Oxidation of benzenediols by hexabromoiridate(IV): kinetics at ambient andelevated pressures

The kinetics of oxidation of several benzenediols by the hexabromoiridate(I V) 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 degrees C the second-order rate constants (the reaction is first order in each reactant concentration), vary from 1.26 x 10(2) to 9.3 x 10(4 ) dm(3) 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 f aster reactions. The Delta S double dagger values do not vary over a wide r ange; the reaction rates are governed more by the enthalpy barrier. Applica tion of pressure (up to 125 MPa) causes significant rate accelerations, giv ing rise to Delta V double dagger values in the -17 to -26 cm(3) mol(-1) ra nge, consistent with the large, negative Delta S double dagger values. This indicates that the rate limiting step is largely characterised by an incre ase in species ordering and electrostriction, and in the present case sligh tly less than for the corresponding reactions with the less bulky hexachlor oiridate(IV) ion.