1040
K. W. WATKINSAND H. C. MOSER
Reaction of Tritium Atoms with Films of Solid Ethylene. Disproportionation and Combination of Ethyl Radicals at 63
by K. W. Watkins and H. C. Moser Department of Chemistry, Kansas State University, Manhattan, Kaneas
(Received October 1 , 1964)
Films of solid ethylene a t 63°K. were bombarded with tritium atoms. Products (ethylene, et,hane, and butane) of disproportionation and combination reactions of ethyl radicals were observed. Conditions were selected under which the only apparent react.ions of ethyl radicals were disproportionation and combination. Under these conditions, no contribution from tritium atom addition to ethyl or product ethylene was evident. A disproportionation to combination ratio (kdlkc) of 0.77 f 0.03 was observed for C2H4T C2H4T,and a ratio of 0.83 f 0.04 was observed for G H 4 T C2H6. Two methods of calculating the tritium isotope effect in the disproportionation of ethyl radicals gave k H / k T = 1.7 f 0.7 and k ~ / = k 2.3 ~ f 0.5. These small primary kinetic isotope effects are consistent with the mechanism discussed by Bradley and Rabinovitch that involves a “loose” transition state for disproportionation in which little or no binding occurs.
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Introduction Hydrogen atoms produced by the atomization of molecular hydrogen at a hot tungsten filament react with films of solid olefins. Reactions with solid properie2~3and solid cis-2-bu tene4 are among those that have been studied. When the hydrogen atoms reacted before reaching thermal equilibrium with the solid, hydrogen atom abstraction and nonterminal addition occurred along with terminal a d d i t i ~ n . ~When the hydrogen atoms were thermalized to the temperature of the olefin film (77 or 90”K.),hydrogen atoni addition to the terniirial carbon atom of the olefin appeared to be the only initial reaction. Under conditions of rapid olefin diffusion all of the products in the propene and cis-Zbutene system were explained by disproportionation and combination reactions of isopropyl2 and secbutyl4 radicals, respectively. 111 the present paper, the results of hydrogen and tritium atom reactions with films of solid ethylene are reported. Here, as in the above cases, the products of reacation can be explained best by the existence of alkyl radical interniediates, Le. , ethyl radicals. We have measured disproportionation to combination ratios (kd,/lc,) for two types of ethyl radicals, C2H4T and CzD4T,at 63°K. Ratios at other temperatures for ethyl5 and isopropyl4radicals have been reported in the literaThe Journal of Physical Chemistry
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ture. I n both cases k d / k , increased with decreasing temperature. By labeling with deuterium, it was shown that disproportionation of ethyl radicals involves a transfer of a hydrogen atom from the CH3 group of one radical to the CH2 group of the other radicaLeJ A transition state which involves a “loose” association of radicals has been p r o p o ~ e das~ being ~ ~ more consistent with the large A factor for disproportionation than the rigid transition state of a “head to tail” mechanism analogous to hydrogen abstraction reactions. The magnitudes of (1) Presented in part at the 148th National .Meeting of the American Chemical Society, Chicago, Ill., Sept. 1964. Work performed under Contract AT(l1-1)584 with the U. S. Atomic Energj Commission. From the Ph.D. Thesis of K. W. Watkins to be submitted to the Graduate School, Kansas State University. (2) R. Klein, M. D. Scheer. and J. G. Waller, J . Phys. Chem., 64, 1247 (1960). (3) H. B. Tun and H. C. Moser, ibid., 67, 2806 (1963). (4) R. Klein, M. D. Scheer, and R. Kelley, ibid.. 68, 598 (1964). ( 5 ) P. S. Dixon, A. P. Stefani, and M. Szwarc, J . Am. Chem. Soc., 85, 2551 (1963).
(6) M. H. Wijnen and E. W. R. Steacie. Can. J . Chem., 29, 1092
(1951). (7) J. R. McNesby, C. M. Drew, and A. S. Gordon, J . P h y s . Chem., 5 9 , 988 (1955). ( 8 ) J. N. Bradley, J . Chem. P h y s . , 35, 748 (1961). (9) J. N. Bradley and B. S. Rabinovitch, ibid., 36, 3498 (1962).
RE.4CTIOXS
OF
TRITIUM ATOMSWITH FILMS O F S 3 L I D ETHYLENE
the hydrogen-tritium (for C2H4T)and the deuteriunitritium (for C2D4T)primary kinetic isotope effects in the disproportionation of two ethyl radicals should serve as a further test of the mechanism. The kinetic isotope effects k H / k T and k D / k T are reported here for ethyl radical disproportionation a t 63°K.
Experimental Ethylene (99.97%, Phillips research grade) and ethylene-& (9Ooj,, Stohler Isotope Co.) were used without further purification. The reaction flask consisted of a cold finger and a tungsten filament (2.5 X X 1 3 m i . ) placed vertically so that its center was 4 tin. froni the bottoiii of the cold finger. The filament was supported by tungsten leads sealed in glass. Three to five pnioles of ethylene was introduced into the vacuum system and was frozen with liquid nitrogen as a film on the bottom of the cold finger. Sfore liquid nitrogen was then added. By pumping the liquid nitrogen, either with a vacuuni pump or with an aspirator, its teniperature was reduced to 63°K. (Le., its freezing point). At this temperature the vapor pressure of solid ethylene is about torr. Hydrogen containing tritium or carrier-free tritium was introduced into the vacuum system through heated palladium thimbles. The specific activity of the hydrogen was approximately 1 arid 58 c./mniole (carrier free). The hydrogen was atomized by the tungsten filament a t pressures from 1 x l o w 3to 9 >(:10-3 torr. The pressure was measured with a Pirarii gauge which was calibrated with a McLeod gauge. The filament was placed perpendicular to the ethylene film in order to niiriiiiiize heat transfer froni the filament to the ethylene film. Also, most of the hydrogen atonis struck the glass wall before encountering the ethylene film. This allowed the H atonis to lose most of their kinetic energy; thus hydrogen atom abstraction from ethylene to form vinyl radicals was not an important reaction. This was verified by the absence of 1-butene as a product. Only in a very few reactions was a trace of 1-butene observed. The tungsten filament temperature was measured with an optical pyrometer, and corrections were made for emissivity. The reaction products were separated by gas chromatography and counted by an ionization chamber placed in the effluent The ionization chamber was calibrated by noting the response (peak area) f roni t ri t ium-labeled 2,3-diniet hylbu tane as it flowed through the chamber. The 2,3-diniethylbutane was then trapped in toluene scintillator solution and counted in a liquid scintillation spectrometer. Products were separated by a gas chromatography column of Ag?;Os in ethylene glycol in series with a di-
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methylsulfolane column and were identified by coniparison with retention volunies of known compounds.
Results The reaction of tritium atonis with f i l m of frozen ethylene a t 63°K. yields under proper conditions only three products which contain tritium, ethylene, ethane, and butane. Table I shows how the product distribution changed
Table I: Tritium Atom Reactions with CzHa a t 63°K: Reaction time,* min.
,
1 2 3 5 9
Tritium dist., %-----C-C
C=C
11.8 f 0 . 7 9.0 f0.5 7.2 f0.2 5.0 2.9
(A) 31.7 f 2 . 2 32.3 f 1 . 8 34.9 f 2 . 8 37.9 46.2
11.8 f 0 . 7 8.5 f0.6 6.3
31.7 f 2 . 2 37.4 f 2 . 3 42.0
Rel. C-C-C-C
8H
act.
56.4 f 1 . 8 58.7 f 1.3 57.9 f 2 8 57.1 50.9
(1.0) 1.5 1.7 2.7 3.0
56.4 f 1.8 54.1 f 1.7 51.7
(1.0) 1.5 2.2
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HZ HT pressure,' torr
x 108
(B) 1 4 9
+ H T pres-
a Filament temperature 1830°K. * Constant HO Constant reaction time of 1 min. sure of 1 0 - ~torr.
with increasing reaction time and increasing hydrogen pressure for a tungsten filament temperature of 1830°K Table I1 shows that when the filament temperature is reduced to 1400°K. the product distribution is constant in time arid also that changing the tritium pressure makes little difference. Table I11 shows the average product distributions, with standard deviations, obtained from the reactions of H and T with C2H4,T with C2H4, and T with C2D4 carried out under the conditions given in Table 11.
Discussion Reactions of Atoms and Radicals. The three products observed from reactions of tritium atoms with solid ethylene are considered to result from reactions 1-5. Hydrogen addition to the olefin and subsequent T
+ C2Hj --+ CzH4T*
C2H4T*
(1)
+ RI --+C2H4T + SI
(2)
(10) D. C . Nelson, P. C. Ressler, J r . , and R. C. Hawes, Anal. Chem.,
35, 1575 (1963).
Volume 69, ,\'umber
S
March 1965
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