Unimolecular Reactions of 1,1,1-Trichloroethane, 1,1,1-Trichloropropane, and 3,3,3-Trifluoro-1,1,1-trichloropropane: Determination of Threshold Energies by Chemical Activation

Abstract

The recombination of CCl3 radicals with CH3, CH3CH2, and CF3CH2 radicals was used to generate CH3CCl3, CH3CH2CCl3, and CF3CH2CCl3 molecules with approximately 87 kcal mol(-1) of vibrational energy in a bath gas at room temperature. The competition between collisional deactivation and unimolecular reaction by HCl elimination was used to obtain the experimental rate constants for each molecule. These experimental rate constants were matched to calculated statistical unimolecular rate constants to assign threshold energies to the three HCl elimination reactions. The models needed for the calculations of the rate constants were obtained from molecular structure calculations using density functional theory (DFT) with the hybrid density-functional MO6-2X recommended by Truhlar for transition states. The assigned threshold energies are 52 ± 2, 50 ± 2, and 52 ± 2 kcal mol(-1) for CH3CCl3, CH3CH2CCl3, and CF3CH2CCl3, respectively, and the CH3 and CF3 groups have only a minor effect on the threshold energies for HCl elimination. The DFT calculated threshold energies are in agreement with the experimentally assigned values. The addition of Cl atoms to the same carbon atom lowers the threshold energy for HCl elimination in the CH3CH2Cl, CH3CHCl2, and CH3CCl3 series. This trend, which is the opposite of that for CH3CH2F, CH3CHF2, and CH3CF3, is discussed in terms of transition-state structure and correlated with the relative stabilities of CH3CH2(+), CH3CHCl(+), and CH3CCl2(+) ions; the relative stabilities are based on the hydride affinities obtained from calculations. Comparison of the reactions of CH3CCl3 and CH2ClCHCl2 shows that the threshold energy is much higher for the isomer with chlorine atoms on both carbon atoms.

Unimolecular Rate Constants for the HF and HCl Elimination Reactions from Chemically Activated CF2ClSH

“…Similar trends are observed for HCl elimination that accounts for the much higher E 0 (HCl) for CF 2 ClCH 3 . Finally, as we have previously noted for haloalkanes, the out‐of‐ring C–X bond length calculated for the carbon losing the halogen is somewhat shorter than in either the reactant or product and that effect is observed for the thiols and trifluoromethanol.…”

“…Typically , our experimental rate constants have error limits of ±20–30%, but these are obviously much greater because of uncertainty associated with (a) calibrating the MS detector, (b) calculating k M , (c) scatter of the experimental data, and (d) the effect of cascade deactivation. We believe the largest uncertainty arises from the scatter in the data.…”

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 Hg( 3 P 1 ) atoms abstract I atoms and Cl-atoms to generate the desired radicals. 23 Some free Cl-atoms also are produced and CF 3 CH=CH 2 serves to control the Cl-atom concentration. 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).…”

Carbocation stability as predictor for electrophilic addition of HCl to chlorinated ethenes and propenes in the gas phase

“…However, neither of these compounds contains a chlorine substituent and to provide a more demanding system for evaluating our computational approach, we computed the activation energies for the elimination of HCl from 1,1,1‐trichloroethane and 1,1,1‐trichloropropane, ie, the reverse of HCl addition to 1,1‐dichloroethene and ‐propene. The resulting values are 236 and 231 kJ/mol, to be compared with the experimental results of 218 and 209 kJ/mol . Thus, our computed reaction barriers are still too high and now by about 20 kJ/mol.…”