Kinetics and mechanisms of the reactions of chromium(III) and heptane-3,5-dione in aqueous solution

Abstract

The equilibra and kinetics of mono-complex formation of chromium(III) with heptane-3,5-dione have been investigated spectrophotometrically in aqueous solution at 50 °C and 0.5 mol dm^–3 NaClO₄. The mechanism proposed to account for the kinetic data involves pathways in which both Cr³⁺ and Cr(OH)²⁺ react with the enol tautomer of the ligand with rate constants of 6.60 × 10^–2 dm³ mol^–1 s^–1 and 1.42 dm³ mol^–1 s^–1, respectively. By comparing rate laws derived on the basis of the theory of coupled sets of first-order reactions with rate laws derived using the steady-state approximation, a useful confirmation of the validity of this mechanism has been provided. The assumptions made in the steady-state approach that the keto–enol equilibrium is maintained and that the protolytic reactions are rapid compared with complex formation are valid.