Kinetics of the Reaction Between HBr and C2F5Br Determination of the Bond Dissociation Energy Z>(C2F5—Br)

Abstract

The kinetics of the overall thermal reaction between HBr and C2F5Br, HBr + C2F5Br ⇌ Br2+ C2F5H (3, –3) were studied in the range 365–516°C. Reactions involved are Br + Br + M ⇌ Br2+ M kc, Br + C2F5Br ⇌ Br2+ C2F5(4, –4), C2F5+ HBr → C2F5H + Br. (5) The rate law expected from this mechanism was confirmed and the experimental rate constant for the bromine-atom transfer reaction (4) is given by log k4/(cm3 mol–1 s–1)=(14.30 ± 0.16)–(24070 ± 530)/θ(10) where θ= 2.303 RT/cal mol–1. These studies also yield values of k–4/k5, which when combined with previous results, lead to log(k–4/k5)=(0.65 ± 0.20)+(2360 ± 500)/θ. We propose that the value of E4 in eqn (10) should be modified to 22.8 kcal mol–1 which leads to a value of the bond dissociation energy D(C2F5—Br)= 68.6 kcal mol–1. The relation of this result to D(C2F5—H) and to ΔH°3, obtained from previous studies of equilibrium (3), is discussed. New data on equilibrium (3) are presented.