k(1b)k(10)/k(-1b) = (8.9 +/- 3.1) x 10(-36) exp{(4140 +/- 200)/T} cm(6) molecule(-2) s(-1) was obtained for the combination of the rate constants for the adduct formation (k(1b)) and adduct decomposition (k(-1b)) reactions and for the reaction C3H6Br + Br-2 --> C3H6Br2 + Br (10). From these kinetic data, the enthalpy of reaction 1b and the heat of formation of the C3H6Br radical were calculated: Delta H(1b)degrees = -(7.7 +/- 1.4) kcal mol(-1) and Delta H(f)degrees(C3H6Br) = 23.8 +/- 1.4 kcal mol(-1). In addition, the rate constant of the reaction Br + C3H5 --> C3H5Br (6) was measured: k(6) = (2.0 +/- 0.5) x 10(-10) cm(3) molecule(-1) s(-1), at T = 298 K and 1 Torr total pressure.