Oxygen exchange between iodate and water: catalysis by hydrogen and hydroxide lons

Abstract

The 18O-exchange reaction between iodate ions and water has been investigated using a rapid chemical-quenching technique. Over the range 2.1 < pH < 12.5 the exchange occurs by hydrogen- and hydroxide-ion catalysed paths according to the rate law (i). At 5 ° C and I= 1.0 mol dm–3(Li[ClO4]) the rate constants are k0 < 0.01 s–1, R/[IO3–]=k0+k1[H+]+k2[H+]2+k3[OH–](i)k1=(3.20 ± 0.11)× 103 dm3 mol–1 s–1, k2=(1.38 ± 0.24)× 105 dm6 mol–2 s–1 and k3=(2.49 ± 0.16)× 102 dm3 mol–1 s–1 respectively. Activation parameters from data at 5–25 °C are ΔH1‡= 61.5 ± 3.8 kJ mol–1,ΔS1‡= 43.1 ± 13.0 J K–1 mol–1, ΔH3‡= 47.4 ± 1.7 kJ mol–1, and ΔS3‡=–28.3 ± 5.9 J K–1 mol–1. The following values for the kinetic solvent deuterium-isotope effect were obtained (5 °C) : klH/klD= 2.1, k2H/k2D= 0.49, and k3H/k3D= 1.7. The normal isotope effect suggests proton transfer as the rate-determining steps corresponding to k1 and k3, whereas the inverse isotope effect on k2 can be explained by a rapid acid–base pre-equilibrium. These results provide further evidence for associative substitution mechanisms with iodine(V).