Substituent Effects and Threshold Energies for the Unimolecular Elimination of HCl (DCl) and HF (DF) from Chemically Activated CFCl₂CH₃ and CFCl₂CD₃

Combination of CFCl 2 and methyl- d 0 and - d 3 radicals form CFCl 2 CH 3 - d 0 and - d 3 with 100 and 101 kcal/mol of internal energy, respectively. An upper limit for the rate constant ratio of disproportionation to combination, k d / k c, for Cl transfer is 0.07 ± 0.03 for collision of two CFCl 2 radicals and 0.015 ± 0.005 for CH 3 and CFCl 2 radicals. The chemically activated CFCl 2 CH 3 undergoes 1,2-dehydrochlorination and 1,2-dehydrofluorination with rate constants of 3.9 × 10 9 and 4.9 × 10 7 s -1, respectively. For CFCl 2 CD 3 the rate constants are 8.7 × 10 8 s -1 for loss of DCl and 1.1 × 10 7 s -1 for DF. The kinetic isotope effect is 4.4 ± 0.9 for HCl/DCl and appears to be identical for HF/DF. Threshold energies are 54 kcal/mol for loss of HCl and 68 kcal/mol for HF; the E 0 's for the deuterated channels are 1.4 kcal/mol higher. Comparison of these threshold energies with other haloethanes suggests that for HF and HCl elimination the transition states are developing charges of different signs on the carbon containing the departing halogen and that chlorine and fluorine substituents exert similar inductive effects.