Kinetics of the acetone-photosensitized addition of HBr to ethylene

Abstract

The kinetics of the acetone-sensitized photoaddition of HBr to ethylene are consistent with the free radical chain mechanism of Vaughan, Rust, and Evans. In the presence of excess propane the combination of bromine atoms (reaction [4]) is the main termination step, and bromoethyl radicals formed by the addition of bromine atoms to ethylene (reaction [2]) are collisionally stabilized. They undergo thermal decomposition (reaction [−2]) or abstract a hydrogen atom from hydrogen bromide (reaction [3a]) to form the product, ethyl bromide. The rate constant ratio, k−2/k3a, increases from 5.6 × 1018 molecules cm−3 at 15.3 °C to 11.5 × 1018 molecules cm−3 at 45.0 °C. This corresponds to E−2 − E3a = 4.5 ± 0.5 kcal mole−1, and is in accord with the thermochemical data. Within experimental error, k2 and k−2 are independent of propane pressure in the range 260 to 390 Torr. Also, [Formula: see text] is equal to 5.5 ± 0.3 × 108 cm3/2 molecule−1/2 s−1/2, independent of temperature over the range 15.3 to 45.0 °C, and E2 = 0.0 ± 0.5 kcal mole−1. In the absence of propane, and at ethylene pressures in excess of 400 Torr, the combination and disproportionation of bromoethyl radicals is the main termination process (reaction [7]). The value of [Formula: see text] is 8.9 ± 0.1 × 10−10 cm3/2 molecule−1/2 s−1/2 at 35.1 °C, in agreement with kinetic data for related reactions.