Photochlorination of methane and fluoroform. Dissociation energy D(CF3—H) and entropy of CF3 radical

Abstract

The kinetics of the vapour-phase chlorinations of CH₄, CF₃H and C₂F₅H have been studied using a competitive technique. For the reactions, Cl+CF₃H→HCl+CF₃(3) Cl+CH₄→HCl+CH₃(4) Cl+C₂F₅H→HCl+C₂F₅(5) we obtain, k₄/k₅=(12.4±0.7) exp (1,450±40)/RT, k₅/k₃=(1.16±0.11) exp (3,080±70)/RT. The activation energy differences are combined with other data to give D(CF₃—H)= 106.4±0.5 kcar mole^–1 at 298°K. This agrees with the value D(CF₃—H)= 106.2±0.8 kcal mole^–1 from oul previous bromination work. These results are compared with other estimates of D(CF₃—H) in the literature.

The data on the pre-exponential factors of reactions (3) and (4) are combined with data on the reverse reactions to obtain the entropy of the CF₃ radical. A similar calculation is made using the reactions, Br+CF₃H⇌HBr+CF₃. The results agree reasonably well with estimates of [graphic omitted] based on spectroscopic data. The preexponential factors of reactions (3) and (4) have been calculated by means of transition state theory.