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–1The 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.