Conclusive evidence for autocatalytic behaviour of manganese(II) ions in the oxidative degradation of ondansetron by permanganate in aqueous sulfuric acid medium – a kinetic and mechanistic approach

•First conclusive evidence for an autocatalytic oxidation of ondansetron was given.•The ondansetron and permanganate reactions are autocatalysed by Mn(II) product.•The choice of initial rates are made due to autocatalytic nature of the reaction.•The oxidation products are different from those obtained by hepatic metabolism.•Negative temperature dependence of rate is due to pre-equilibrium steps (Scheme 2).

The oxidation of ondansetron by permanganate in aqueous sulfuric acid medium has been investigated spectrophotometrically under the pseudo-first order condition at 25 °C and at constant ionic strength, I = 0.60 mol dm−3. The reaction between ondansetron and permanganate in acid medium exhibits 2:1 stoichiometry. The products are identified and characterised by IR and GC–MS spectral studies. The identified oxidation products are 1-methyl-2-((E)-4-(2-methyl-1H-imidazol-1-yl)but-3-enyl)-1H-indole-3-carboxylic acid and Mn(II), which are different from those obtained by hepatic metabolism. The reaction is autocatalysed due to one of the product, Mn(II). The reaction is first order with respect to MnO4− and less than unit order with respect to ondansetron, acid and Mn(II) concentrations. The oxidation reaction in acid medium proceeds through a permanganate–ondansetron complex that decomposes slowly in a rate-determining step followed by other fast steps to give the products. The reaction constants involved in different steps of the mechanism are calculated at different temperatures. The activation parameters with respect to the slow step of the mechanism are computed, and thermodynamic quantities are also determined.

Graphical abstractThe characteristic sigmoid profile showing autocatalytic behaviour of manganese(II) ions in the oxidative degradation of ondansetron by permanganate in aqueous sulfuric acid medium.Figure optionsDownload full-size imageDownload as PowerPoint slide