Kinetic investigation on the hydrolysis of aryl(fluoro)(phenyl)-lambda(6)-sulfanenitriles

A kinetic investigation on the hydrolysis of aryl(fluoro)(phenyl)-lambda (6 )-sulfanenitriles was carried out in some aqueous and mixed aqueous-organic solutions. The pH-rate profiles showed that the hydrolysis consists of pH- independent, acid-catalyzed and base-catalyzed reactions. The neutral hydro lysis of fluoro-lambda (6)-sulfanenitriles was found to proceed via an S(N) 1 or an S-nitrilosulfonium cation-like transition state, which is character ized by a large negative Hammett rho -value (rho = -1.76 in water; -1.85 in CH3CN/H2O(1/4, v/v); -2.35 in TFE/H2O (1/1, v/v)), relatively large m-valu es (ca 0.83 for fluoro(diphenyl)-lambda (6)-sulfanenitrile; ca. 0.82 for fl uoro (p-nitrophenyl)(phenyl) -lambda (6)-sulfanenitrile against the solvent ionizing power Y-OTs-values in acetonitrile-water), a common ion effect in TFE/H2O, and a small salt effect. The large negative activation entropies (-60--101 J K-1 mol(-1)) were presumed to be due to strong solvation of F- with H2O in the reaction systems. The ease of ion dissociation of the S-F b ond was examined by a theoretical calculation in a DFT method, to show that the S(N)1-like transition state is caused by a facile tendency of dissocia tion of the S-F bond of fluoro-lambda (6)-sulfanenitriles. The acid-catalyz ed hydrolysis was found to proceed via a more cation-like transition state involving a concerted proton transfer to the fluorine atom and breaking of the sulfur-fluorine bond in the lambda (6)-sulfanenitrile. The alkaline hyd rolysis probably takes place via an S(N)2 mechanism.