Bond dissociation energies from equilibrium studies. Part 1.—D(CF3—Br), D(C2F5—Br) and D(n-C3F7—Br)

Abstract

The gas-phase equilibria: Br₂+CF₃H ⇌ HBr+CF₃Br (1) Br₂+C₂F₅H ⇌ HBr+C₂F₅Br (2) Br₂+n-C₃F₇H ⇌ HBr+n-C₃F₇Br (3) have been studied over the ranges 410–608°C (1), 359–569°C (2) and 340–528°C (3). Equilibria were approached from both sides. A third-law treatment of the values of K₁ gives ΔH^°₁=–4.59 ± 0.25 kcal mole^–1 so that ΔH^°_f(CF₃Br)=ΔH^°_f(CF₃H)+11.5 ± 0.30 kcal mole^–1, all at 298°K. The use of our previous value of D(CF₃—H) leads to D(CF₃—Br)= 69.4 ± 0.8 kcal mole^–1. The third-law method cannot be applied to equilibria (2) and (3) but we estimate that ΔH^°₂= 5.1 ± 1.0 and ΔH^°₃= 5.0 ± 1.0 kcal mole^–1. Taking D(C₂F₅—H)=D(n-C₃F₇—H)= 103.5 ± 2.0, we obtain, D(C₂F₅—Br)=D(n-C₃F₇—Br)= 67.0 ± 2.5, all in kcal mole^–1 at 298°K. The various bond dissociation energies are compared with those of the corresponding alkanes and alkyl bromides.