Reinvestigation of the Unimolecular Reactions of CHF₂CHF₂: Identification of the 1,1-HF Elimination Component from Addition of :CFCHF₂ to trans-2-Butene

Abstract

The recombination of ·CHF radicals in a room-temperature bath gas was used to generate CHFCHF* (where * indicates vibrational excitation) molecules with 96 kcal mol of vibrational energy. The CHFCHF* molecules decompose by four-centered 1,2-HF elimination and by three-centered 1,1-HF elimination reactions to give HF and either CHF═CF or :CFCHF, respectively. The 1,1-HF component was identified by trapping the :CFCHF carbene with trans-2-butene that forms 1-fluoro-1-difluoromethyl-2,3-dimethylcyclopropane. The total rate constant for the decomposition of CHFCHF* was 6.0 × 10 s, and the rate constant for the 1,1-HF pathway forming the carbene, as measured by the 1-fluoro-1-difluoromethyl-2,3-dimethylcyclopropane yield, was 1.4 × 10 s. On the basis of matching the experimental rate constants to calculated statistical rate constants, the threshold energies for the four-centered and three-centered reactions are 78 and ≤85 kcal mol, respectively.

Analysis of the Five Unimolecular Reaction Pathways of CD2ClCHFCl with Emphasis on CD2Cl(F)C: and CD2Cl(Cl)C: Formed by 1,1-HCl and 1,1-HF Elimination

“…Several examples (C 2 D 5 CHF 2 , CD 3 CHFCl, CHF 2 CHF 2 ) demonstrate the relation between 2,1-DF and 1,1-HF elimination, and CD 2 ClCHFCl seems to fit the same pattern, even though HF and DF elimination reactions are only 10% of the total unimolecular reaction. The 1,1-HF/2,1-DF branching ratios are 0.3 to 0.4 for ⟨ E ⟩ = 90–96 kcal mol –1 .…”

“…Giving less weight to points below 1/ P = 0.2 gives a ratio of 1.2–1.1. Upon the basis of numerous − studies, product alkenes from four-centered elimination of HX from haloalkanes do not retain sufficient vibrational energy for cis/trans isomerization, and ratios in Figure a are expected to be pressure-independent. The cis-isomers of CDClCDF and CDClCDCl are lower in energy by about 0.8 kcal mol –1 with equilibrium constants (cis/trans) of about 1.6 at 600 K. Because the transition states for 2,1-DX elimination resemble those of the alkene products, a cis/trans ratio of 1.4–1.2 would be expected for 2,1-DCl and 2,1-DF elimination.…”

“…Our laboratory has investigated unimolecular reactions of halogenated alkanes using the chemical activation technique coupled with electronic structure calculations; 1,2-HX (X = F, Cl, Br) elimination reactions − and halogen interchange reactions − have been characterized and threshold energies assigned for several systems. Our recent efforts have been directed toward 1,1-HX elimination reactions − of 1,1-dihalomethanes, -ethanes, and -propanes. These reactions proceed with conservation of spin to give singlet halocarbenes and HX.…”

Experimental and Computational Studies of Unimolecular 1,1-HX (X = F, Cl) Elimination Reactions of C2D5CHFCl: Role of Carbene:HF and HCl Adducts in the Exit Channel of RCHFCl and RCHCl2 Reactions

“…The current study of C 2 D 5 CHFCl provides additional examples of carbene:HX adducts. Low vibrational frequencies of the transition state enable 1,1-HX elimination reactions to compete with traditional 2,1-HX elimination reactions at high temperatures or high vibrational energies. − For most chemical activation systems, the carbene product has sufficient energy to isomerize to an olefin. Several examples of 1,1-HF elimination reactions have been successfully characterized by experimental and computational studies. − According to computational results, , hydrogen-bonded carbene:HF adducts with dissociation energies between 6 and 10 kcal mol –1 exist in the exit channels.…”

“…Low vibrational frequencies of the transition state enable 1,1-HX elimination reactions to compete with traditional 2,1-HX elimination reactions at high temperatures or high vibrational energies. − For most chemical activation systems, the carbene product has sufficient energy to isomerize to an olefin. Several examples of 1,1-HF elimination reactions have been successfully characterized by experimental and computational studies. − According to computational results, , hydrogen-bonded carbene:HF adducts with dissociation energies between 6 and 10 kcal mol –1 exist in the exit channels. Calculated threshold energies for 1,1-HF elimination are a few kcal mol –1 higher than the enthalpy of reaction, and those calculated values are in accord with threshold energies derived from comparing experimental rate constants with statistical rate constants obtained from models of transition states.…”