in the dark for 4 days. During this period the evolution of gases ceased and a clear solution was obtained. The reaction mixture was diluted with water (50 ml) and neutralized with solid sodium bicarbonate. In addition, 30 ml of water was added and the whole extracted with six portions of 50 ml of chloroform. The combined extracts were dried over anhydrous sodium sulfate and then evaporated to dryness to obtain 0.83 g of a residue.By thin layer chromatography on silica gel, using commercially available plates (DC-Fertigplatten Kieselgel F 254, Merck) and developing them with ethyl acetate, it could be shown that the obtained product was a mixture of the starting compound (Rt 0.59) 'and 3-azidopyridazine 1-oxide (Rt 0.45) (detection in uv light).A solution of 300 mg of the crude product in 3 ml of chloroform was separated on a chromatoplate with the above-mentioned absorbent, and the part containing the azide was separated and the compound eluted with chloroform. Upon evaporation of the solvent 12 mg of the residue which consisted of practically pure 3-azidopyridazine 1-oxide10 was obtaind (yield 2.4%); mp 151-152°and mixture melting point with an authentic specimen obtained from nitrosation of 3-hydrazinopyridazine 1-Kinetics of Deuteration of Imidazole 1141 oxide12 was undepressed. Moreover, ir spectra of both products were identical [2179 and 2146 cm-1 (Na) and 1263 cm-1 (N-O)].

The kinetics of the gas phase reaction of pentafluoroethyl iodide with hydrogen iodide have been studied spectrophotometrically from 478 to 560 K. The rate-determining step was found to be C2F5I + I -C2F5 + 12 for which log ( k / L mol-' s-') = (1 1.2 f 0.3) -(16.8 f 0.8)/0 (0 = 2.3RT in kcal/mol) was determined. This result was combined with established thermochemical data to yield the C-I bond dissociation energy, DHO(CF3CF2-I) = 52.5 * 1 kcal/mol, and the enthalpy of formation of pentafluoroethyl, AHfo(CF$F2,g,298) = -213 f 1.3 kcal/mol. This also led to an evaluation of the T bond dissociation energy in tetrafluoroethylene, DTO(CF~=CF~) = 52.3 f 2 kcal/mol. This value is shown to be in good agreement with the thermochemical data on tetrafluoroethylene.In 1949, Lacher et a1.I showed that the heat of addition of chlorine to tetrafluoroethylene was nearly 14 kcal/mol more exothermic than that to ethylene, and it has since been generally found that addition reactions to tetrafluoroethylene are more exothermic than those to e t h~l e n e .~.~ This has been almost exclusively attributed to the destabilization of the C-C T bond by fluorine s u b~t i t u t i o n ,~-~ though recently some evidence has been presented indicating that u bond stabilization may be important too.'In this paper, we wish to report on a kinetic study of the gas phase reaction of pentafluoroethyl iodide with hydrogen iodide, from which we derive the heat of formation of the pentafluoroethyl radical and, thereby, the ?r bond dissociation energy in tetrafluoroethylene.8 W e shall further show that this value of the T bond energy is in quantitative agreement with the thermochemical data for tetrafluoroethylene. Experimental SectionPentafluoroethyl iodide was obtained from the Pierce Chemical Co. and was vacuum distilled at dry ice temperatures before use. Gasliquid chromatographic analysis on a 0.6 X 305 cm column packed with 30% dimethylsulfolane on Chromosorb W indicated that the sample, after distillatio., was better than 99% pentafluoroethyl iodide. Hydrogen iodide, obtained from the Matheson Gas Co., was similarly distilled from a dry ice bath to a liquid N2 trap and degassed at liquid N2 temperatures before use. Iodine, obtained from the J. T. Baker Chemical Co., was sublimed in vacuo before use.The kinetics of the reaction of C2FsI with HI were followed spectrophotometrically by observing the appearance of 12 at 500 nm. The stoichiometry of the reaction was established from measurements at 225,270, and 500 nm. Consequently, the absorption constants, a = cjRT in OD Torr-', were determined for C~F S I , HI, and 12 at these wavelengths and at each reaction temperature.The experimental apparatus and procedure has been described in detail previously and will not be repeated here.9 Resultswell-established mechanism given by reactions 1 to 4.1°T h e reaction of CF3CF2I with H I is expected to follow the 12 + M e 21 + M KI,for which the overall reaction is:The stoichiometry of this reaction was established a t temperatures from 500 to 560 K by meas...

35)The experimental results with w h i i we are primarily interested are the values of D/S for c-C&T and c-C&T measured for samples simultaneously containing c-C4H8 and c-CP8.4 In these mixtwes both mdecules are e x w to the same tritlum atom flux and the DIS values have been measured for equivalent experimental conditions. However, the D/S values for C-CI H~T in pwe c-C4Hs3 and in C-CIH~-C-C& mixtures" are the same, M i t i n g the T atom flux and the deactivation processes of c-C"!+T do not differ appreciably in the two systems. (36) The distance of the H or D atom from the cyclobutane center of mass is (37) T. Valencich and D. L. Bunker, ref 29, find at a T-atom energy of 200 kcallmol Walden inversion substlutions resuits in the ejected atoms canyirig an average fractiin of 0.20 of the incident energy with a largest fraction of 0.48. For substiution with retention the fractions are 0.25 and 0.62, respectively. These fractions change very little over a 500 kcal/mol range of T-atom energies: e.& for retained substitutions the smallest average fraction of energy carried by the ejected atom is at 130 kcallmol and equals 0.22, with a largest average fraction at 400 kcal/md equal to 0.36. (38) P. J. Kunb. E. M. Nemeth. J. C. Polanyi, and W. H. Wong, J. Chem. phys., 52, 4654 (1970); f que+ 0.57, 0.26, 0.42, and 0.38 at 46, 92, 138, and 277 kcal/mol, respectively. The equilibrium constants for the gas-phase reaction CF3CH3 + I2 = CFsCH2I+ HI have been determined spectroscopically over the temperature range 730-775 K. The entropy of CF3CHzI was estimated and combined with known entropies for the other reagents and the experimental equilibrium constants to yield Mr0(750) = 16.0 f 0.5 and f l r 0 ( 2 9 8 ) = 15.3 f 0.5 kcal mol-'. This result yields DH02g8(CF3CHpH) = 106.7 f 1.1 kcal mol-', indicating a marked strengthening of the C(sp3)-H bond dissociation energy with 6fluorine substitution. This value for the C-H bond dissociation energy was combined with known thermochemistry to yield the heat of formation of the 2,2,2-trifluoroethyl radical, .Wfo(CF&H2, g, 298) = -123.6 f 1.2 kcal mol-'.

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