Kinetics of the reaction between HBr and CF3Br. Determination of the bond dissociation energy D(CF3—Br)

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 K_c, 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).