Threshold Energies and Substituent Effects for Unimolecular Elimination of HCl (DCl) and HF (DF) from Chemically Activated CF2ClCH3 and CF2ClCD3

Article abstract of DOI:10.1021/j100375a037

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) x 10⁹ and (0.10 +/- 0.02) x 10⁹ s^-1, respectively, a branching ratio of 25:1 in favor of HCl.The CF2ClCD3 has rate constants of (0.78 +/- 0.12) x 10⁹ for loss of DCl and 0.033 +/- 0.005 x 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.