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The oxidation of lactic acid (H2L) by chromium(VI) has been found to proceed in two measurable steps, both of which give pyruvic acid as the primary product: 2Cr(VI) + 2CH3CHOHCOOH 1Í4-2CH3COCOOH + Cr(V) + Cr(III); Cr(V) + CH3CHO-HCOOH ii-Cr(III) + CH3COCOOH. Buildup and decay of chromium(V) intermediates accompany the decay of chromium(VI).Analysis for the intermediate [Cr(V)] in runs with excess lactic acid gives first-order rate constants: 2k6 = [H2L](fc0 + feH [H+] + ¿L[H2L]), with k0 = 0.0007 M"1 11s"1, kH = 0.013 M"2 s"1 and kL = 0.0021 M"2 s"1 at 298 K; k5 = [H2L](fcA + fcB[H+]2), with kA = 0.0023 M"1 s"' and ZcB = 0.027 M"3 s"1 at 298 K. Activated states and mechanisms have been deduced for each step and compared with similar oxidations of other -hydroxy acids and oxalic acid and cooxidations of oxalic acid and alcohols. The significant buildup of chromium(V) has consequences for the interpretation of many previously studied chromium(VI) oxidations.1

Attempts to rationalize the kinetics of cytochrome c oxidation catalyzed by solubilized mitochondrial cytochrome c oxidase (ferrocytochrome c-.oxygen oxidoreductase, EC 1.9.3.1) have been based on assumptions of productive complex formation (Michaelis-Menten approach). However, the range of substrate concentrations used has not, in general, been sufficient to establish a general rate equation. Data adequate to derive such a rate expression are presented, as well as a method for estimation of constants which appear in the rate law deduced and reported herein. It is shown that either of two types of mechanisms, one assuming productive complex formation, as opposed to the other postulating dead-end complex formation, accurately predict the rate equation as deduced from experiment.

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