1,2-FCl Rearrangement as an Intermediate Step in the Unimolecular 1,3-HCl Elimination from Chlorofluoropropanes

Abstract

Calculations at the B3PW91/6-311+G(2d,p) level of theory have been performed on a series of chlorofluoropropanes in order to account for the chemistry of the molecules CF 2 ClCF 2 CH 3 and CF 2 ClCF 2 CD 3 , chemically activated in the gas phase, which form novel elimination products, CF 3 CFdCH 2 or CF 3 CFdCD 2 , formally a 1,3-HCl or DCl elimination together with a 1,2-fluorine migration. The proposed mechanism involves an initial 1,2-FCl rearrangement, with an activation energy of 62.5 kcal/mol, giving CF 3 CFClCH 3 , which is 3.3 kcal/mol lower in energy than CF 2 ClCF 2 CH 3 . Subsequently CF 3 CFClCH 3 eliminates HCl with a barrier height of 55.4 kcal/mol. This mechanism accounts for both the unimolecular kinetics and the small kinetic isotope effect. A concerted transition geometry has been characterized for the 1,2-FCl rearrangement of each molecule of the type CF 2 ClCXFCY 3 , where X and Y are H, D, or F; in each case the rearrangement leads to a more thermodynamically stable rearrangement product CF 3 CXClCY 3 .

Computational study of the threshold energy for the 1,2-interchange of X and R (X, R = halogens, pseudohalogens, and monovalent hydrocarbon groups) on CH2XCH2R

“…These halogen-interchange processes are also known as type-1 dyotropic [14][15][16][17][18][19] rearrangement reactions and a number of computational studies have appeared for interchange of halogens. 2,12,[14][15][16][17] In conjunction with our experimental work for CF 2 ClCF 2 CH 3 and CF 2 ClCF 2 CD 3 1 we showed 2 that replacing the four hydrogen atoms of CF 2 ClCHFCH 3 with fluorine caused the threshold energy computed using B3PW91/6-311+G(2d,p) to increase from 230 to 290 kJ/mol for the 1,2-FCl interchange reaction. It was also noted that the C-C moiety of the transition state resembled an alkene.…”

“…11 Our experience is that when the E 0 (interchange) ≤ 290 kJ/mol the interchange reaction will be competitive with other decomposition processes for our chemical activation method that forms reactants with 350-420 kJ/mol of internal energy. [1][2][3][4][5][7][8][9][10][11] Scanning Tables 1 and 2 suggests that the interchange of Cl or Br might be observed with all of the groups in Table 1 results confirm E 0 (Cl-CH 3 ) = 259 kJ/mol for the CH 3 -Cl interchange reaction for 2,2-dimethyl-1chloropropane; i.e., two methyl groups replace two hydrogen atoms on C-2 of 1-chloropropane. 20 In addition, reactions with E 0 (interchange) > 290 kJ/mol might be observed for photochemical and thermal activation methods that prepare reactants with energy greater than 400 kJ/mol.…”

“…In previous studies we have demonstrated the existence of a series of unimolecular, gas-phase reactions involving the interchange of two halogen (F, Cl and Br) atoms on adjacent carbon atoms. [1][2][3][4][5] This interchange mechanism has been supported by experimental and computational evidence for CF 2 ClCF 2 CH 3 , 1, 2 CF 2 ClCF 2 CD 3 , 1, 2 CHF 2 CH 2 Cl, 6,7 13 and CF 2 ClCH 2 F 13 and can be competitive with unimolecular 1,2-HX (X = F, Cl, or Br) eliminations. These halogen-interchange processes are also known as type-1 dyotropic [14][15][16][17][18][19] rearrangement reactions and a number of computational studies have appeared for interchange of halogens.…”

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).…”