Chemical Activation Study of the Unimolecular Reactions of CDCDCHCl and CHClCHCl with Analysis of the 1,1-HCl Elimination Pathway.

Chemically activated CDCHCl molecules were generated with 88 kcal mol of vibrational energy by the recombination of CD and CHCl radicals in a room temperature bath gas. The competing 2,1-DCl and 1,1-HCl unimolecular reactions were identified by the observation of the CDCD═CHCl and CDCD═CDCl products. The initial CDCDC-Cl carbene product from 1,1-HCl elimination rearranges to CDCD═CDCl under the conditions of the experiments. The experimental rate constants were 2.7 × 10 and 0.47 × 10 s for 2,1-DCl and 1,1-HCl elimination reactions, respectively, which corresponds to branching fractions of 0.84 and 0.16. The experimental rate constants were compared to calculated statistical rate constants to assign threshold energies of 54 and ≈66 kcal mol for the 1,2-DCl and 1,1-HCl reactions, respectively. The statistical rate constants were obtained from models developed from electronic-structure calculations for the molecule and its transition states. The rate constant (5.3 × 10 s) for the unimolecular decomposition of CHClCHCl molecules formed with 82 kcal mol of vibrational energy by the recombination of CHCl radicals also is reported. On the basis of the magnitude of the calculated rate constant, 1,1-HCl elimination must contribute less than 15% to the reaction; 1,2-HCl elimination is the major reaction and the threshold energy is 59 kcal mol. Calculations also were done to analyze previously published rate constants for chemically activated CDClCHCl molecules with 86 kcal mol of energy to obtain a better overall description of the nature of the 1,1-HCl pathway for 1,1-dichloroalkanes. The interplay of the threshold energies for the 2,1-HCl and 1,1-HCl reactions and the available energy determines the product branching fractions for individual molecules. The unusual nature of the transition state for 1,1-HCl elimination is discussed.