Substituent Effects and Threshold Energies for the Unimolecular Elimination of HCl (DCl) and HF (DF) from Chemically Activated CFCl₂CH₃ and CFCl₂CD₃

Abstract

Combination of CFCl2 and methyl-d 0 and -d 3 radicals form CFCl2CH3-d 0 and -d 3 with 100 and 101 kcal/mol of internal energy, respectively. An upper limit for the rate constant ratio of disproportionation to combination, k d/k c, for Cl transfer is 0.07 ± 0.03 for collision of two CFCl2 radicals and 0.015 ± 0.005 for CH3 and CFCl2 radicals. The chemically activated CFCl2CH3 undergoes 1,2-dehydrochlorination and 1,2-dehydrofluorination with rate constants of 3.9 × 109 and 4.9 × 107 s-1, respectively. For CFCl2CD3 the rate constants are 8.7 × 108 s-1 for loss of DCl and 1.1 × 107 s-1 for DF. The kinetic isotope effect is 4.4 ± 0.9 for HCl/DCl and appears to be identical for HF/DF. Threshold energies are 54 kcal/mol for loss of HCl and 68 kcal/mol for HF; the E 0's for the deuterated channels are 1.4 kcal/mol higher. Comparison of these threshold energies with other haloethanes suggests that for HF and HCl elimination the transition states are developing charges of different signs on the carbon containing the departing halogen and that chlorine and fluorine substituents exert similar inductive effects.

Isomerisation of CF2ClCH2Cl and CFCl2CH2F by Interchange of Cl and F Atoms with Analysis of the Unimolecular Reactions of Both Molecules

“…The F atoms in CF 2 ClCH 3 and CFCl 2 CH 3 also hardly affect E 0 (HCl) relative to CH 2 ClCH 3 . These trends were noted with surprise by Holmes and coworkers [24,25] in 1990, but the summary in Figure 1 and studies of the CF 2 ClCH 2 F$CF 3 CH 2 Cl system [7] and CF 2 ClCHFCH 3 [9c] illustrate the same tendency. In addition to following the major experimental trends in E 0 (HF), the DFT calculated E 0 (HF) have similar absolute values as the experimental results.…”

“…These threshold energies are compared to the threshold energies of CF 2 ClCH 3 and CFCl 2 CH 3 . The latter were obtained from analysis of previously published chemical activation data, [24,25] as well as consideration of thermal activation studies. [26][27][28][29] In anticipation of the results, the substitution of fluorine or chlorine atoms atom for a hydrogen atom on the carbon atom, C H , from which the H atom departs, always seems to raise the threshold energy for HCl and HF 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.…”

“…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).…”

Pyrolysis of 1,1-dichloro-1-fluoroethane in the absence and presence of added propene or CCl4: A computer-aided kinetic study

“…The value of E is to be compared with the values of 56.0 and 58.5 kcal/mol obtained from chemical activation experiments [21] and ab initio quantum chemical calculations [22], respectively. Note that the molecular elimination of HF has a 14 kcal/mol higher E [21]. This means that at a mean temperature of 675 K the ratio of concentrations [CCl 2 " CH 2 ]/[CFCl " CH 2 ] should be on the order of 10 Ϫ4 .…”