Threshold Energies and Unimolecular Rate Constants for Elimination of HF from Chemically Activated CF₃CH₂CH₃ and CF₃CH₂CF₃:  Effect of CH₃ and CF₃ Substituents at the β-Carbon and Implications about the Transition State

Abstract

Chemically activated CF 3 CH 2 CF 3 was prepared with 104 kcal/mol of internal energy by the combination of CF 3 CH 2 and CF 3 radicals, and chemically activated CF 3 CH 2 CH 3 was prepared with 101 and 95 kcal/mol by combination of CF 3 and CH 2 CH 3 radicals and by combination of CF 3 CH 2 and CH 3 radicals, respectively. The experimental rate constants for unimolecular 1,2-dehydrofluorination were 1.2 × 10 5 s -1 for CF 3 CH 2 CF 3 and 3.2 × 10 6 s -1 for CF 3 CH 2 CH 3 with 95 kcal/mol and 2.0 × 10 7 s -1 with 101 kcal/mol of energy. Fitting the calculated rate constants for HF elimination from RRKM theory to the experimental values provided threshold energies, E 0 , of 73 kcal/mol for CF 3 CH 2 CF 3 and 62 kcal/mol for CF 3 CH 2 CH 3 . Comparing these threshold energies to those for CF 3 CH 3 and CF 3 CH 2 Cl illustrates that replacing the hydrogen of CF 3 CH 3 with CH 3 lowers the E 0 by 6 kcal/mol and replacing with CF 3 or Cl raises the E 0 by 5 and 8 kcal/mol, respectively. The CF 3 substituent, an electron acceptor, increases the E 0 an amount similar to Cl, suggesting that chlorine substituents also prefer to withdraw electron density from the β-carbon. As the HF transition state forms, it appears that electron density flows from the departing hydrogen to the β-carbon and from the β to the R-carbon, to the R-carbon from its substituents, but the R-carbon releases most of the incoming electron density to the departing fluorine. The present work supports this scenario because electron-donating substituents, such as CH 3 , on either carbon would reduce the E 0 as they aid the flow of negative charge, while electron-withdrawing substituents such as Cl, F, and CF 3 would raise the E 0 for HF elimination because they hinder the flow of electron density.

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.…”

“…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). The data show an increase in the threshold energy for loss of HF, E o (HF), of 13-30 kJ mol À1 for replacement of each H on the a-carbon by a F: CH 2 F-CH 3 , 242.7 kJ mol À1 [41]; CHF 2 -CH 3 , 255.2 kJ mol À1 [47]; CF 3 -CH 3 , 284.5 kJ mol À1 [42]. The a-carbon contains the halogen and the b-carbon the hydrogen that are eliminated.…”

“…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).…”

“…The situation is less clear for PVDF. The nature of the substituents at the a-and b-carbon exerts a strong effect on E o (HF) [42]. However, since PVDF presents an alternate succession of CH 2 and CF 2 groups, it seems reasonable to assume that the average of the threshold energies of CF 3 -CH 2 -CF 3 and CH 3 -CF 2 -CH 3 (E o (HF) = 265.7 kJ mol À1 ) might safely represent the minimum energy required for HF release from the polymer.…”

Chemistry with ADF

“…The "activation strain" may be conceived as a good estimate of the barrier for the corresponding thermal elimination (i.e., without the assistance of a base) of HF from fluoroethane which occurs indeed in a syn-fashion. 213,214 So it is the TS interaction E # int with the base that leads to the preference for anti-elimination. To understand this, we take a closer look at the substrate's LUMO, the acceptor orbital in the interaction with the HOMO of the base.…”