Kinetics of the oxidation of formate by iodine in the water-methanol and the water-ethanol mixed solvents

The kinetics of the iodine-formate reaction gives an identical rate equation in several water-alcohol mixed solvents: rate=k[HCOO-] [I3-]/(a[I-] + 1). The values of k/M-1 s-1 and 10-3 a/M-1 are 5.5 and 16 in ethanol at 350C, 3.6 and 12 in methanol at 300C, and 0.06 and 0.7 in water at 300C, respectively. The values of a are comparable to the formation constants of I3- in the water-alcohol mixtures. The rate-determining step is the reaction between an iodine molecule and a formate ion. The activation parameters, ΔHsup(not equal)/kJ mol-1 and ΔSsup(not equal)/JK-1 mol-1, are 71 and -1 in the 0.95 mole fraction of methanol, 60 and -32 in the 0.92 mole fraction of ethanol, and 75 and -20 in water, respectively. The entropy term contributes to the acceleration by alcohol in the water-methanol mixtures, whereas both the entropy term and the enthalpy term contribute to the acceleration in the water-ethanol mixtures. These effects of the mixed solvents suggest that the specific interactions of the solvent molecule with the reactants and the activated complex, such as a specific solvation or a hydrogen bond, play an important role in the rate-determining step. (auth.)