Bond dissociation energies from equilibrium studies. Part 5.—The equilibria Br2+ CH2F2⇌ HBr + CHF2Br and Br2+ CH3F ⇌ HBr + CH2FBr. Determination of D(CHF2—Br) and ΔH °f(CHF2Br, g)

Abstract

The equilibrium, Br₂+ CH₂F₂⇌ HBr + CHF₂Br (4), has been studied in the temperature range 371–479°C, with equilibrium being approached from both sides. Third-law calculations lead to ΔH^°₄=–39.9 ± 0.3 kJ mol^–1 at 298 K from which, ΔH^°_f(CHF₂Br, g)=–425.3 ± 0.9 kJ mol^–1, D(CHF₂—Br)= 289 ± 8 kJ mol^–1, at 298 K. The value of ΔH^°_f(CHF₂Br) is combined with published kinetic data on the pyrolysis of CHF₂Br to give, ΔH^°_f(CF₂)=–194 ± 9 kJ mol^–1 at 298 K.

The system, Br₂+ CH₃F ⇌ HBr + CH₂FBr, was also studied but true equilibrium was not achieved owing to unexpected formation of CH₃Br as a major product. Possible mechanisms involving breaking of a C—F bond and conversion of CHF₂Br or CH₃F to CH₃Br are discussed.