The kinetics and mechanism of the hydrolysis of p-nitrophenyl isothiocyanate promoted by soft metal ions

Abstract

The hydrolysis of p-nitrophenyl isothiocyanate in aqueous acid solution is powerfully promoted by the Ag⁺ and hg²⁺ ions. The organic product is p-nitroaniline. The mercury-promoted reaction is first order in isothiocyanate and in [Hg²⁺], and has ΔH^‡= 56 ± 2 kJ mol^–1, ΔS^‡=–53 ± 4 J K^–1 mol^–1, and k_H2O/k_D2O= 1.12 ± 0.05 at 25.0 °C. The behaviour with silver is similar: promotion is principally first order in [Ag⁺], but in H₂O there is evidence for a small kinetic term in [Ag⁺]². ΔH^‡= 59 ± 2 kJ mol^–1 and ΔS^‡=–88 ± 3 J K^–1 mol^–1 for the first-order reaction, and ΔH^‡= 25 ± 4 kJ mol^–1 and ΔS^‡=–202 ± 12 J K^–1 mol^–1 for the route second order in [Ag⁺]. Use Of D₂O removes the second-order term, and k_H2O/k_D2O= 1.07 ± 0.06 for the route involving one Ag⁺ ion. A mechanism of hydrolysis is suggestea that involves an isothiocyanate–metal ion pre-equilibrium, followed by a rate-determining attack of water to give the soft metal derivative of the corresponding thiocarbamic acid as a rapidly decomposing intermediate.