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