2185
COMMUNICATIONS TO THE EDITOR shown in Figure 1. This demonstrates that propane 1,2-dz (CH2D'CHD .CHa) is predominantly formed at the beginning of the reaction. These results indicate that propme-ds is formed by the simple addition of deuterium to the double bond of normal propylene or to the propyl group in the surface. Consequently, it is concluded that the hydrogenation and isomerization proceed through different reaction intermediates over zinc oxide. Acknowledgment. The authors wish to thank Dr. Shuji Saito for his continuous help during the measurement of microwave spectra. SAQAMI CHEMICAL RESEARCH CENTER SHUICHINAITO* KONDO OHNUMA, SAQAMIHARA, KANAQAWA-KEN TOSHIHIKO JAPAN MASARU ICHIKAWA DEPARTMENT OF CHEMISTRY THEUNIVERSITY OF TOKYO HONQO, BUNKYO-KU, TOKYO, JAPAN RECEIVED MARCH16, 1972
KENZITAMARU 4
TIME
(MINI
Figure 1. Transmission changes in a solution containing 2,4-pentanedione (0.05 M), MnS04 (0.017 M ) , and KBrOa (0.07 M ) in 1 M HzS04at 25", 8-om light path. Magnetic stirring was stopped a t A and restarted at B.
Kinetic Oscillations in the Oxidation of 2,4-Pentanedione by Bromate Ion, Catalyzed by Manganese(I1) Publication costs borne completely by The Journal of Physical Chemistry
Sir: The increasing interest which is currently being shown in the oscillatory kinetics of Belousov's reaction (bromate oxidation of malonic acid)'-& prompts us to report our observations on a related system in which the organic acid is replaced by 2,4-pentanedionea Several novel features make the diketone oxidation worthy of attention, particularly since in constrast to both the malonate-bromate reaction and the well-known hydrogen peroxide-iodate reaction, no permanent gas is evolved, so that experiments can be conducted in a materially (if not thermally) "closed" situation. Our chief findings may be most succinctly summarized as follows. (i) I n acid solution, with Mn(I1) present, the bromate ion oxidation of 2,4-pentanedione proceeds in a periodic fashion. Fluctuations in the absorbance of the solution begin to build up immediately on mixing the reagents, as Figure &shawa+with only a very brief induction period. (ii) The reaction is affected by stirring in the sense that the oscillations damp away more rapidly if stirring is discontinued. We originally thought it possible that dissolved oxygen could be responsible for this effect in a system which was open to the atmosphere. However, a stirring effect exactly similar to that shown in Figure 1 was observed in a sealed system where the components had been rigorously degassed by standard
X
(nml
Figure 2. Spectra of the transient intermediates, obtained by the flow method. The initial concentrations after mixing for KBr08, MnSOd, and 2,4-pentanedione were (A) 0.052, 0.037, 0.022 M and (B) 0.30, 0.42, and 0.025 M, respectively, in 1.5 M HZSO4.
freeze-thaw-pump methods under high-vacuum conditions before mixing. We therefore conclude that the mechanism of this reaction includes at least one heterogeneous step which involves neither gas evolution nor the presence of atmospheric oxygen. The origin of the heterogeneity may be similar to that suggested by Kasparek and Bruicel for the cerium-catalyzed malonate-bromate reaction, namely transient formation (1) G.J. Kasparek and T. C. Bruice, Znorg. Chem., 10, 382 (1971). (2) A. N. Zaikin and A. M. Zhabortinski, Nature (London) 225, 535 (1970). (3) R. M.Noyes, R. J. Field, and E. Koros, J. Amer. Chem. Soc., 94,1394(1972). The Journal of Phu,aical Chemistry, Vol. 76,No. 16, 197.9
COMMUNICATIONS TO THE EDITOR
2186
of a colloidal form of a complex of the catalyst ion. In addition, the temperature fluctuations recently found in reactions of this type4raise the possibility that the systems are thermally (rather than chemically) heterogeneous. (iii) Since there is no well-defined induction period, a simple flow method was used to obtain spectra of the colored intermediates. In these experiments, the components, after mixing, entered a flow-through cuvette in the sample compartment of a spectrophotometer. The two typical spectra shown in Figure 2 were obtained with a constant flow rate such that the reaction was monitored close to the maximum of the second or third oscillation. As Figure 2 shows, the color changes appear to be the result of two broad bands with maxima at 410 nm (yellow intermediate) and 475 nm (pink intermediate), respectively, which oscillate in phase but
The Journal of Physical chemistry, Vol. Y6,No. 16, 19YP
with relative intensities which depend on initial concentrations of the reactants. The nature of the intermediates responsible for these spectra has not yet been established, although the position and shape of the pink band correspond closely to the spectrum of Mn(II1) a t the same acid concentration. More detailed studies of this system, including characterization of the organic products, are continuing in this laboratory.
Acknowledgment. This work was supported by the Simmons College Fund for Research. U.Franck and W. Geiaeler, Naturwkenschaften, 58, 62 (1971). DEPARTMENT OF CHEMISTRY PETER G . BOWERS* SIMMONS COLLEQE KARENE. CALDWELL 02115 DEBORAH F. PRENDERGAST BOSTON,MASSACHUSETTS RECEIVED APRIL12, 1972 (4)
