Bromination of Hydrocarbons. II. Photochemical Bromination of Ethane and Ethyl Bromide. Carbon-Hydrogen Bond Strength in Ethane

Abstract

The kinetics of the photochemical bromination of ethane was determined. The mechanism is very similar to the analogous brominations of hydrogen and methane: (1) Br 2 +hν= Br + Br (2) Br + C 2 H 6 = C 2 H 5 + HBr (3) C 2 H 5 + Br 2 = C 2 H 5 Br + Br (4) C 2 H 5 + HBr = C 2 H 6 + Br (5) Br + Br +M= Br 2 +M. Rate constants computed by assuming various efficiencies for ethane as the third body in step (5) are presented, as well as those calculated by considering heterogeneous recombination of bromine atoms. The rate of bromination of ethyl bromide is about the same as that for ethane, but its exact determination was precluded by the presence of rapidly brominated impurities. The experimental activation energy, 13.6±0.5 kcal., is quite insensitive to the several variations in mechanism discussed, and indicates a value of 98±2 kcal. for the upper limit of the strength of the C2H5–H bond. This figure is in good agreement with absolute values reported by other workers.