Sept. 5, 1957
RADIATION-INDUCED EXCHANGE OF Clz WITH CC14
explanation. This is, we suggest (with Dodson and co-workers), that the continuing increase in exchange rate a t high [Cl-]/ [Tl(III)] ratios is due to rapid exchange between TlCL- and one or more of the weakly associated thallium(1) complexes, such as T1C1, TlClZ-, TIC&=and possibily TlClI.’ I n view of these findings on the effect of chloride on the exchange, we find it extremely difficult to take seriously arguments based solely on coulombic attractions or repulsions, when we come to the consideration of possible mechanisms. In our previous work, such arguments were considered, but now we are inclined to account for the high exchange rate in sulfuric acid as compared to perchloric acid, on the basis that sulfate ion may actually participate in the electron transfer process. We believe that the rapid exchange reactions in sulfuric acid are *TlSO.t+ 4- T1’ = *TI+ f TlS04’
(5)
and *Tl(SOn)i- f TlSOI- = *TlS04- $- Tl(SO4)n-
(6)
We suggest that exchange is accomplished by transfer of electrons through the sulfate bridge in the comdexes (7)
and
L
J
b
(This would be formally equivalent to the transfer of a sulfate radical, so4-, from the thallium(II1) to thallium(1).) We believe this proposal is further supported by the absence of an (SO4’) term in the rate law, since (7) H. Fromherz and K. H. Lih, 2. p h y s i k . Chem., 6 1 5 3 , 335 (1931); Kuo-Hao H u a n d A. B. Scott, THISJOURNAL, 77, 1380 (1955); J . Chem. Phys., as, 1830 (1955).
[CONTRIBUTIONFROM
THE
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the symmetrical, bridged complexes (such as i and 8) can only occur for odd numbers of sulfates in the complex. The product species are chemically identical with the reactants. We also believe that we are now able to offer possible explanations for the seemingly anomalous effects of chloride and sulfate ions in other oxidation or reduction reactions involving either thallium(1) or thallium(II1). It has been reportedq that the Tl(1)-Ce(1V) reaction is fast in aqueous HCl, but is very slow in sulfuric acid and, in fact, sulfate retards the reaction markedly even in the presence of chloride. Further, we are toldg8’O that the reduction of Tl(II1) by Fe(I1) in HClO4 is accelerated by sulfate and retarded by chloride. If we assume, as we have above, that Tl(II1) chloride complexes are much stronger than the sulfate complexes and remember that TlS04- is slightly stronger than TlC111t12and the higher chloride c ~ m p l e x e swe , ~ believe that i t is possible to account, in part, for these effects. I n the oxidation of thallium(1) by cerium(1V) in HC1, the addition of sulfate effectively reduces the concentrations of the various chlorocomplexes, which react readily with the cerium chloro complex, by forming T1S04-. IVe are somewhat puzzled by the lack of reaction between Ce(SOI)3=and T1SO4in the absence of chloride. In the reduction of thallium(II1) by iron(II), the chloride ion strongly complexes thallium(II1) and removes the reacting species (ie., TlOHff and TIf3) from solution, while sulfate, which forms weak complexes, does not, and any of these complexes that do form, may also react readily with iron (II). (8) P. A. Shaffer,J. Phrs Chem., 40, 1021 (1936). (9) C. E. Johnson, Jr., THISJ O U R N A L , 74, 959 (1952). (10) K. G. Ashurst and W. C. S. Higginson, J . Chem. Soc., 343 (1946) . Soc., 49, 619 (11) R. B. Bell and J. H. B. George, T ~ a n s Favaday (1953). (12) V. S. K. Iiair and G. H. Noncollas, J . Chem. Soc., 318 (1952).
EASTLANSING. MICHIGAN
LOS ALAMOSSCIENTIFIC LABORATORY O F THE UNIVERSITY
OF
CALIFORNIA]
Radiation-induced Exchange of Chlorine with Carbon Tetrachloride] BY JOHN W. SCHULTE RECEIVED JANUARY 26, 1957 The chemical and virtual changes in CC14 containing C1, under the influence of Co60 y-rays have been investigated. Two reactions are observed: the exchange of Clz with cc14, and the decomposition of CC4 to form CzCll and Cln. The rates of both reactions are indey, ndent of the chlorine concentration and directly proportional to the dose rate. The former reaction is interpreted as a measure of the “radical yield” in the system, and it is observed to occur with an efficiency corresponding to 3.5 f 0.35 molecules of chlorine being brought into exchange with CC1, for every 100 e.v. absorbed. The latter reaction is interpreted as a measure of molecular yield and takes place with an efficiency of 0.80 =k 0.06 molecule of C2Cls and Clz being formed for every 100 e.v. absorbed. Spontaneous exchange and exchange under the influence of sunlight and ultraviolet light also were noted.
Introduction Previous work2 indicated that in the work on CHC13 02,most of the information obtained re-
+
(1) Work performed under the auspices of the U.S. Atomic Energy Commission. (2) J. W. Schulte, J. F. Suttle and R. Wilhelm, THISJOURNAL, 1 5 , 2222 (1953).
ferred to the chain-carrying processes rather than the Primary act. BY using a system, cc14 -k C ~ Pi ,t was hoped that the reactions observed would be simpler and a direct consequence of the primary act, such a system G values for “radical yields’’ and “molecular yields” analogous to those determined for water might be measured.
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JOHN
W. SCHULTE
Vol. 79
The rate of exchange of tagged chlorine with CCI4 fragments produced by irradiation appeared to be a promising reaction to use as a measure of the “radical yield.” The evidence for a “molecular reaction” is the formation of the stable products CIS and C2C&which build up linearly with the dose, a t a rate which is unaffected by the accumulation of these products. The early work of Rollefson and Libby3 indicated that little or no exchange occurs when solutions of tagged CI, (37-minute half-life) in CC14 are illuminated with a quartz mercury arc. In the present study, in which C1236(4 X 105 year halflife) was dissolved in CCla, it was found that exchange does take place a t measurable rates in the dark, in sunlight and when samples are exposed to ultraviolet light or y-radiation. This disagreement may be attributed to: incomplete purification of CCld by Rollefson and Libby3 (ie., the C1 atoms formed may have reacted with impurities to give inorganic products); the difference in exciting radiation ( light used in the present study included light of shorter wave length) ; and possibly the low-precision counting equipment then available may have obscured the results which were observed by this author. Seely and Willard4 observed that Brz and CzBr6 are formed in CBr4 illuminated at 90” with light of 4100 and 4500 A. By using radioactive Br2, they found that the photo-activated exchange of free Br2 with the Br in CBr4 is much greater than the quantum yield for the formation of Brz and CzBre. Other workers6p6have measured the thermal and photochemical exchanges between Brz and CCLBr.
Consequently, the vacuum system contained two liquid nitrogen cold traps between the oil diffusion pump and the section of the manifold where the samples were prepared. Two stopcocks (in series), located between the diffusion pump and the manifold, remained closed when the vacuum system was not in operation. Since usually negligible quantities of HCl were observed in the samples, it appears that contamination by hydrocarbons was held to a minimum. About 35 ml. of CClr and 1-3 ml. of stock solution were taken for each set of five samples. These solutions were poured into a 50-ml. round-bottom flask having a standard taper joint for connection to the vacuum line. About 1 g. of PZOSwas added t o prevent traces of moisture from being distilled into the sample tube; a glass wool plug was inserted in the neck to keep the PzOs in the flask during distillation. The CCL-CI2 solution was frozen with liquid Nz and pumped down to a pressure of
