THE KINETICS AND MECHANISM OF THE HYDROLYSIS OF PARA-NITROPHENYL ISOTHIOCYANATE PROMOTED BY SOFT METAL-IONS

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 DELTA-H double-ended-dagger = 56 +/- 2 kJ mol-1, DELTA-S double-ended-dagger = -53 +/- 4 J K-1 mol-1, and k(H2O)/k(D2O) = 1.12 +/- 0.05 at 25.0-degrees-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. DELTA-H double-ended-dagger = 59 +/- 2 kJ mol-1 and DELTA-S double-ended-dagger = -88 +/- 3 J K-1 mol-1 for the first-order reaction, and DELTA-H double-ended-dagger = 25 +/- 4 kJ mol-1 and DELTA-S double-ended-dagger = -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 suggested 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.