Kinetic Investigation on the Hydrolysis of Aryl(fluoro)(phenyl)-λ6-sulfanenitriles

Abstract

Abstract A kinetic investigation on the hydrolysis of aryl(fluoro)(phenyl)-λ6-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 hydrolysis of fluoro-λ6-sulfanenitriles was found to proceed via an SN1 or an S-nitrilosulfonium cation-like transition state, which is characterized by a large negative Hammett ρ-value (ρ = −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-values (ca. 0.83 for fluoro(diphenyl)-λ6-sulfanenitrile; ca. 0.82 for fluoro(p-nitrophenyl)(phenyl)-λ6-sulfanenitrile against the solvent ionizing power YOTs-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 bond was examined by a theoretical calculation in a DFT method, to show that the SN1-like transition state is caused by a facile tendency of dissociation of the S–F bond of fluoro-λ6-sulfanenitriles. The acid-catalyzed 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 λ6-sulfanenitrile. The alkaline hydrolysis probably takes place via an SN2 mechanism.