Kinetics and mechanisms of oxidation by metalions. Part XI(1). Kinetics of the osmium(VIII)-catalysed oxidation of glycols by alkaline hexacyanoferrate(III) |
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Authors: | Ramesh C. Kapoor Raj N. Mehrotra Shared K. Vajpai Purnima Chaudhary |
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Affiliation: | (1) Department of Chemistry, University of Jodhpur, 242001 Jodhpur, India;(2) Department of Chemistry, Kumaun University, 263002 Naini Tal, India;(3) Department of Biochemistry, V A Medical Centre, 4801 Linwood Boulevard, 64128 Kansas City, MO, USA |
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Abstract: | Summary The kinetics of the OsVIII-catalysed oxidation of glycols by alkaline hexacyanoferrate(III) ion exhibits zerothorder dependence in [Fe(CN)63–] and first-order dependence in [OsO4]. The order with respect to glycols is less than unity, whereas the rate dependence on [OH–] is a combination of two rate constants; one independent of and the other first-order in [OH–]. These observations are commensurate with a mechanism in which two complexes, [OsO4(H2O)G]– and [OsO4(OH)G]2–, are formed either from [OsO4(H2O)(OH)]– or [OsO4(OH)2]2– and the glycol GH, or by [OsO4(H2O)2] and [OsO4(H2O)(OH)]– and the glycolate ion, G–, which is in equilibrium with the glycol GH through the reaction between GH and OH–. Hence there is an ambiguity about the true path for the formation of the two OsVIII-glycol complexes. A reversal in the reactivity order of glycols in the two rate-determining steps, despite the common attack of OH– ion on the two species of OsVIII-complexes, indicates that the two complexes are structurally different because S changes from the negative (corresponding to k1 1) to positive (related to k2). |
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