Oscillatory oxidation of benzaldehyde by air. I. Experimental observations

Abstract

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 details