Oxidation of benzenediols by hexabromoiridate(IV): kinetics at ambient and elevated pressures†

Abstract

The kinetics of oxidation of several benzenediols by the hexabromoiridate(IV) ion have been studied spectrophotometrically by the stopped-flow method. In 0.010 mol dm^–3 HClO₄ and an ionic strength of 0.10 mol dm^–3 (NaClO₄) at 25.0 °C the second-order rate constants (the reaction is first order in each reactant concentration), vary from 1.26 × 10² to 9.3 × 10⁴ dm³ mol^–1 s^–1. The enthalpies of activation range from about 44 kJ mol^–1 for the slowest reacting substrate to about 20 kJ mol^–1 for the faster reactions. The ΔS ^‡ values do not vary over a wide range; the reaction rates are governed more by the enthalpy barrier. Application of pressure (up to 125 MPa) causes significant rate accelerations, giving rise to ΔV ^‡ values in the –17 to –26 cm³ mol^–1 range, consistent with the large, negative ΔS ^‡ values. This indicates that the rate limiting step is largely characterised by an increase in species ordering and electrostriction, and in the present case slightly less than for the corresponding reactions with the less bulky hexachloroiridate(IV) ion.