Isotope Effects and Final Products in Pyrolysis of Halomethanes and Perfluoroalkyl Iodides

Abstract

Final products of isothermal pyrolysis of CF 2 HBr, CF 2 ClBr, CH 3 I, CH 2 I 2 , CHI 3 , i-C 3 F 7 I, and C 4 F 9 I were determined and mechanisms of their formation were proposed. The enthalpy of formation of the free biradical CFBr .. (D f H 0 0 = 80 + 20, D f H 0 298 = 60 + 20 kJ mol 31 ) was estimated. The dissociation energies E D,0 (ID 2 C3D), E D,298 (ID 2 C3D), and E D,0 (IH 2 C3D) equal to 437 + 6, 444 + 6, and 435 + 4 kJ mol 31 , respectively, were determined.Over almost half a century the standard enthalpies of formation of methyl and ethyl radicals have been accepted [133] to equal values (1) and (2).These values as well as some others have been used as the basis of the interpolation network [4,5] of thermochemical data, including many dozens of molecules and free radicals. However, recently new experimental methods [6,7] have led to much higher values (3) and (4). D f H 0 298 (CH 3 . ) = 146.7 + 0.4 kJ mol 31 , (3) D f H 0 298 (C 2 H 5 . ) = 121.0 + 1.7 kJ mol 31 .The sharp difference between the new (4) and old (2) enthalpies of formation of the free ethyl radical frustrates the entire system [4, 5] of thermochemical data. Some authors [8310] still try to use the old values (1) and (2), accepting a compromise value (5), but the rigorous quantummechanical calculation [11] provides an even higher value (6), which makes hopeless not only preservation of the obsolete value (2), but also of the compromise value (5). D f H 0 298 (C 2 H 5 . ) = 112 + 4 kJ mol 31 , (5) D f H 0 298 (C 2 H 5 . ) = 125 kJ mol 31 .Later we found out the reason for the fact that our experiments [10] on iodoethane pyrolysis and also the earlier work [12] on iodoethane pyrolysis in the presence of iodine hydride invariably resulted in the underestimated data (2) or (5). It appeared [13] that iodoethane pyrolysis does not yield equal amounts of ethane and ethylene by reaction (7) [12,14] or butane by reaction (8) [10], but proceeds by a more complicated mechanism [13] to give unequal amounts of ethylene and ethane (1 : 2) and other products.

Kinetics of Isothermal Pyrolysis of Iodomethanes in the Gas Phase

“…4). Table 4 shows that scheme (E) predicts that the yields of methane, dicarbon, and molecular iodine should not exceed 1%, which is not the case [12]. On the contrary, scheme (F) predicts more than 95% yield of methane and C 5 , in agreement with the experimental data.…”

“…However, recently, in direct measurements [12] of the composition of products of iodomethane and diiodomethane pyrolysis, we detected no recombination products R 2 even under the mildest pyrolysis conditions. Instead we detected methane in the volume of the vessel and carbyne on its walls.…”

“…At the same time, iodomethane is consumed to 80 390% under all the investigated pyrolysis conditions [12], and pyrolysis products mainly consist of methane, elemental iodine, and elemental carbon (carbyne). Furthermore, scheme (C) predicts a high yield of diiodomethane comparable with that of iodomethane, which is not the case either [12]. On the contrary, in full agreement with the ex- Thus, scheme (D) seems to be the only adequate of the four examined schemes of iodomethane pyrolysis.…”

“…It is logical to assume that only three reactions (4), (12), and (33) take place in the initial relaxation phase of the iodomethane pyrolysis. In the isothermal kinetic experiment (Fig.…”

“…Only methane, carbyne, and molecular iodine were detected [12] among final products of diiodomethane pyrolysis as is the case with iodomethane [scheme (61)]:…”