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Published November 1990 | public
Journal Article

Oscillatory oxidation of benzaldehyde by air. I. Experimental observations


In 90% aqueous acetic acid at 70 °C, the oxidation of benzaldehyde to benzoic acid by air is catalyzed by a mixture of cobalt(II) and bromide. Jensen has shown that the concentration of cobalt(III) undergoes major oscillations during this reaction. Consistent with these prior observations, we find that the rate of formation of cobalt(III) increases exponentially until at about 10^(-5) M s^(-1) it becomes comparable to the maximum rate at which O_2 can be transported from atmosphere to solution. A virtually discontinuous change of behavior then causes the concentration of cobalt(III) to decrease at about 10^(-4) M s^(-1); the rate of this decrease is almost constant and independent of [Co^(III)] until most of this species has been consumed. We have examined most of the subsystems that we could prepare and study independently. Among other observations, we find that increasing acidity increases the rate at which bromide catalyzes the oxidation of benzaldehyde by cobalt(III). We also find that free Br_2 is formed and can be entrained by gas flow. We propose a detailed molecular mechanism for the oscillations that seems to be consistent with most of what we know about the behavior of the subsystems, and we have initiated modeling computations with encouraging results. We also suggest additional experiments and computations that could further test the validity of our proposed mechanism.

Additional Information

© 1990 American Chemical Society. Received February 22, 1990. Revised Manuscript Received July 3, 1990. Based in part on the Ph.D. thesis of Zhi Yuan, University Oregon, August 1987. This research was supported in part by Grants No. CHE8405518 and 8717791 from the National Science Foundation to the University of Oregon. A.J.C. was on leave from the Department of Chemistry, University of Mar del Plata and CONICET, Argentina. E.G. was on leave from the Department of Chemistry, University of Mar del Plata and CIC, Argentina. We are particularly indebted to Drs. James Jensen, Mark Roelofs, and Edel Wasserman of the du Pont Company who encouraged us to undertake an independent study of the reaction that they had discovered and who provided us with results of their own further investigations. Dr. John Guslander of the University of Oregon provided much assistance during efforts (not reported here) to model various hypothetical mechanisms. He has also developed a skeleton mechanism with only three independent variables that reproduces some (but not all) of the features of this system and that is of mathematical interest in its own right. That work will be reported elsewhere.

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