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 kH2O/kD2O= 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 kH2O/kD2O= 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.