Kinetics of the reaction between HBr and C2F5Br. Determination of the bond dissociation energy D(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 k_c, 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.