Kinetics and Mechanism of Hexachloroplatinate(IV) Reduction by Some Neutralized α-Hydroxy Acids in a Carbonate-Hydrogencarbonate Buffer Medium

Abstract

Abstract The kinetics of hexachloroplatinate(IV) reduction by some neutralized α-hydroxy acids such as glycolic, lactic, α-hydroxyisobutyric, mandelic, atrolactic, and benzilic acids in a carbonate-hydrogencarbonate buffer medium have been investigated. Platinum(IV) is reduced by the substrates to platinum(II) in a one-step two-electron process, whereas the substrates are oxidized to give formaldehyde, acetaldehyde, acetone, benzaldehyde, acetophenone, and benzophenone for the respective reactions. The pseudo-first-order rate constant is independent of the initial [platinum(IV)] as well as [OH-]. The reaction rate increases with increasing [substrate], but decreases with increasing chloride concentration. The reactions obey the following rate expression: −d[PtIV]t/dt=(kKe[A−][PtIV]t )/([Cl−]+Ke[A−])· The reactions proceed through an initial 1 : 1 complex formation between the reactants, followed by decomposition of the complex to give the respective reaction products via C-C bond cleavage. The reactivity of the α-hydroxycarboxylate towards Pt(IV) are as follows: atrolactic acid > mandelic acid > benzilic acid > α–hydroxyisobutyric acid > lactic acid > glycolic acid. Thermodynamic parameters for the decomposition step have been evaluated. The mechanism of the reactions is discussed.