Abstract:
The kinetics of the overall thermal reaction CF3Br + HBr → CF3H + Br2(5) were studied in the range 368–537°C. Reactions involved are Br + Br + M ⇌ Br2+ M Kc, Br + CF3Br ⇌ Br2+ CF3(6, –6), CF3+ HBr → CF3H + Br. (7) The rate law was in accord with this mechanism. The rate constant for the transfer of Br by reaction (6) is given by log k6/(cm3 mol–1 s–1)=(13.91 ± 0.12)–(25 170 ± 340)/θ where θ= 2.303RT/cal mol–1. From this and previous work, log(k–6/k7)=(0.749 ± 0.036)–(1870 ± 90)/θ. The difference in activation energies E6–E–6=ΔH°6 is used to derive a value of the bond dissociation energy D(CF3—Br) in good agreement with that obtained previously by a different method. We recommend D(CF3—Br)= 70.6 kcal mol–1 at 298 K. Studies of the kinetics of reactions such as (5) should provide in favourable cases a route to the determination of bond dissociation energies D(R—Br) and also provide Arrhenius parameters for reactions analogous to (6).
