Hydrofluorochlorocarbons (HCFCs) replaced chlorofluorocarbons (CFCs) as refrigerants and are released into the atmosphere via industrial processes. Our research group has been investigating reaction mechanisms, rate data, and threshold energies for many HCFCs. Experimental rate constants will aid in the understanding of the effect of halogen substitution on threshold energies for unimolecular HCl eliminations. Threshold energies, the amount of energy required for a reaction to proceed, and rate constants for CH3CCl3, CH3CH2CCl3 and CF3CH2CCl3 are of interest because they will tell researchers how long these molecules persist in the atmosphere, which reaction will occur first, and the mechanism determines whether toxic compounds will be formed as products. Changing the number of carbons and halogen substituents systematically modifies the molecules being studied. Observing numerous molecules allows identification of trends in the elimination reactions. The recombination of CCl3 radicals with CH2X (X = H, CH3 and CF3) radicals was used to generate CH3CCl3, CH3CH2CCl3 and CF3CH2CCl3 molecules with approximately 90 kcal mol-1 of vibrational energy in a room temperature bath gas. The competition between collisional deactivation and unimolecular reaction was used to obtain the experimental rate constants for 1,2-HCl elimination of 8 x 109 s-1, 1.5 x 108 s-1 and 7 x 106 s-1 for X = H, CH3 and CF3, respectively. These experimental rate constants were matched to calculated statistical unimolecular rate constants to assign threshold energies to the three HCl elimination reactions to determine the effect of electron-donating and electron-withdrawing substituents. Substituent effects to be discussed include the impact of the addition of Cl-atoms to the same carbon and the effect of electron-donating and withdrawing substituents on the carbon losing the hydrogen.
