ChemInform Abstract: Trifluoromethylfluorocarbene Formation and Reactions under C2F5SiF3 Pulsed Adiabatic Compression Pyrolysis.

Abstract

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Chain elongation during thermolysis of tetrafluoroethylene and hexafluoropropylene: Modeling of mechanistic hypotheses and elucidation of data needs

“…However, others [28] have suggested that the product so labeled was really 2-C 4 F 8 ; if so, the C 4 F 8 isomer ratio was 2-> i-. Buravtsev and coworkers [5,28,30] used adiabatic compression to achieve high-temperature pulses and also observed the successive formation of c-C 4 F 8 , C 3 F 6 , and C 4 olefins over the range 700-1100 • C by GC analysis; the C 4 F 8 composition was 2-:i-:1-≈ 66:33:1. Hence we might tentatively conclude that the 2-:i-ratio is >1 in the "high-temperature regime" compared with 1 in the mediumtemperature regime.…”

“…These differences between perfluorocarbons and hydrocarbons can be traced largely to differences in r H, which reflect the "instability" of C 2 F 4 and the enthalpic penalty of having fluorine substituents on sp 2 -compared with sp 3 -hydridized carbons [30]. While C 2 F 4 is "unstable," the ground-state singlet carbene, 1 CF 2 :, possesses special stability [13,52,53] compared with the ground-state triplet carbene, 3 CH 2 ; the singlet-triplet splitting in CF 2 : is very large (54 kcal/mol) [75].…”

“…Similarly under adiabatic compression conditions at ≈1100-1500 • C and low conversion, C 2 F 4 was the only initial product, with smaller amounts of C 4 olefins appearing only at higher temperature. The latter fraction was in this case dominated by the 2-isomer, much more so than that from C 2 F 4 at somewhat lower temperature [28,30]. The Table 1 Experimental, estimated, and computed gas-phase standard-state thermochemical parameters; values are in kcal/mol or cal/mol K as appropriate; see Table S-1 contrast of no C 2 F 4 being detected from C 3 F 6 in the mediumtemperature regime with its becoming the major product in the high-temperature regime (cf.…”

“…Data for S 298 are more uniform but there is again some spread for the larger species. "Experimental" thermochemical parameters from calorimetric or equilibrium data relevant to this study are compiled in Table S-1 [13,20,30,; also listed are "estimated" values, often less reliable because they have been obtained by fitting a kinetic model for which the mechanism is assumed. Selected "average" values used herein are given in Table 1.…”

“…Option C begins with a 1,2-F shift across the double bond in C 2 F 4 in step (13) to give CF 3 CF: which then rapidly combines in step (-14) with CF 2 : produced in step (1). For the reverse decomposition of C 3 F 6 , Buravtsev and coworkers [28,30] and Hynes and coworkers [63,100] appear to have associated their values of k 10 ( Fig. 12) with rate-limiting dissociation step (14).…”