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

Abstract

Chemically activated CF(3)CFClCH(3), CF(3)CFClCD(3), CF(3)CFClCH(2)D, and CF(3)CFClCHD(2) molecules with 94 kcal mol(-1) of internal energy were formed by the combination of CF(3)CFCl radicals with CH(3), CD(3), CH(2)D, and CHD(2) radicals, which were generated from UV photolysis of CF(3)CFClI and CH(3)I, CD(3)I, CH(2)DI, or CHD(2)I. The total (HF + HCl) elimination rate constants for CF(3)CFClCH(3) and CF(3)CFClCD(3) were 5.3 x 10(6) and 1.7 x 10(6) s(-1) with product branching ratios of 8.7 +/- 0.6 in favor of HCl (or DCl). The intermolecular kinetic isotope effects were 3.22 and 3.18 for the HCl and HF channels, respectively. The product branching ratios were 10.3 +/- 1.9 and 11.8 +/- 1.8 (10.8 +/- 3.8 and 11.6 +/- 1.7) for HCl/HF and DCl/DF, respectively, from CF(3)CFClCH(2)D (CF(3)CFClCHD(2)). The intramolecular kinetic-isotope effects (without correction for reaction path degeneracy) for HCl/DCl and HF/DF elimination from CF(3)CFClCH(2)D (CF(3)CFClCHD(2)) were 2.78 +/- 0.16 and 2.98 +/- 0.12 (0.82 +/- 0.04 and 0.91 +/- 0.03), respectively. Density function theory at the B3PW91/6-311+G(2d,p) and B3PW91/6-31G(d',p') levels was investigated, and the latter was chosen to calculate frequencies and moments of inertia for the molecules and transition states. Rate constants, branching ratios and kinetic-isotope effects then were calculated using RRKM theory with torsional motions treated as hindered internal rotations. Threshold energies for HF and HCl elimination from CF(3)CFClCH(3) were assigned as 61.3 +/- 1.5 and 58.5 +/- 1.5 kcal mol(-1), respectively. The threshold energy for Cl-F interchange was estimated as 67 kcal mol(-1). The difference between the transition states for HCl and HF elimination is discussed.

Highly Selective Dehydrochlorination of 1,1,1,2‐Tetrafluoro‐2‐chloropropane to 2,3,3,3‐Tetrafluoropropene over Alkali Metal Fluoride Modified MgO Catalysts

“…Holmes et al. have calculated the threshold energies for HF and HCl elimination from HCFC‐244bb, which are 61.3 and 58.5 kcal mol −1 , respectively . This small energy difference suggests the selective control of above reaction is not easy.…”

Concerted and sequential pathways of proton-coupled electron transfer in hydrogen halide elimination

“…In this work, we describe an apparently simple and fundamental organic reaction, hydrogen halide elimination (or dehydrohalogenation), where both concerted and sequential mechanisms of proton coupled electron transfer are operative. Dehydrohalogenation reactions of haloalkanes have been extensively studied [16], [17], [18], [19], [20], [21], [22], and the role of charge transfer has previously been implicated [20]. For example, in the series of monohaloethanes C2H5X (X = F, Cl, Br, I), it was shown that the barrier to HX elimination correlates more strongly with the heterolytic (C − X → C + + X: − ) than homolytic (C − X → C + X ) bond dissociation energy [21].…”

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

“…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.…”