Low-Pressure Study of the Reactions of Br Atoms with Alkenes. 3. Reactions with 2-Methyl-2-butene, 2,3-Dimethyl-2-butene, and 1-Hexene

Abstract

The kinetics of the reactions of Br atoms with 2-methyl-2-butene (C5H10), 2,3-dimethyl-2-butene (C6H12), and 1-hexene have been studied by the mass spectrometry discharge-flow method at 0.5−2.0 Torr of He over the temperature range T = 233−320 K. The occurrence of both hydrogen atom abstraction and addition channels has been observed for all the reactions. The Arrhenius expressions k1a = (5.2 ± 2.5) × 10-16 exp{(1910 ± 250)/T} and k2a = (8.4 ± 6.3) × 10-16 exp{(1950 ± 290)/T} cm3 molecule-1 s-1 have been obtained (at 1 Torr total pressure of He) for the addition reactions Br + C5H10 ↔ C5H10Br (1a,−1a) and Br + C6H12 ↔ C6H12Br (2a,−2a), respectively. Considering k7 and k8 as the rate constants for the reactions of Br2 with C5H10Br and C6H12Br, respectively, the following expressions have been determined: k1ak7/ k-1a = (1.4 ± 0.8) × 10-35 exp{(5250 ± 300)/T} and k2ak8/k-2a = (6.2 ± 6.8) × 10-35 exp{(5190 ± 510)/T} cm6 molecule-2 s-1. Kinetic data have been also obtained for the H atom abstraction channels: k1b = (2.1 ± 0.2) × 10-11 exp{−(655 ± 40)/T}, k2b = (2.4 ± 0.6) × 10-11 exp{−(75 ± 130)/T}, and k3b = (3.25 ± 0.4) × 10-12 exp{−(425 ± 70)/T} cm3 molecule-1 s-1, for the reactions of Br with 2-methyl-2-butene, 2,3-dimethyl-2-butene, and 1-hexene, respectively. From this kinetic study, the enthalpies of reactions 1a and 2a and the heats of formation of the adduct radicals C5H10Br and C6H12Br have been estimated: ΔH°1a = −(10.0 ± 1.6), ΔH°2a = −(9.8 ± 2.1), ΔH°f(C5H10Br) = 6.8 ± 1.6, and ΔH°f(C6H12Br) = 0.1 ± 2.5 (in kcal mol-1).