Kinetics and mechanisms of oxidation by metalions. Part XI(1). Kinetics of the osmium(VIII)-catalysed oxidation of glycols by alkaline hexacyanoferrate(III)

Summary The kinetics of the Os^VIII-catalysed oxidation of glycols by alkaline hexacyanoferrate(III) ion exhibits zerothorder dependence in Fe(CN) ₆ ^3– ] 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 Os^VIII-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 Os^VIII-complexes, indicates that the two complexes are structurally different because S changes from the negative (corresponding to k₁₁) to positive (related to k₂).