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.