1812
NOTES From the exchange between I? and
and abstraction.' CF3I'
The Rate of Reaction of Active Nitrogen with Perfluorobutene-2
exp [- 15,800
Ic(exchange) =
=k
700/RT] 1. mol-' sec-l while other kinetic studies4 lead to
by N. nladhavan and W. E. Jones Department of Chemistry, DaEhousie University, Halifax, Nova Scotia, Canada (Receiced J u l y 1.4, 1967)
k(abstraction) = 1010.6exp [ - 17,60O/RT] 1. mol-' sec-l The activation energy for abstraction, 17.6 kcal/mol, is consistent with the recent determination of the heat of formation of CF31.5 The smaller values for the Arrhenius parameters for the exchange could imply that a t least part of the exchange proceeds by direct substitution with a lower energy of activation (in the exchange of iodine with alkyl iodides in the liquid phase, the rates of abstraction and substitution are similar6%'). However, the results were not sufficiently precise to establish this with any certainty, and the present results do not suggest that direct substitution is important in these systems. If the exchange of 1, with C2FJ takes place exclusively by abstraction, kl = 10'0.0 exp [- 13,500 800/ R T ] 1. mol-' sec-' and the bond dissociation energy D(C2Fs-I) = El - E-, D(1-I). For the reaction of CF3radicals and Iz,E-' is zero8 and is probably zero for CzFS radicals also. We calculate that D(CZF5-I) is 49.6 0.8 kcal/mol. This is some 4 kcal/mol less than D(CF3-I) and this difference is about equal to that between D (CH3-I) and D(C2H5-I).g*'o The frequency factor of kl, 1 X 1010 1. mol-' sec-1 is of the same order as that for the 12-CF31 system and is normal for halogen atom reactions of this type. For the similar abstraction reactions with CH31, C2H;I, and C3H71, the frequency factors are 2.5 X loll, 4.2 X lo1', and 1.1X 10" 1. mol-' sec,-l respectively11,l and are greater than the collision frequencies, suggesting a relatively loose transition state. It would seem that this is not a characteristic of the reactions of iodine atoms with fluorinated iodides.
*
+
*
Acknowledgments. This work mas supported in part by a grant from the Australian Institute of Kuclear Science and Engineering. (4) R. K. Boyd, E. W.Downs, J. S. Gow, and C. Horrex, J . P h y s . Chem., 67, 719 (1963). (5) C. A. Goy, A . Lord, and H. 0. Pritchard, ibid.,71, 1086 (1967). (6) R. A. Herrmann and R. M.Noges, J . Amer. Chem. Soc., 78, 5764 (1956). (7) J. E. Bujake, M.WT.T. Pratt, and R. M. Noyes, ibid.,83, 1547 '1961). (8) J. C. Amphlett and E. Whittle, Trans. Faraday Soc., 62, 1662 (1966). (9) J. A. Kerr, Chem. Rev., 66, 465 (1966). (10) I n ref 1, thevalue of D(CFs-I) from the exchange results is given as 51.5 kcal/mol. From the photochemical activation energy, D(CF3I) is 52.0 =I=0.8 kcal/mol, in slightly better agreement with the value of 53.5 kcal/mol obtained from the data of ref 4 and 5. (11) J. H. Sullivan, J . P h y s . Chem., 65, 722 (1961). (12) M. C. Flowers and 8. W. Benson, J . Chem. Phys., 38, 882 (1962).
T h e Journal of Physical Chemistry
The reactions of active nitrogen with hydrocarbons and their derivatives have been studied by many workers. However, no studies have been reported of reactions with completely fluorinated hydrocarbons. We have found that when perfluorobutene-2 is admitted to a stream of active nitrogen, a bright red flame is produced. This flame is similar to that found in the hydrocarbonactive nitrogen reactions but appears slightly more intense. The flame lends itself readily to determination of the rate constant of the reaction by the diffusionflame technique first developed by Hartel and Polanyi' and applied to the reaction of active nitrogen and ethylene by Greenblatt and Winklerez By this method, perfluorobutene-2 is allowed to escape through a small jet and diffuses into a stream of active nitrogen. At steady-state conditions, the partial pressure of active nitrogen is constant throughout the reaction space and drops to zero at the jet. Under these conditions, the rate constant may be expressed identically as obtained by HarteI and Polanyil as
k = - - (In - PflPo)2 D d 2 / 4 p?r~ where pf and PO are the partial pressures of perfluorobutene-2 a t the jet and a t the edge of the flame, respectively; p x is the partial pressure of nitrogen atoms; d is the diameter of the flame; and D is the diffusion coefficient of perfluorobutene-2 into the gas mixture.
Experimental Section Perfluorobutene-2, ethylene, propylene, and nitric oxide were obtained from Matheson of Canada, Ltd., and purified by bulb to bulb distillation. X t r i c oxide was further purified by condensation in and evaporation from caroxite. Gas chromatographic analysis indicated that the perfluorobutene-2 contained 98% trans and 2% cis isomers with no impurities. Purified nitrogen from Canadian Liquid Air Corp. was further purified by passage through a column of heated copper at 400" and through a liquid nitrogen trap. The nitrogen was activated either by a glow discharge excited by a 125-W microwave unit operating at 2450 RfHz or by a condensed electric discharge. The reaction was studied in a fast-flow system simi(1) H. v. Hartel and M. Polanyi, 2. Phys. Chem. (Leipsig), B11, 97 (1930). (2) J. H. Greenblatt and C. A. Winkler, Can. J . Res., B27, 732 (1949).
1813
NOTES lar to that described by Levy and Winkler.3 The major difference was the type of reaction vessel, which for the present experiments was made from Pyrex tubingof 60 mm i.d. and 45 cm length. The reactants were admitted to the active-nitrogen stream in the center of this reaction vessel from a jet of 1.5-mm diameter. The flow of the fluorocarbon or hydrocarbon was adjusted until a spherical diffusion flame was obtained. This flame was photographed in a darkened room with a scale attached to the reaction vessel. This allowed the flame diameter to be determined within an error of The various partial pressures required for calculation of the rate constants were obtained from the total pressure of the gas mixture and the individual flow rates. The flow rate of perfluorobutene was determined by means of gas chromatography. A quantity of the gas was trapped directly following a measurement and under the conditions of the measurement. Quantitative determination of the amount of gas was made on a 1.5-m column of silica gel coated with 301, squalane. The limiting pressure or pressure of the reactant at the edge of the flame was obtained by reducing the reactant flow rate until the small reaction flame was practically indistinguishable from the yellow background of active nitrogen. The concentration of nitrogen atoms was measured flow rates by the NO gas titration t e ~ h n i q u e . ~ The J of NO, Nz, and the hydrocarbons were determined by evacuation of a known volume through the capillary flow meter. A knowledge of the molecular diameters of the diffusing gases was required for theo evaluation of D.6 The values of 3.75, 7.4, 3.5, and 5.8 A for the molecular diameters of nitrogen, perfluorobutene-2, ethylene, and propylene, respectively, were used. The diameter of molecular nitrogen rather than atomic nitrogen has been used, since the amount dissociated is only a small percentage of the total flow in both the microwave and condensed discharges. The diameter of perfluorobutene-2 was obtained from viscosity measurements by comparison of its flow time through a fine capillary with that of gases of known viscosities. The other diameters were obtained from the literature.
Results and Discussion Rate measure.ments of the reaction of perfluorobutene-2 with active nitrogen produced both in the condensed discharge and microwave discharge were made for several flow rates of fluorocarbon. I n order to check the accuracy of the equipment and measurements, the reactions of ethylene and propylene with active nitrogen produced in the condensed discharge were also studied. The values determined for the various parameters necessary for the calculation of the rate constants and the rate constants are given in Table I.
Table I: Experimental Data for the Calculation of k a loapf?
103p0,
IO~PN,
torr
torr
torr
d, om
10-3k, torr-1 see-1
10 -I%, c c mol-'
seo-1
+
2.70 2.70 2.70
N CzHd, Condensed Discharge (Temp = 313'K," D = 97.4 emz sec-l) 5.15 0.95 0.162 122 0.162 122 5.84 0.74 0.162 122 5.36 0.63
1.76 1.76 1.76
N C3He,Condensed Discharge (Temp = 313"K, D = 48.3 om2 sec-l) 1.3 0.171 122 2.54 0.171 122 2.41 1.5 0.171 122 2.80 1.1
2.20 2.43 4.32
N C4F8-2,Condensed Discharge (Temp = 313'K, D = 29.8 em2 sec-l) 0.162 122 2.30 1.3 0.162 122 2.03 1.5 0.162 121 2.29 1.1
1.6 1.6 1.6 0.6
N C4F8-2, Microwave Discharge (Temp = 298"K, D = 27.4 em2 sec-l) 3.71 2.8 0.12 19 0.12 19 4.37 2.0 2.0 0.12 19 4.39 0.04 19 3.00 4.6
1.9 1.5 1.2
+
2.5 2.9 2.2
+
2.5 3.4 3.9
+
5.2 3.7 3.7 8.5
mol sec-l; N flow rate, 13.5 X a Nz flow rate, 187 X 10-6 mol see-1 for condensed discharge; N flow rate, 2.0 X 10-6 mol sec-1 for microwave discharge; total pressure, 1.75 torr. p f indicates partial pressure of hydrocarbons or fluorocarbon All temperatures are k 3 " . where appropriate.
*
A comparison of the average rate constants determined in this work with those found by other studies for the active nitrogen-hydrocarbon reactions are given in Table 11. It can be seen from the values tabulated that the measurements of the present work agree well within the accuracy of the technique. Also listed in Table I1 are the average values obtained for the rate of the active nitrogen-perfluorobutene reaction. The measurement of the flame diameters is rather difficult, since the flames do not normally have a sharp cutoff in intensity near their edges. This was especially true of the hydrocarbons, since their flames tended to be somewhat diffuse and nonspherical. The fluorocarbon flame, however, was much better suited to diff usion-flame measurements. It was easily obtained in a near-perfect spherical shape with a relatively sharp cutoff. An example of such a flame is shown in Figure 1. For the active nitrogen-ethylene reaction, the original (3) E. M.Levy and C. A. Winkler, Can. J . Chem., 40, 686 (1962). (4) (a) A. A. Westenberg and N. de Hass, J . Chem. Phgs., 40, 3087 (1964); (b) L. Elias, ibid., 42, 4311 (1965). (5) J. T. Herron, J . Phys. Chem., 69, 2736 (1965). (6) J. 0.Hirschfelder, C. F. Curtiss, and R. B. Bird, "Molecular Theory of Gases and Liquids," 2nd ed, John Wley and Sons, Inc., New York, N. Y.,p 14. Volume 72, Number 6 M a y 1968
NOTES
1814
.... . . ' II
The Variation of Partial Molar Volume of Some Tetraalkylammonium Iodides with Temperature in Aqueous Solutions'
I
by Ram Gopal and Mohd. Aslam Siddiqi2 DeparlmenfOJ Chemkfru. Lucknow Uniorrailu. Laeknou. U . P.. Indio (Rrcn'ucd Aanaal 4. IOR7)
measurement of the rate, which was made by diffusionflame technique gave values of 1.45 X 1O1O and 2.29 X 1O1O a t 298"I
