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)
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.