Kinetics and mechanism of the oxidation of iminodiacetate, nitrilotriacetate and ethylenediaminetetraacetate by trans-cyclohexane-1,2-diamine-N,N,N′,N′-tetraacetatomanganate(III) in aqueous media

Abstract

Kinetic studies of the oxidation of iminodiacetate (ida), nitrilotriacetate (nta) and ethylene-diaminetetraacetate (edta) by trans-cyclohexane-1,2-diamine-N,N,N′,N′-tetraacetatomanganate(III), [Mn^III(cdta)]^–, have been made in aqueous solution in the range pH 3.0–10.0 with varying reductant concentrations at constant ionic strength, I= 0.50 mol dm^–3(NaClO₄), and temperature, 30°C. All the reactions are first order both in complex and reductant concentration, and follow the general rate law –d[Mn^III]/dt=k_obs[Mn^III]=(k_d+k_s[R])[Mn^III], where k_d denotes the autodecomposition rate of the complex, k_s the electron-transfer rate and R is the reductant irrespective of the nature and type of the reacting species. The complex [Mn^III(cdta)]^– showed an interesting behaviour in acidic and alkaline media. For ida oxidation both the aqua- and hydroxo-forms of the complex are reactive and an inner-sphere mechanism has been proposed. However, for nta and edta oxidations, the aqua form is the sole reacting species and an outer-sphere mechanism has been proposed. The rate parameters and proton equilibrium constants of the complex and reductants were obtained by fitting of the experimental data by appropriate rate equations using computer-fit programs. Thus the reactivities of all the species of the polycarboxylates available in the reaction solutions have been evaluated individually. The reactivity orders are Hida^– < ida^2–, H₂nta^– < Hnta^2– < nta^3– and H₃edta^– < H₂edta^2– < Hedta^3– < edta^4–.