Kinetics of the gas‐phase reaction CH 3 F + I 2 ⇆ CH 2 FI + HI: The CH bond dissociation energy in methyl and methylene fluorides

The kinetics of the gas‐phase reaction of CH 3 F with I 2 have been studied spectrophotometrically from 629 to 710 K, and were 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\documentclass{article}\pagestyle{empty}\begin{document}$$ \log k_4 (M^{ - 1} \cdot s^{ - 1}) = (11.3 \pm 0.1) - (30.8 \pm 0.2)/\theta $$\end{document}where θ = 4.575 T /1000 kcal/mol. The errors represent one standard deviation. The experimental activation energy E 4 = 30.8 ± 0.2 kcal/mol was combined with the assumption E 3 = 1 ± 1 kcal/mol and estimated heat capacities to obtain\documentclass{article}\pagestyle{empty}\begin{document}$$ \Delta H_r^\circ (4,g,298K) = 30.0 \pm 1{\rm kcal}/{\rm mol} $$\end{document}The enthalpy change at 298 K was combined with selected thermochemical data to derive\documentclass{article}\pagestyle{empty}\begin{document}$$ DH^\circ ({\rm CH}_{\rm 2} {\rm F} - {\rm H}) = 101.2 \pm 1{\rm kcal/mol} $$\end{document}The kinetic studies of ĊHF 2 and CH 2 F 2 have been reevaluated to yield\documentclass{article}\pagestyle{empty}\begin{document}$$ DH^ \circ \left( {{\rm CHF}_{\rm 2} - {\rm H}} \right) = 103.2 \pm 1\,{{{\rm kcal}} \mathord{\left/ {\vphantom {{{\rm kcal}} {{\rm mol}}}} \right. \kern-\nulldelimiterspace} {{\rm mol}}} $$\end{document}These results are combined with literature data to yield the CH, CF, and CCl bond dissociation energies in their respective fluoromethanes, and the effect of α‐fluorine substitution is discussed.