KINETICS AND MECHANISM OF THE HYDROLYSIS OF ALLYLIC AND TERTIARY ALKYL ISOTHIOUREAS IN AQUEOUS, MIXED AQUEOUS AND MICELLAR SYSTEMS

Previously it has been shown that aryl isothioureas and an allylic isothiourea hydrolyze by an S(,N)1 mechanism. This investigation was undertaken to determine the solvent and micellar effects on the hydrolysis of this allylic isothiourea and two tertiary alkyl isothioureas as well as two tertiary alkyl sulfonium compounds.^ It was expected that isothioureas would exhibit reactivity in mixed aqueous solvents similar to tertiary alkyl sulfonium compounds, because of their similarities-both hydrolyze by carbon-sulfur bond cleavage and both are positively charged S(,N)1 reactants. According to the scheme proposed by Ingold, charged S(,N)1 reactants should exhibit a small increase in reactivity with decreasing solvent polarity. Isothioureas do hot exhibit the expected behavior, but rather show a significant decrease in reactivity in lower polarity media. This reversal in reactivity arises from the difference in polarity of the leaving groups of isothioureas and sulfoniums.^ The S(,N)1 hydrolysis kinetics of isothioureas was also studied in various surfactant systems and compared to the kinetic behavior of tertiary alkyl sulfonium salts in the same surfactants solutions. Anionic alkyl sulfate surfactants, above their critical micelle concentration, stabilize the isothiourea approximately a hundred-fold, while cationic and nonionic surfactants have little effect. Sulfonoium compounds exhibit the opposite reactivity behavior in the same anionic surfactant systems with decreases in stability of two to three-fold. These differences in stability between isothioureas and sulfoniums parallel their stability differences in mixed aqueous media. Since cationic isothioureas and sulfoniums are probably bound in the Stern layer near the surface of the anionic micelle, their reactivity would be governed by the medium polarity in this region. It has been proposed that water near an ionic micelle surface exhibits a dielectric constant that is significantly lower than its normal bulk value. The reactivity observations for isothioureas and sulfoniums in micellar systems and mixed aqueous media correlate well with this proposal and lends further support to such a model for the state of water around ionic micelle surface.