Elucidation of Concentrated Salt Effects on the Solvolysis Reactions of Typical SN1 and SN2 Substrates in Sulfolane–Water Mixed Solvent

Abstract

Abstract In a 50 vol % sulfolane–H2O mixed solvent, extensive studies have been carried out of the concentrated salt effects on the solvolysis reaction rates of aliphatic halides and related compounds (RX). In the solvent system, the “pseudo” first-order rate constants (k/s−1) of typical SN1 substrates, 1-adamantyl chloride and bromide, or 2-adamantyl bromide, were increased exponentially by the addition of MClO4 (M = Li and Na) and M(ClO4)2 (M = Mg and Ba); the extent of the cation effects increased as Na+ < Li+ < Mg2+ ≈ Ba2+, which was attributed to the “chemical” interaction between the metal ions and the leaving-group anions in the “aqueous” solution modified by the mixed-organic solvents and concentrated salts. Based on the Raman spectra, we discuss the distortion of the bulk water structure and the alteration of properties of water into those of a “non-aqueous” solvent with the addition of the organic solvents and concentrated salts (LiClO4 and Et4NBr) as well as with increasing temperature. However, the addition of non-metallic salts, i.e., Et4NX (X− = ClO4−, Cl−, Br−, and tosylate), caused decreases in the rates substantially, regardless of the leaving group (Cl or Br): the deceleration effects increasing as ClO4− < Br− < Cl− < TsO−. The deceleration in the solvolysis rates with Et4NX and R4NBr (R = Pr and n-Bu) should be caused by mainly the further decrease in water activity of the solvents. In addition, we would like to report solvolyses accompanied by “ion exchange” reactions for the typical SN2 substrates, hexyl chloride, bromide, and tosylate, in the sulfolane–H2O solvent.