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) : k_l^H/k_l^D= 2.1, k₂^H/k₂^D= 0.49, and k₃^H/k₃^D= 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).