2272
NOTES
REACTIONS O F TRITIUM ATOMS WITH FROZEN CYCLOPROPANE' BY H. C. MOSERAND R. D. SHORES Department o/ Chemistry, Kansas State University, Manhattan, Kansas Recezved May 81, 1066
Tritium atoms produced by the atomization of molecular hydrogen containing tritium a t a hot tungsten filament have been found to react with frozen hydrocarbons.2 One of the principal reactions is the exchange of T for H, and this occurs simultaneously with other reactions requiring less activation energy such as hydrogenation of unsaturated compounds. Investigation of tritium atom reactions with cyclopropane was undertaken to help elucidate the energy effects. Hydrogen atoms of different energies have been reported to react differently with cyclopropane.3-6 Experimental Cyclopropane (Matheson Co.) usually was purified by gas chromatography to remove a small amount of propene impurity. A 15-ft. dimethylsulfolane column was used for the separation. Ethane (Matheson C.P.) and propane (Matheson instrument grade) were used without further purification. Molecular hydrogen containing tritium was produced by the reduction of tritiated water (specific activity 18 mc./ mmole) by metallic zinc0 at 650". A 4.3-cm. diameter, thin-walled, Pyrex flask was used as the reaction vessel. Suspended from molybdenum leads at the center of the flask was a 3.1 cm. X 0.008cm. or3.1 cm. X 0.005 cm. tungsten filament. The maximum filament to wall distance was 3 cm. An optical pyrometer was used to measure the temperature of the filament a t its center. Approximately two thirds of the filament was a t the reported temperature, and the remainder was a t a lower temperature. A Pirani gage was used to measure the pressure. The gage was frequently calibrated with a McLeod gage. mmole of cyclopropane From 5.7 X 10-6 to 4.2 X was introduced into the system and frozen out as a film on the inside surface of the reaction flask by immersing the vessel in liquid nitrogen. Hydrogen was introduced through a high vacuum needle valye which was used to maintain a pressure of 5 fi during reaction. Following reaction, products were transferred to a sampler a t -195'. The amount of tritium incorporated, exclusive of any present as methane, was measured by counting a known fraction of the gaseous sample with an ionization chamber. Products were separated by gas chromatography with a 15-ft. dimethylsulfolane column operated at room temperature. Methane usually was pumped off with hydrogen remaining after reaction. In a few experiments methane and some hydrogen were adsorbed on silica gel at - 195" and separated by gas chromatography using a column packed with Linde Type 5A Molecular Sieve. Following separation the products were passed through a proportional counter placed in the effluent stream.
Results Table I gives results of reactions run at three different filament temperatures. The hydrogen pressure was maintained a t 5 p during a 5 min. reaction time with 0.042 mmole of purified cyclopropane. The amount of tritium incorporated increased with filament temperature. Exclusive of tritium present in labeled methane, 0.26 pc. was contained in the (1) Work performed under Contract AT(ll-1)-584 with the U.S. Atomic Energy Commission. (2) R. D. Shores and H. C. Moeer, J . Phys. Chem., 66, 570 (1961). (3) H. I. Schiff and E. W. R. Steacie, Can. J. Chem., 29, 1 (1951). (4) J. K. Lee, B. Musgrave, and F. S. Rowland, ibid., 88, 1756 (1960). ( 5 ) D. W. Setser, B. 8. Rabinowitch. and E. G. Spittler, J. Chem. Phus., 8 5 , 1840 (1961). (6) K. E. Wilsbach, I,. Kaplan, and W. G. Brown, Science, 118, 522 (1953).
Vol. 66
products from reaction with unpurified cyclopropane a t 1750' (0.008 cm. X 3.1 cm. filament). Table I1 gives results of reactions with different amounts of purified cyclopropane for a 5 min. reaction time, 1750' filament temperature, and 5 p hydrogen pressure. Assuming uniform deposition, film thicknesses for these reactions were approximately 6 X 6X and 4 X em. TABLE I PRODUCTS OF TRITIUV ATOMREACTIONSWITH FROZEN CYCLOPROPANE Filament temp., OC.
1180
1520
1750
73 27
37 15
12 23 37 6 5 6 4 7
% Tritium in Products" Ethane Propane Cyclopropane Isobutane %-Butane 2-Methylbutane n-Pen tane C6 Does not include methane.
14 8
6 7 5 7
TABLE I1 EFFECT OF FILM THICKNESS ON PRODUCTS OF TRITIUM ATOM REACTIONS WITH FROZEN CYCLOPROPANE 5.7 x 5.7 x 1.1 x 4.2 x Amt. of cyclopropane (mmoles)
10-4
10-2
10-1
10 25 28 10 9 7 5 5
5 32 37 8
12 23 37 6 5 6 4 7
% Tritium in products" Ethane 25 Propane 26 Cyclopropane 14 Isobutane 13 n-Butane 10 2-Methylbutane 8 n-Pentane 3 C0 1 Does not include methane.
7 6 3 2
Methane was found in each sample analyzed for it. In some cases the methane contained more than 30% of the incorporated tritium. The percentage of tritium in methane decreased in reactions run a t the higher filament temperatures. The reactions mere not noticeably affected by eliminating thermionic emission from the filament with the use of a negatively charged grid. In a larger reaction vessel (250-cc. volume) the filament was surrounded by an open-structured, wire grid. The negative pole of a 67.5-volt battery was connected to the grid and the positive pole to one end of the filament. Yields for hydrogen exchange reactions were measured for propane and cyclopropane under the same conditions. Relative yields of 3 and 1 were obtained, respectively, for 3-min. reactions. The yield of the hydrogen exchange reaction on cyclopropane was measured under various experimental conditions. I n addition to the effects of filament temperature and film thickness (Tables I and 11), the yield was greatly reduced by introduction of 50 p helium as a moderating gas. Also, when the surface of the cyclopropane was covered with a film of propane or ethane, the yield for the exchange of T for H in cyclopropane was reduced to an undetectable level.
Nov., 1962
NOTES
2273
atoms and cyclopropane react with low collision efficiency without the formation of deuterated c~~clopropane.~ On the other hand exchange has been observed a t 135' presumably through the formation of cyclopropyl radicalse6 The relative yield of labeled cyclopropane is diminished when the thickness of cyclopropane is reduced to a few molecular layers (Table 11) while yields from cracking and isomerization increase. Deactivation of excited cyclopropane and propane apparently is facilitated by an increase in the thickness of frozen cyclopropane. This behavior is dependent upon the diffusion of cyclopropyl radicals and hydrogen atoms into the frozen film. Rapid diffusion of hydrogen atoms in frozen propene has been proposedlO and diffusion of cyclopropyl radicals in frozen cyclopropane also is reasonable on the basis of general diffusion behavior. l1 The excitation energy accompanying addition of a hydrogen atom to cyclopropyl is about 30 kcal. more than the 65 kcal. activation energy required for isomerization.12 Unless deactivated through collisions, the excited cyclopropane should have a high probability of undergoing isomerization to form propene. In small amounts frozen propene reacts with hydrogen atoms to form principally methane, ethane, and propane.13
Discussion Tritium Atom Production.--New information on the mechanism of atomization of hydrogen on tungsten has been reported by Brennan and Fletcher7 and Hickmott.8 Brennan and Fletcher have proposed, for the lower range of temperatures used in the present study, that equilibrium is maintained between adsorbed and gaseous hydrogen and that atomic and molecular hydrogen desorb in their equilibrium ratio as determined by the temperature of the surface and the pressure of the undissociated gas. On this basis a MaxwellBoltzmann distribution of hydrogen atom energies is consideired reasonable. A large fraction of the hydrogen atoms that leave the filament should have a collision free path- to the film of c,yclopropane. The mean free path for hydrogen atoms in the reaction vessel is estimated to be about 2 cm. for a pressure of 5 p measured a t the Pirani gage. Mechanism and Energy Effects.-Atom-radical and radical-radical combinations of hydrogen, methyl, ethyl, propyl, isopropyl, and cyclopropyl free radicals can account for all of the products observed. Hydrogen abstraction to form cyclopropyl and hydrogen atom addition to form propyl are probably the initial reactions of hydrogen atoms and cyclopropane. Subsequent reactions resulting in the formation of other radicals are mentioned later on in the discussion. Exchange of ?' for H in cyclopropane can occur only with energetic tritium atoms and therefore must be initiated on the surface of the frozen cyclopropane film. Thermalized D atoms do not exchange for hydrogen in cyclopropane a t room t e m p e r a t ~ r e but , ~ exchange has been observed6 in the Hg (3P1) photosensitization of deuteriumcyclopropane mixtures at 135' and in reactions of recoil tritons with cy~lopropane.~Our results from reactions with added helium and from reactions when the cyclopropane surface was covered with a frozen film of ethane or propane also indicate that only energetic atoms react by exchange. I n our experimental approach, this requirement is met for surface rem5ons where the pressure in the reaction vessel is low enough to allow a collision free path from the fdament to the walls. There is both a lower activation energy and lower collision efficiency for the formation of propyl radicals than for the formation of cyclopropyl (see Table I) because the relative yield of labeled cyclopropane increases with filament temperature while that for propane decreases. Hydrogen (or tritium) atom addition to cyclopropyl would give excited cyclopropme which could deactivate or could isomerize to florm propene. Hydrogen atom addition to a propyl radical would give excited propane which could deactivate or crack to form methyl and ethyl radical^.^ Isomerization and cracking must occur only on the surface where the probability for collision or deactivation is the lowest. This mechanism is in qualitative agreement with results from the gas phase where a t room temperature deuterium
Mixtures of barium oxide freshly dried 20 hr. at 1050' BUCUO and a vanadium oxide were thoroughly ground in appropriate ratios, pelletized, and heated in vctcuo. The mixtures were prefired, reground, and refired to emure homogeneity. Annealing procedures subsequently were carried out in evacuated silica amopules. Magnesium oxide single crystal was employed as refractory material. Above 1350" two pellets were stacked, the lower one to be discarded after heating. The samples were checked on constancy of weight after each heat treatment. Whole number mole ratio mixtures of BaO and, respectively, V0l.00, VzOs, VOZ, and V206 were studied subsequently. Of the BaO,
(7) D. Brennan and P. C. Fletcher, Proc. Roy. Soc. (London), AWO, 389 (1959). ( 8 ) T. W. Eickmott, J . Chem. Phys., 8 3 , 810 (1960). (9) C. H.Heller and A. S. Gordon,J . Phys. Cham., 64,390 (1960).
(1) This work forms a part of the Doctoral Thesis of U. Spitabergen to be submitted t o the University of Leiden, The Netherlands. (2) P. Hambling, Acta Cryst., 6 , 98 (1953).
(IO) R. Klein, M. ID. Scheer, and J. G. Waller, ibid., 64, 1247 (1960). (11) A. M. Bass and H. P. Broida, "Formation and Trapping of Free Radicals," Academic Press, Inc., New York, N. Y., 1960,p. 77. (12) H. 0.Pritchard, R. G. Sowden, and A. F. Trotman-Dickenson, Proc. Roy. SOC.(London), A317, 563 (1953). (13) Han Bo Yun, M.S. Thesis, Kansas State University, 1962.
HIGH TEMPERATURE DISPROPORTIOPL'ATION O F LOWER VSNADIUM OXIDES REACTING WITH BARIUM OXIDE1 BY U. SPITSBERGEN AND P. W. J. JANSEN Laboratory for Inorganic end Physical Chemistry, University of Leiden, Leiden, The Netherlanda Received December
I 4 1061
Products of solid state reactions at temperatures between 1000 and 1450' of mixtures of barium oxide and various vanadium oxides have been studied by X-ray identification. Powder photographs were taken in a Guinier-de Wolff focusing camera, R = 229.2 mm. vith CuKa1,, radiation, X = 1.5418 8. Samples were mized with KCl (Analytical Reagent, azo = 6.2919 A. as given by €Iambling2) as a reflection standard. in
