Chemical Activation Study of the Unimolecular Reactions of CD₃CD₂CHCl₂ and CHCl₂CHCl₂ with Analysis of the 1,1-HCl Elimination Pathway

Chemically activated C₂D₅CHCl₂ molecules were generated with 88 kcal mol^-1 of vibrational energy by the recombination of C₂D₅ and CHCl₂ 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 CD₃CD═CHCl and CD₃CD═CDCl products. The initial CD₃CD₂C-Cl carbene product from 1,1-HCl elimination rearranges to CD₃CD═CDCl under the conditions of the experiments. The experimental rate constants were 2.7 × 10⁷ and 0.47 × 10⁷ s^-1 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^-1 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 × 10⁷ s^-1) for the unimolecular decomposition of CHCl₂CHCl₂ molecules formed with 82 kcal mol^-1 of vibrational energy by the recombination of CHCl₂ 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^-1. Calculations also were done to analyze previously published rate constants for chemically activated CD₂ClCHCl₂ molecules with 86 kcal mol^-1 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.