Substituent effects and threshold energies for the unimolecular elimination of HCl (DCl) and HF (DF) from chemically activated CFCl2CH3 and CFCl2CD3

Combination of CFCl2 and methyl-d(0) and -d(3) radicals form CFCl2CH3-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 CFCl2 radicals and 0.0l5 +/- 0.005 for CH3 and CFCl2 radicals. The chemically activated CFCl2CH3 undergoes 1,2-dehydrochlorination and 1,2-dehydrofluorination with rate constants of 3.9 x 10(9) and 4.9 x 10(7) s(-1), respectively. For CFCl2CD3 the rate constants are 8.7 x 10(8) s(-1) for loss of DCl and 1.1 x 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 Eo'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.