Effects of CF₃ and CH₃ Groups on the Threshold Energy for the Unimolecular Interchange Reaction of Cl- and F-Atoms in CF₃CHFCH₂Cl and CH₃CHFCH₂Cl

Abstract

The recombination reactions of CH2Cl radicals with CF3CHF and with CH3CHF radicals were used to generate CF3CHFCH2Cl and CH3CHFCH2Cl molecules with 90-92 kcal mol(-1) of vibrational energy. The experimental rate constants for elimination of HCl and HF and the interchange of Cl and F atoms were measured and compared to RRKM calculated rate constants to assign the threshold energy for each unimolecular reaction channel. The Cl/F interchange reaction is approximately 18% of the total unimolecular reaction for both molecules. The product branching ratios and some rate constants also could be measured for the unimolecular reactions of the rearranged molecules, CF3CHClCH2F and CH3CHClCH2F. The most important result is that the CH3 group lowers the threshold for Cl/F interchange relative to CH2FCD2Cl, as expected for an electron-density donating group, and the CF3 group, an electron-density withdrawing group, increases the threshold energy relative to CH2FCD2Cl. The CH3 and CF3 groups alter the threshold energies of the HCl and HF elimination reactions in such a way so as to maintain the same branching fraction for the interchange reaction. The results from density functional theory using the B3PW91 method with the 6311+G(2d,p) and G-31G(d',p') basis sets are used to discuss the trends in threshold energies for the Cl/F interchange and the HF and HCl elimination reactions.

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

“…The experimentally assigned E 0 values for HF elimination from fluoroalkanes were usually in close agreement with the DFT calculated values; however, the calculated E 0 's for HCl elimination from chloroalkanes were often 2−5 kcal mol −1 lower than the experimentally favored values. 3,6,16,18,19,41 For that reason, we decided to use the MO6-2X method in the present work and, indeed, the calculated E 0 are higher than from the B3PW91 method as shown in Table 2. Calculations also were done with the cc-pVDZ basis set.…”

“…The enthalpy of reaction associated with the formation of the 1,1,1-trichloride molecules is needed to assign the energy of each molecule. Our standard procedure ,,, to obtain the average vibrational energy for molecules formed by radical recombination at 298 K has been to add the thermal energy contributed by the two radicals (3 RT + the vibrational energy of each radical) to the enthalpy change for the recombination reaction at 0 K. Because the low vibrational frequencies associated with the CCl 3 group broaden the thermal energy distribution, we decided to average the RRKM rate constant over the thermal distribution function, f ( E ), to obtain ⟨ k E ⟩, as well as use the more convenient ⟨ E ⟩ defined above to find k ⟨ E ⟩ for comparison with the experimental rate constant. The energy distribution functions were obtained in the usual way using the properties of the transition states for dissociation to the radicals .…”

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

“…Certain substituents can raise the threshold energy for 2,1-HX elimination, which has the effect of enhancing the 1,1-HX elimination component. However, the possibility of competing halogen-atom interchange reactions, if different halogen atoms are located on adjacent carbon atoms, should be remembered when considering the unimolecular mechanisms for more complex haloalkanes. − We have recently characterized 1,1-HF elimination from 1,1-difluoroethane, 1,1-difluoropropane, and 1,1-chlorofluoroethane using the chemical activation experimental technique with deuterium labeling to identify the pathways. The experimental results were coupled with electronic-structure calculations to characterize models for the transition states and assigned the threshold energies, E 0 .…”

Characterization of the 1,1-HF Elimination Reaction from the Competition between the 1,1-HF and 1,2-DF Unimolecular Elimination Reactions of CD3CD2CHF2

“…In recent years we have studied the unimolecular 1,2-HX (X = F, Cl, Br) elimination reactions in conjunction with measuring the rates of the halogen interchange reactions of 1,2-dihaloalkanes using chemical-activation techniques. − Transition-state models, threshold energies, and substituent effects were characterized. The 1,1-dihalide class of haloalkanes were not studied because the competition with 1,1-HX elimination reactions made mechanistic interpretations more difficult.…”