Bromination of fluoroalkanes. Part 5.—Kinetics of forward and reverse reactions in the system Br2+ i-C3F7H⇌HBr + i-C3F7Br

Abstract

The kinetics of the forward and reverse reactions in the gas-phase system Br₂+ i-C₃F₇H⇌HBr + i-C₃F₇Br have been studied. The slow steps for the forward and reverse reactions, respectively, are Br + i-C₃F₇H→HBr + i-C₃F₇(2), Br + i-C₃F₇Br→Br₂+ i-C₃F₇. (–3)

The Arrhenius parameters obtained, for the range 421–534 °C, are log A₂/cm³ mol^–1 s^–1= 12.66 ± 0.34, E₂= 81.2 ± 3.3, log A_–3/cm³ mol^–1 s^–1= 13.80 ± 0.29, E_–3= 86.3 ± 4.2 kJ mol^–1

The competitive brominations of mixtures of i-C₃F₇H + C₂F₅H and i-C₃F₇H + n-C₃F₇H have been studied over the ranges 249–430 and 220–402 °C, respectively. Each system yielded Arrhenius parameters for reaction (2) which are in excellent agreement with those given above. The results lead to the following bond dissociation energies at 298 K: D(i-C₃F₇—H)= 433.3 ± 2.4 kJ mol^–1, D(i-C₃F₇—Br)= 273.8 ± 4.7 kJ mol^–1.

Attempts were made to measure D[(CF₃)₃C—H] using competitive photobromination and photochlorination. However, (CF₃)₃C—H is so ureactive that only the approximate results D[(CF₃)₃C—H]≈ 456 kJ mol^–1 was obtained. The trends in C—H and C—Br bond dissociation energies are compared in alkanes, fluoroalkanes and the corresponding bromides.