THRESHOLD ENERGIES AND SUBSTITUENT EFFECTS FOR UNIMOLECULAR ELIMINATION OF HCL (DCL) AND HF (DF) FROM CHEMICALLY ACTIVATED CF2CLCH3 AND CF2CLCD3

Chemically activated CF2ClCH3-d0 and -d3 were prepared with 101 and 102 kcal/mol of internal energy, respectively, by the combination of CF2Cl with methyl-d0 and -d3 radicals at 300 K. The CF2ClCH3 reacts by loss of HCl and HF with rate constants of (2.5 ± 0.4) × 109 and (0.10 ± 0.02) × 109 s-1, respectively, a branching ratio of 25:1 in favor of HCl. The CF2ClCD3 has rate constants of (0.78 ± 0.12) × 109 for loss of DCl and 0.033 ± 0.005 × 109 sec-1 for loss of DF. The combined primary and secondary isotope effect was 3.2 ± 0.9 for HCl/DCl elimination and 3.0 ± 0.9 for loss of HF/DF. RRKM theory was used to model these unimolecular rate constants to determine the threshold energy barriers, E0's, for the four-centered elimination reactions. The E0's were 55 ± 2 kcal/mol for dehydrochlorination and 69.5 ± 2 kcal/mol for dehydrofluorination. These threshold barriers are in sharp disagreement with prediction based on trends in the E0's for a series of mono-, di-, and trisubstituted chloroethanes and a similar series of fluoroethanes. An analysis of the E0's for the chloroethanes, fluoroethanes, and chlorodifluoroethane suggests that the carbon-halogen bond in the transition state changes from a nearly neutral C-Cl bond for HCl loss to a C-F bond that has much greater charge separation when HF is eliminated. © 1990 American Chemical Society.