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

Abstract

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 x107 and 0.47 x107 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 x 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. Based upon 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 CD2Cl-CHCl2 molecules with 86 kcal mol-1 of energy in order 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.

Experimental and Computational Studies of Unimolecular 1,1-HX (X = F, Cl) Elimination Reactions of C2D5CHFCl: Role of Carbene:HF and HCl Adducts in the Exit Channel of RCHFCl and RCHCl2 Reactions

“…Following irradiation, the contents of the vessels are transferred to a vacuum system and injected into a gas chromatograph with mass spectrometer as the detector (Shimadzu 2010 GC-MS mass spectrometer and 105m RTX-200 column). These experiments are similar to previous studies from this laboratory and additional details are given in those references. ,,, The desired measurements are ratios of the concentrations of the five products from reaction at various pressures. The experimental rate constants, k exp , are defined by the relation k exp / k M [M] = D i /S, where k M is the collision rate constant, [M] is the bath gas concentration, and D i /S is the ratio of a decomposition product to stabilized C 2 D 5 CHFCl.…”

“…The details needed to calculate k M are given in footnote b . The rate constants ,, for CD 3 CHFCl*, C 2 D 5 CHF 2 , and C 2 D 5 CHCl 2 * are included for comparison to C 2 D 5 CHFCl*. The uncertainty in the rate constants for C 2 D 5 CHFCl* was increased at least to 10% to account for the uncertainty in k M .…”

“…Calculated threshold energies for 1,1-HF elimination are a few kcal mol –1 higher than the enthalpy of reaction, and those calculated values are in accord with threshold energies derived from comparing experimental rate constants with statistical rate constants obtained from models of transition states. In contrast to 1,1-HF reactions, 1,1-HCl elimination is more difficult to understand, although dissociation energies of carbene:HCl adducts are smaller than those for carbene:HF. − Calculated threshold energies of 1,1-HCl elimination transition states are similar to energies of their corresponding adducts, as are the geometries at the maximum and minimum positions on the potential. Thus, calculated 1,1-HCl threshold energies are below the enthalpy of reaction.…”

“…Gas-phase, unimolecular, 1,1-HX (X = F, Cl) elimination reactions of 1,1-dihaloalkanes give singlet-state halocarbenes + HX as products. − The halogen atom stabilizes the singlet halocarbene, which is the ground state, and the enthalpy of these reactions is typically about 70 kcal mol –1 . In addition to interest in kinetics of these reactions, − adducts between carbenes and HX molecules provide examples of hydrogen bonding to a carbon atom, and such adducts are beginning to receive attention − in the hydrogen-bond literature. The current study of C 2 D 5 CHFCl provides additional examples of carbene:HX adducts.…”

“…Reactions 2a , 2c, , and tend to favor the Z­( cis )-isomers of 1-chloro- and 1-fluoropropene because the threshold energy is slightly lower than for the E­( trans )-isomers. The cis – trans isomerization of chloro- and fluoropropene-d 5 following reactions and was not observed, and the question of energy disposal from 1,1-HX reactions ,, is not of interest. Yields of the Z- and E-isomers were combined for analysis of the reactions of C 2 D 5 CHFCl*.…”