Low-Pressure Study of the Reactions of Br Atoms with Alkenes. 2. Reactions with Ethene and trans-2-Butene

Abstract

The reactions of Br atoms with ethene and trans-2-butene have been studied by the mass spectrometry discharge-flow method, at a total pressure of helium 0.5−2.0 Torr in the temperature range 233−320 K. The expression = (3.0 ± 1.2) × 10-33 exp{(730 ± 220)/T} cm6 molecule-2 s-1 has been obtained for the low-pressure limit of the rate constant for the addition reaction Br + C2H4 + M ↔ C2H4Br + M (1,−1). The expressions k1k5/k-1 = (3.1 ± 0.6) × 10-36 exp{(3540 ± 280)/T} cm6 molecule-2 s-1 and k1k6a/k-1 = (1.8 ± 0.9) × 10-35 exp{(3390 ± 270)/T} cm6 molecule-2 s-1 have been obtained for the combinations of the rate constants of reactions (1,−1) and reactions Br2 + C2H4Br → Br + C2H4Br2 (5) and Br + C2H4Br → C2H4Br2 (6a). For the reaction of Br atoms with trans-2-butene, both the addition channel, Br + C4H8 ↔ C4H8Br (2a,−2a), and the H atom abstraction channel, Br + C4H8 → C4H7 + HBr (2b) have been observed and the following kinetic data were obtained: = (2.1 ± 0.8) × 10-32 exp{(1300 ± 90)/T} cm6 molecule-2 s-1, k2ak9/k-2a = (3.8 ± 1.7) × 10-36 exp{(4630 ± 230)/T} cm6 molecule-2 s-1 (where k9 is the rate constant of the reaction C4H8Br + Br2 → C4H8Br2 + Br) and k2b = (5.8 ± 1.7) × 10-12 exp{−(960 ± 160)/T} cm3 molecule-1 s-1. Thermochemical data around room temperature could be derived from these kinetic measurements; the enthalpies of reactions 1 and 2a are = −(6.8 ± 1.6) kcal mol-1 and = −(8.8 ± 1.5) kcal mol-1, and the heats of formation of the adducts C2H4Br and C4H8Br are = 32.4 ± 1.6 kcal mol-1 and = 20.5 ± 1.8 kcal mol-1.