762
NOTES
NiC12, and chloroacetic acid on the Hz yield in ethanol. Their results show curves which are similar to those in Figures 1 4 . They interpreted these in terms of G(e-80,v)fl= 0.90 f 0.10 and the scavenging of electrons in the spurs to give G(e-),, = 0.75 f 0.10. Thus they found a total of G(e-)T = 1.65 f 0.2. The present results and calculations are consistent with G(e-molv)fl= 1.2, G(e-solv)gr = 1.9, and G(e-solv)T = 3.1. The fact that a much higher value for the total yield of solvated electrons was found in the present work may indicate that the rates of reaction of the solutes used by Hayon and Moreau with the solvated electrons in ethanol are lower than they presumed. By comparison with the results of Adams and Sedgewick for the effect of acidJZathey concluded that increases in over-all yield on addition of some of the scavengers were due to scavenging of solvated electrons in the spurs. It is felt that an incomplete understanding of the true effects of acid on the radiolysis may have resulted in an incorrect comparison. There is ail apparent disagreement between the proposed mechanism and the previously accepted2,?, l7 yield of free-ion solvated electrons in y irradiated ethanol. [G(free ion) = 0.9 0.1, compared to the present estimate of 1 2.1 Although the present estimate might be too high, it is quite possible that the previous estimates are too low because of the lack of good plateaus in the scavenger plots (see Figure 2 as well as ref 2 and 3). I n effect, the concentration of scavenger required to react with all free-ion solvated electrons is not radically different frorn that which will begin to react in the
*
spurs. In comparison, the case of water is quite difO 16 4 l - l sec-I has been ferent. A value of / C ~ , ~ - + H ~ = quoted.18 This corresponds to a half-life of the order of 1 msec. Thus in very pure water the solvated electron has a long lifetime and may be scavenged efficiently by a low concentration of scavenger. This suggests why very good scavenger plots may be obtained in aqueous systems and not in ethanolic ones. Taub, et ul.," estimated that G(e-,"Iv)fi = 1.0 f 0.3, which is consistent with the present value of 1.2. In conclusion, the reaction mechanism advanced may be used to explain the yields of primary reducing species and their variation under differing conditions. The simple mechanism for nonhomogeneous kinetics is as successful in treating the results as the limitations of the method could permit. This agreement must be consideied further evidence in favor of the theory.* The shape of the scavenp ,. curves is a function of the lifetime of the free-ion solvated electrons in the medium. The lifetime of free-ion solvated electrons in ethanol appears to be approximately 3-7 psec compared to 1 msec in water. The existence of four distinguishable hydrogen precursors has been demonstrated. Species Y might be an excited molecule, but the identity of species X is unknown.
(17) I. A. Taub, D. A. Harter, M. C. Sauer, and L. M. Dorfman, J . Chem. Phys., 41, 979 (1964). (18) E. I. Hart, quoted by J. Rabani in "Solvated Electron," Advances in Chemistry Series, No. 50, American Chemical Society, Washington, D. C., 1965, p 251.
NOTES
Disproportionation and Combination Reactions of Isopropoxy Radicals with Nitric Oxide
For the isopropoxy-nitric oxide pair, k l / k 2 was found to be 0.15 at 26" and 0.20 a t 77" from results of photoly(CHs)&HO
by Barbara E. Ludwig and G. R. McMillan Department of Chemistry, Western Reserve University, Cleueland, Ohio 4.4106 (Received August 1, 1966)
Meager information is available about the temperature dependence of disproportionation and combination reactions of simple radica,ls other than the alkyls. The Journal of Physical Chemistry
+ NO +CH3COCHS + HNO 4(CH3)zCHOKO
(1) (2)
sis of diisopropyl peroxide in the presence of nitric oxide.' The data showed considerable scatter, especially a t 77", and we agree with Arden, Phillips, and Shaw2 (1) G. R. MclClillan, J . Am. Chem. Sac., 83, 3018 (1961). (2) E. A. Arden, L. Phillips, and R. Shaw, J. Chem. Soc., 5126 (1964).
NOTES
763
~
Table I : Pyrolysis of Diisopropyl Peroxide-Nitric Oxide Mixtures, Typical Data a t 120' -Pressures, [(CHdzCHOIz
4.5 6.0 6.4 7.3 10.1 13.2 a
Of peroxide.
mm-
NO
70 decompna
(CHa)zCHONO
Yield, molecules X 10-18 (CHa)zCO
(CHa)zCHOH
(CHa)*CO*/ (CHs)zCHONO
22.4 40.7 26.9 2.8 38.6 41.4
7.5 4.3 3.0 5.6 4.5 2.2
3.11 1.87 1.14 3.94 4.68 2.24
0.702 0.470 0.474 1.11 1.28 0.575
0.570 0.429 0.482 0.980 0.826 0.479
0.191 0.204 0 .295c 0.225 0.232 0.211
* Corrected, see text.
Packed cell.
that the ratios obtained at the two temperatures are identical within experimental uncertainty. The ratio of rate constants for disproportionation to combination of ethoxy radicals with nitric oxide has been reported to display negligible temperature dependence, the values being 0.30 at 95" and 0.31 at 135°.2 Because these constants are important for interpretation of the primary photochemistry of alkyl nitrites, nitrates, and nitro compounds, the isopropoxy-nitric oxide reaction was studied over a wide temperature range. Diisopropyl peroxide1j3was purified by fractional distillation and gas chromatography. Even with careful exclusion of room light,' it was impossible to reduce the acetone content of the samples below about 0.05%. Mixtures of the peroxide vapor with nitric oxide were pyrolyzed in quartz vessels of approximately 200 ml volume. A cell was packed with quartz tubing to increase the surface:volume ratio by a factor of 9. Unreacted nitric oxide and products volatile a t - 160" were pumped away, and the condensable fraction was analyzed by gas chromatography at 32" on a 12-ft column of tricresyl phosphate on Chromosorb W. The main products found were isopropyl nitrite, acetone, and isopropyl alcohol, with traces of acetaldehyde and me1hyl nitrite. Results are shown in Table I for some of the 120" experiments. The isopropyl alcohol is probably formed in large part by hydrolysis of isopropyl nitrite and was therefore added as a correction to the isopropyl nitrite yield before calculation of the last column in Table I. The isopropyl alcohol yield at a given temperature did not depend on the variables discussed below. Some hydrolysis of alkyl nitrites always is observed on the columns used in this study. I n one experiment, a small amount of t-pentyl nitrite was mixed with peroxide and nitric oxide and the mixture was pyrolyzed at 120" in the usual manner. The gas chromatogram showed considerable t-pentyl alcohol. Over the ranges indicated, the following variables were without effect on the acetone: isopropyl nitrite ratio : pressure of diisopropyl peroxide, 2.7-14.6
100
120
140 T, 'C.
160
180
Figure 1. Temperature dependence of acetone: isopropyl nitrite ratio.
mm; pressure of nitric oxide, 2.8-46.9 mm; pressure of nitric oxide: pressure of peroxide, 0.35-8.58; per cent decomposition of peroxide 2.0-12.1%; time of reaction, 0.7-1440 min; added nitrogen, 200 mm. The effect of teinperature is shown in Figure 1 for temperatures at which secondary pyrolysis of isopropyl nitrite is negligible as shown by experiments on isopropyl nitrite-nitric oxide mixtures. The mechanism of thermal decomposition of diethyl peroxide in the presence of nitric oxide has been discussed by Arden, Phillips, and Shaw.2 The ratio of rate constants for disproportion to combination of ethoxy radicals with nitric oxide was calculated simply as the ratio of yields of the disproportionation and combination products. I n the present system such a calculation is not generally valid. Figure l indicates a change in mechanism in the temperature range studied. Some side process, which the packed-cell results suggest (3) W. A. Pryor, D. & Huston, I. T. R. Fiske, T. L. Pickering, and E. Ciuffarin, J. A m . Chem. Soc., 86, 4237 (1964).
Volume 71, Number 3 February 1967
764
YOTES
to be a surface reaction, makes a temperature-dependent and M-MX3, where JrT represents alkali, alkaline earth contribution to the product ratio below 150". At and rare earth metals, and X represents halide ions. higher temperatures, this process is apparently overThis report on the phase equilibria in the KzS-K system deals with an extension of the work to the first whelmed, for the acetone to isopropyl nitrite ratio representative of systems hrT-nIzY, where Y is a doubly becomes temperature independent above about 160". This constancy together with independence of the ratio charged chalcogenide ion, and where the anion-toof the other variables suggests the ratio be identified cation ratio is as much as a factor of 6 smaller than in the previous other extreme, the I\rT-,lIXa systems. with kl/kz. The value thus obtained, 0.17, is the same as the grand average of low-temperature results (26The significance of the stoichiometry for the electrical conductivity, i e . , for the mobility of the metal elec79") , 1 The disproportionation :combination ratio may therefore be taken as 0.17 over the range 26-180". trons, was discussed earlier.3 The isopropoxy-nitric oxide pair is evidently similar to The KzS was prepared by slowly adding triply subalkyl-alkyl reactions in showing an inappreciable diflimed, carbon-free sulfur to excess potassium in an ference in activation energies between the two procinert atmosphere at a controlled temperature of 180esses. 200". The excess potassium was then removed by As has been pointed out,2 a value of 0.17 for k l / k 2 a t distillation at 300-350" under high vacuum. The final 120" would fit well with a correlation showing the disproduct was analyzed for K, S, and 0 and found to be proportionation to combination ratios for alkoxy better than 99% IGS with the major impurities being radicals and nitric oxide to be proportional to the I< and KzO. The potassium contained less than 0.02% number of available hydrogen atoms, in contrast to of other cations. All handling operations were perresults on alkyl-alkyl reactions. Such a correlation formed under helium in a drybox. would not be expected from data on reactions of alkoxy The phase diagram was determined by thermal radicals with methyl radicals, which indicate that analysis (cooling curves). Inconel capsules with isopropoxy disproportionates with methyl about twice thermocouple wells were used as cor tainers and a Ptas fast relative to combination as does m e t h ~ x y . ~ Pt-lO% Rh thermocouple was used in conjunction with a Rubicon potentiometer acd a Brown recorder Furthermore, a referee has pointed out that the corto measure temperatures. The Inconel capsules were relation would be significant only if all the alkoxynitric oxide disproportionation :combination ratios were sealed under helium in a welding drybox. A rocking temperature independent, and this must be considered furnace permitted mixing of the components before questionable in view of the apparent absence of diseach cooling curve was run. proportionation at 25" in the reaction of methoxy and The K-KzS phase diagram is shown in Figure 1. ethoxy radicals with nitric oxide The melting point T , of 948" is considerably higher Acknowledgment. Grateful acknowledgment is made than the value of 912" reported in the l i t e r a t ~ r e . ~ However, the latter value mas determined with KzS of support of this work by the Div'sion of Air Pollution, Bureau of State Services, U. S. Public Health Service. which contained at least 5% impurity. The critical solution temperature T , lies at about goo", or about 50" (4) Summarized in G. R. McMillan and J. G. Calvert, "Oxidation below the melting point of the KzS. The monotectic and Combustion Reviews," Vol. I, C. F. H. Tipper, Ed., Elsevier T,,, is 883" and the monotectic concentratemperature Publishing Co., Amsterdam, 1965,p 123. ( 5 ) A. R. Knight and H. E. Gunning, Can. J . Chem., 39, 1231,2466 tion is 21 mole % K. The K2S-K system thus resembles (1961). the KBr-K system except that T , for the former lies about 180" higher. The end of the miscibility gap at 63 mole % K is somewhat uncertain due to the very rapid increase in Miscibility of Metals with Salts. VII. The the sulfide solubility in the liquid metal just below the Potassium-Potassium Sulfide System' monotectic temperature. This rapid increase is, of by A. S. Dworkin and M. A. Bredig Oak Ridge National Laboratory, Oak Ridge, Tennessee (Received September 8, 1966)
so1utions2 Our previous studies Of have dealt with systems of the type M-RIX, M-RIIX2, The Journal of Physical Chemistry
(1) Research sponsored by the U. S. Atomic Energy Commission under contract with the Union Carbide Corp. (2) M. A. Bredig, "Mixtures of Metals with Molten Salts," in "Molten Salt Chemistry," 11.Blander, Ed., Interscience Publishers, Inc., New York, N. Y.,1964. (3) M. A. Bredig, J . Chem. Phys., 37, 914 (1962). (4) J. Goubeau, H. Kolb, and H. G. Krall, 2. Anorg. Allgem. Chem., m , 4 5 (1938).
