The kinetics and equilibrium of the gas‐phase reaction CH 3 CF 2 Br + I 2 = CH 3 CF 2 I + IBr the C‐Br bond dissociation energy in 1,1‐difluorobromoethane

The kinetics and equilibrium of the gas‐phase reaction of CH 3 CF 2 Br with I 2 were studied spectrophotometrically from 581 to 662°K and determined to be consistent with the following mechanism:A least squares analysis of the kinetic data taken in the initial stages of reaction resulted in log k 1 (M −1 · sec −1 ) = (11.0 ± 0.3) ‐ (27.7 ± 0.8)/θ where θ = 2.303 RT kcal/mol. The error represents one standard deviation. The equilibrium data were subjected to a “third‐law” analysis using entropies and heat capacities estimated from group additivity to derive Δ H r ° (623°K) = 10.3 ± 0.2 kcal/mol and Δ H r r (298°K) = 10.2 ± 0.2 kcal/mol. The enthalpy change at 298°K was combined with relevant bond dissociation energies to yield DH°(CH 3 CF 2 ‐ Br) = 68.6 ± 1 kcal/mol which is in excellent agreement with the kinetic data assuming that E 2 = 0 ± 1 kcal/mol, namely; DH°(CH 3 CF 2 ‐ Br) = 68.6 ± 1.3 kcal/mol. These data also lead to ΔH f °(CH 3 CF 2 Br, g, 298°K) = ‐119.7 ± 1.5 kcal/mol.