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.
