Bromination of fluoroalkanes. Part 4.—Kinetics of thermal bromination of fluoroform and pentafluoroethane

Abstract

The thermal brominations of CF₃H and C₂F₅H have been studied in the ranges 361–431° and 323–458°C respectively. The kinetics agree with the reactions [graphic omitted] From measurements of initial rates, log k₃(cm³ mole^–1 sec^–1)=(13.107 ± 0.035)–(22,320 ± 110)/2.303 RT, log k₄(cm³ mole^–1 sec^–1)=(13.026 ± 0.020)–(19,305 ± 61)/2.303 RT. The inhibition by added HBr was studied with both systems. For Br₂+ CF₃H, the values of k_–3/k₉ obtained agree with previous independent data, the combined result being, log (k_–3/k₉)=(–0.58 ± 0.04)–(2,130 ± 90)/2.303 RT. For the system Br₂+ C₂F₅H, log (k_–4/k₁₅)=(–0.45 ± 0.06)–(2,750 ± 250)/2.303 RT. The data for reactions (3) and (4) are combined with previous competitive results to give for the reference reactions, Br + CH₄→ HBr + CH₃, Br + CH₃Br → HBr + CH₂Br, log k_CH4(cm³ mole^–1 sec^–1)=(13.991 ± 0.049)–(18,580 ± 140)/2.303 RT, log k_CH3Br(cm³ mole^–1 sec^–1)=(14.000 ± 0.058)–(16,310 ± 170)/2.303 RT. Arrhenius parameters are tabulated for all bromination reactions studied to date.

From Arrhenius parameters for bromination reactions, the following bond dissociation energies are obtained: D(CF₃—H)= 106.7 ± 0.5, D(C₂F₅—H)= 102.6 ± 1.2 and D(CH₃—H)= 104.3 ± 0.7, all in kcal mole^–1 at 298°K. These figures are compared with results from other sources and recommended values of the three quantities are, D(CF₃—H)= 106.4 ± 0.5, D(C₂F₅—H)= 102.9 ± 1.0 and D(CH₃—H)= 104.2 ± 0.5 kcal mole^–1.