REACTIONS OF TRITIUM ATOMSWITH TRITIUM-LABELED ISOPROPYL RADICALS
1137
Reactions of Tritium Atoms with Tritium-Labeled Isopropyl Radicals at 63 OK1
by K. W. Watkins and H. C. Moser Department of Chemistry, Kansas State University, Manhattan, Kansas
(Received October 11, 1965)
DT, tritiated propene, and tritiated propane were obtained as products of reactions of tritium atoms with a solid mixture of propene-& and isopentane (mole ratio 1:lOOO) at 63'K. These products are interpreted as having resulted from disproportionation and combination reactions of T atoms with i-CsDeT radicals. Disproportionation to combination ratios, kd/k,, for tritium atom reactions with i-C3HeT and i-CsDeT radicals at 63'K were determined to be 3.55 0.05 and 3.05 0.10, respectively. These ratios were used to calculate a value of 1.2 for the hydrogen-deuterium kinetic isotope effect, kH/kD, in the disproportionation reaction. The similarities of H atom-alkyl radical and alkyl-alkyl radical disproportionation and combination reactions are discussed.
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Introduction Reactions of hydrogen atoms with solid propene have been studied in our laboratory2n3and by The atoms were produced by the atomization of molecular hydrogen at an incandescent tungsten filament in each case except one in which a microwave discharge was used. Under conditions where the hydrogen atoms reacted before reaching thermal equilibrium with solid propene a t 77 OK, hydrogen atom abstraction and nonterminal addition occurred along with terminal addition.2 When the hydrogen atoms were thermalized to the temperature of the solid propene by collisions with helium, hydrogen atom addition to the terminal carbon appeared to be the only initial r e a c t i ~ n . ~I.n~ the presence of helium all of the products were explained by the disproportionation and combination reactions of isopropyl radicals. Klein, Scheer, and Kellef have recently demonstrated that a sec-butyl radical, produced by the addition of a hydrogen atom to cis-2-butene1can be trapped in 3-methylpentane long enough so that it reacts with another hydrogen atom before diffusing to another butyl radical. The products which they observed indicated that, in addition to the combination of a hydrogen atom and a see-butyl radical, a little known reaction, hydrogen atom-alkyl radical disproportionation, also occurred. Heller and Gordon previously observed this type of reaction in the gas phase for ethylg and isopropyl radicals.lo In the present paper the results of tritium atom re-
actions with tritium-labeled isopropyl radicals are reported. From measurements of the hydrogen yield new evidence in support of hydrogen atom-alkyl radical disproportionation is presented. Also, we have measured the disproportionation to combination ratios for T i-C3H6Tand for T i-C3DsTat 63°K.
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Experimental Section Most of the experimental apparatus and techniques have been described,l1 including the reaction vessel. The bath temperature of 63°K was obt,ained by using a slush of liquid and solid nitrogen. The slush was easily prepared by lowering the vapor pressure of nitrogen over the liquid nitrogen with a vacuum pump. Films composed of 0.5 pmole of propene (Atatheson CP grade) or propene-& (99%, Volk Radiochemical ~
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(1) From the Ph.D. Thesis of K. W. Watkins, Kansas State University, 1965. Work performed under Contract AT(11-1)584 with the U. S. Atomic Energy Commission. (2) H. B. Yun and H. C. Rloser, J . Phys. Chem., 67, 2806 (1963). (3) H. B. Yun and H. C. Moser, ihid., 69, 1059 (1965). (4) R. Klein, M. D., Scheer, and J. G. Waller, ihid., 64, 1247 (1960). (5) Y. P. Lomanov, A. N. Ponomarev, and V. L. Tal'roze, Kinetika i Kataliz, 3 , 49 (1962). (6) A. N. Ponomarev, ihid., 4, 859 (1963). (7) C. G. Hill, Jr., R. C. Reid, and M. W. P. Strandberg, J. Chem. Phys., 42, 4170 (1965). (8) R. Klein, M.D. Scheer, and R. Kelley, J . Phys. Chem., 68, 598 (1964). (9) C. A. Heller and A. S. Gordon, J . Chem. Phys., 36, 2648 (1962). (10) C. A. Heller and A. 9. Gordon, J . Phys. Chem., 64, 390 (1960). (11) K. W. Watkins and H. C. Moser, ihid., 69, 1040 (1965).
Volume 70, Number 4 April 1966
K. W. WATKINSAND H. C. MOSER
1138
Co.) diluted with 500 pmoles of isopentane (99+%, Matheson Coleman and Bell) were treated with T atoms produced by the atomization of Tz (pressure, 1X torr) at a hot tungsten filament. All reactions reported here were performed in the presence of 0.05 torr of helium (Airco assayed reagent grade). Tritium-containing products were counted by an ionization chamber placed in the effluent stream of a gas chromatograph. This technique allowed a complete pmole of products. analysis on as little as 2 X The reaction times were from 5 to 15 sec for a filament temperature of 2025OK. During an average reaction, 2 X pmole of tritium-labeled products was produced. After the reaction, carrier (H2 or D2) was added, and the hydrogen products containing tritium (HT, DT, and Tz) were collected on Molecular Sieve 5A at 77°K. These products were separated by gas chromatography using a 6-fb column of moderately activated alumina at 77'K.12 The T2 used in these experiments was obtained by separation from a sample containing a mixture of HT, T2, and D T (-1%) which had been purified by diffusion through a heated palladium thimble. Separation of HT, T2, and D T was performed by the alumina column, and the Tz was trapped from the helium stream on Molecular Sieve 5A at 77°K. The molecular sieve trap was then attached to the vacuum system, and while maintaining the molecular sieve at 77'K, the helium was pumped off. Upon warm-up the Tz expanded into the vacuum system and was used for production of tritium atoms.
Results The products of tritium atom reactions with films of pure propene-& and highly diluted propene-de are shown in Table I. Each of the values represents an average from four reactions. Standard deviations are indicated. Smaller standard deviations were obtained if the per cent DT was left out of the averages. Table I1 compares the propene and propane products from light and perdeuterated propene. Yields of H T from T atom reactions with CJ& are not reported because some H T was always formed as a product even in the C3D6experiments which indicated that the H T could not be considered to result exclusively from reactions with C3Ha. In experiments where the hydrogen yield (DT) was used as a measure of the T atomisopropyl radical disproportionation reaction, GDs instead of C3H6 was used in order to eliminate the experimental difficulty owing to the formation of H T from parts of the system other than the hydrocarbon film. Thus, any DT formed by the reaction of T The Journal of P h y s h l Chemistry
atoms with the C3D6 film could be considered to have originated only from reactions at the film. ~~
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Table I: Products of Reactions of Moderated Tritium Atoms with Solid Propene-& a t 63°K' % T in productsFilm composition 0.1% propene-&
Product
DT
c=c-c c-c-c c-c-c I c-c--c c=c-c/ c-c-c
100% propene-&
in isopentane
5 . 0 f 0.4 40.2 f 2.4 41.1 f 1 . 2
36.9 f 2 . 8 35.5 f 2.0 27.5 f 0.9
13.6 f 0.8
0
1.o
1.3
' Conditions: filament temperature, 2025'K; helium moderator; and torr of purified Tz.
0.05 torr of
Table I1 : Products of Reactions of Moderated Tritium Atoms with Propene and Pr0pene-d~in Isopentane a t 63'K
Product
c=c-c c-c-c c-c-c I c-c-c +c-c/ c-c-c a
% T in productsn--Fm li composition-0.1% C3D6 0.1% C3HB
55.9 f 1 . 2 44.1 f 1.2
59.7 f 0.6 40.3 f 0.6
0
0
1.27 f 0.04
1.48 f 0.02
Does not include DT or HT.
Discussion Reactions of Atoms and Radicals. The hydrocarbon products shown in Table I from reactions of moderated tritium atoms with a film of pure propene-de at 63°K are consistent with previous work and evidently result from isopropyl radical disproportionation and combination. Labeled isopropyl radicals were formed from tritium atom addition to propene. When propenede was highly diluted with isopentane, the per cent of isopropyl radical dimer, 2,3-dimethylbutane, fell to zero and the per cent DT became equal to that of labeled propene to within experimental error. This effect resulted from changing two experimental conditions: (1) the concentration of propene and (2) a change of the film composition to reduce (12) E. H. Carter, Jr., and H. A. Smith, J. Phys.'Chem., 67,1512(1963).
REACTIONS OF TRITIUM ATOMSWITH TRITIUM-LABELED ISOPROPYL RADICALS
diffusion. Because of the small concentration of propene in the mixed film, isopropyl radicals are formed much farther apart than they are in a pure propene film. Also, the diffusion of isopropyl radicals in or on isopentane is probably slower than in pure propene, and the isopropyl radicals cannot diffuse to each other before encountering a tritium atom. The absence of 2,3-dimethylbutane from the products shows that isopropyl-isopropyl reactions did not occur. Under these conditions isopropyl radicals reacted only with tritium atoms. The products can be explained by reactions 1-4.
+ C3D6 -% CD3CDCDZT T + i-C3D6T -% C3D5T+ DT T + i-CsDaT -% C3D6 + Tz T + i-CaDeT -% C3DeT2 T
(1) (2) (3)
+ t-C4HeD
----+
t-C4H,D2
+H
The ratio of tritium in propene to tritium in propane, propene(T)/propane(T), was used to calculate kd/kc. The ratio k2/k4 was taken as
_kz -- 2propene(T) kq
propane(T)
(7)
Multiplication by 2 corrects for the occurrence of two tritium atoms in propane(?'). Since k3 results in nonlabeled propene, it could not be measured. The value k3 was assumed to be '/skz since there are five deuterium atoms and one tritium atom that can be transferred randomly in disproportionation if isotope effects are neglected. The ratio of disproportionation to combination was calculated from eq 8. For T
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(4)
These experiments with C3D6 represent the first quantitative measurement of the hydrogen formed from H atom-alkyl radical disproportionation. If all of the labeled propene is formed by reaction 2, then an equal quantity of DT must be formed. The data in Table I show that within experimental error D T and propene were formed in equal amounts. Thus, good evidence wax obtained for tritium atom-isopropyl radical disproportionation. Consecutive reactions of tritium atoms with product propene molecules were not evident. The product distributions used to calculate the averages in Table I1 did not vary, even though the tritium incorporated into the products varied over a range of 85-fold for C3H6and a range of sixfold for C3D6. If consecutive reactions were important, the per cent labeled propene should have decreased as more tritium was incorporated into products. Lomanov, Ponomarev, and Tal'roze5 observed the formation of H D when deuterium atoms were treated with solid isobutene at 77°K and attributed H D formation to reaction 5. However, the activation energy D
1139
(5)
requirements are too large for this reaction to occur between atoms and radicals thermalized to 77°K) whereas hydrogen atom-alkyl radical disproportionation, if it is similar to alkyl radical disproportionation, would require essentially no activation energy and could easily occur at 77°K. Also, the equivalent yields of DT and propene reported here would not be expected from reaction 5, which does not form an olefin product. Disproportionation to Combination Ratio. The ratio of disproportionation to combination (kdlk,) is
i-C3H6T,kd/k, was calculated to be 3.55 i 0.05, while for T i-C3D6T,kd/k, was calculated as 3.05 f 0.10. An estimate of the hydrogen-deuterium kinetic isotope effect, kH/kD, on the disproportionation reacition was obtained from values of k d / k c for T C3H6Tand T i-C3D6T. The rate of combination, k,, was assumed to be the same for both radicals. This amounts to neglecting a possible inverse secondary isotope effect. The primary isotope effect was calculated as follows.
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The effect of isotopic substitution on kd/k, is very small and shows that neglecting an isotope effect in calculating kd/k, is a reasonable approximation. Comparison of Radical-Radical and Hydrogen AtomRadical Reactions. Hydrogen atom-alkyl radical disproportionation and combination exhibit several similarities to alkyl-alkyl radical disproportionation and combination. In both cases small hydrogen kinetic isotope effects on disproportionation were observed a t 63°K. In alkyl-alkyl radical reactions this effect was considered to support the concept of a "loose" transition state for d i s p r o p ~ r t i o n a t i o n , ~A~ ~loose ~ association of the H atom and the alkyl radical in the transition state for disproportionation is also proposed here. In both cases the rate of disproportionation increases relative to combination as the temperature decreases, indicating that the activation energy for combination (Ee) is higher than the activation energy for disproportionation (Ed). Using the values kd/k, = 0.2 for deuterium atom-isopropyl radical reactions at Volume 70,Number 4
April 1966
0. D. BONNER AND DAVIDC. LUNNEY
1140
361°K9 and k d / k , = 3.55 for tritium atom-isopropyl radical reactions at 63"K, an activation energy difference of E, - E d = 0.43 kcal/mole was calculated. This calculation was done by making the assumption that the k d / k c ratio is dependent exclusively upon the temperature and is not affected by a change in the phase. A small activation energy difference of 0.42 kcal/mole with combination having the higher activation energy would also explain the difference in k d / k c values for isopropyl radical disproportionation and combination at 373 and 77"K, 0 . W 3 and 5.5, respectively. The value of k d / k c of 0.8 for ethyl radicals
a t 63°K" is consistent with the gas phase value of 0.13 at 273"K14for an activation energy difference of E, Ed = 0.3 kcal/mole which is essentially the same dependence that was first reported in the gas phase.14 Other small activation energy differences of the same magnitude with E, - E d have been reported for isopropyl-isopropyl and sec-butyl-sec-butyl radical disproportionation and combination.8 (13) C . A. Heller and A. S. Gordon, J. Phys. Chem., 60, 1315 (1956). (14) P.S. Dixon, A. P. Stefani, and M. Szwarc, J . Am. Chem. Soc., 85, 2551 (1963).
A Study of Some Concentration Cells with Liquid Ion-Exchanger Membranes
by 0. D. Bonner and David C. Lunneyl Department of Chemistry, University of South Carolina, Columbia, South Carolina
(Received October 11, 1965)
Concentration potentials of cells in which two aqueous electrolyte solutions are separated by a liquid ion exchanger (an oil phase with ion-exchange properties) have been measured for aqueous NaCI, NH,Cl, and CaClz solutions, using organic solutions of dinonylnaphthalenesulfonates as the liquid exchangers, for aqueous HCl solutions using General Mills Aliquat 336 and Humko Kemamine Q-19024 exchangers, and for p-toluenesulfonic acid solutions, using Humko Kemamine Q-1902-C exchanger. The behavior of the cells is analogous to that of concentration cells with solid ion-exchange membranes; their failure to reach the theoretical potential for cells with ideally permselective membranes is explained by considering the solubility of the exchanger in the aqueous phase and transport of water across the organic phase by hydrated ions. Activity coefficients were calculated from data obtained for p-toluenesulfonic acid, and they agree well with activity coefficients determined isopiestically.
Introduction The electrochemicalproperties of cells of the type
have been studied intensively since about 1935, but very little attention has been given to cells in which the membrane is a liquid ion exchanger (an organic solution of a water-insoluble ionogen). In 1953 and The Journal of Physical Chemiatry
1954 Bonhoeff er, Kahlweit, and Strehlowz reported potentials of concentration cells in which two aqueous LiCl solutions were separated by an oil phase consisting of quinine hydrochloride in quinoline. The observed potentials fell far short of those obtainable with conventional ion-exchange membranes. In 1964, Sollner and Shaen3 reported that they had obtained concentra(1) National Science Foundation Graduate Fellow. (2) K. F. Bonhoeffer, M. Kahlweit, and H. Strehlow, Z . Elektrochem., 57, 614 (1953); Z.Physik. Chem. (Frankfurt), 1, 21 (1954).
