Unimolecular Reaction Kinetics of CF₂ClCF₂CH₃ and CF₂ClCF₂CD₃:  Experimental Evidence for a Novel 1,2-FCl Rearrangement Pathway

Abstract

Chemically activated CF 2 ClCF 2 CH 3 and CF 2 ClCF 2 CD 3 , containing 98.5 and 100 kcal/mol of internal energy, respectively, were formed in the gas phase from the combination of CF 2 ClCF 2 and CH 3 or CD 3 radicals, respectively. These radicals were generated from the UV photolysis of CF 2 ClCF 2 I and CH 3 I or CD 3 I. The decomposition products were CF 2 ClCFdCH 2 (CF 2 ClCFdCD 2 ) from a 2,3-HF (DF) elimination and CF 3 -CFdCH 2 (CF 3 CFdCD 2 ) suggesting a 1,3-HCl (DCl) elimination reaction. The 1,3-HCl elimination mechanism appears to be a two-step process; a 1,2-FCl rearrangement, producing CF 3 ClCFClCH 3 (CF 3 CFClCD 3 ), followed by a 2,3-HCl (DCl) elimination. Unimolecular rate constants for CF 2 ClCF 2 CH 3 (CF 2 ClCF 2 CD 3 ) were 5.3 ( 2.1 × 10 5 s -1 (1.8 ( 0.7 × 10 5 s -1 ) for 2,3-HF (DF) loss and 3.6 ( 1.4 × 10 4 s -1 (2.3 ( 0.9 × 10 4 s -1 ) for the 1,2-FCl rearrangement. The branching ratio was 13.5 ( 3 (7.8 ( 1.6) favoring the HF (DF) process. The isotope effect for 2,3-HF/DF was 2.9 ( 0.6, while for the FCl rearrangement, it was considerably smaller at 1.5 ( 0.3. The CF 3 CFClCH 3 and the CF 3 CFClCD 3 , formed by the 1,2-FCl migration, react by loss of HCl (DCl) with rate constants of 2.1 ( 1.3 × 10 7 s -1 (7.9 ( 4.8 × 10 6 s -1 ) and an isotope effect of 2.7 ( 0.8. Theoretical rate constants, branching ratio and isotope effects were calculated using RRKM theory and density functional theory to compute all of the data necessary for the RRKM calculations. The agreement between the experimental and computed kinetic data suggests that the 1,3-HCl elimination is a two-step mechanism consisting of a 1,2-FCl rearrangement followed by a 2,3-HCl elimination.

Thermal stability and bond dissociation energy of fluorinated polymers: A critical evaluation

“…It is interesting to compare our data with published pyrolysis studies of unimolecular elimination experiments on chemically activated mixed halogenated species [37][38][39][40][41][42][43][44][45][46][47][48]. In all these experiments HCl loss was dominant.…”

“…In all these experiments HCl loss was dominant. For CClF 2 -CH 3 Jones et al [39] reported a branching ratio of 25:1 whereas, in the low-pressure limit, for CF 3 -CFCl-CH 3 the reported value was 13.5:1 in favour of HCl [43]. Pyrolysis studies reported branching ratios near 10:1 for CClF 2 -CH 3 .…”

“…These include thermal heating, photolytic excitation, and chemical activation. In the last 20 years, this last technique has been applied intensively by Holmes and co-workers [37][38][39][40][41][42][43][44] for determining the rate constants for elimination of HCl and HF from hydrofluorocarbons and hydrochlorofluorocarbons. The resulting experimental data have been matched to rates calculated using the Rice-Rampsberger-Kassel (RRK) unimolecular theory [45] and the modified RRK version (RRKM) [36] to determine E o (HX), the threshold energy for HF or HCl removal (see Table 5).…”

Rate Constants and Kinetic Isotope Effects for Unimolecular 1,2-HX or DX (X = F or Cl) Elimination from Chemically Activated CF3CFClCH3-d0, -d1, -d2, and -d3

“…3,6 In previous work, the observation of CF 3 CFdCH 2 following excitation of CF 2 ClCF 2 CH 3 6 was claimed as identification of formation of CF 3 CFClCH 3 by the Cl-F interchange reaction. In the current work, the branching between the HF and HCl elimination channels of CF 3 CFClCH 3 was measured, and the preference of HCl elimination was verified.…”

“…8 We now wish to report a comprehensive study of the HCl and HF elimination reactions from the CF 3 CFClCH 3 molecule. This work is part of a systematic study of various fluorochloropropanes, [6][7][8] and the assigned threshold energies for CF 3 CFClCH 3 will be compared to that for HF elimination from CF 3 CF 2 CH 3 .…”