Concentrated salt effects on solvolysis rates in 1,4-dioxane–H2O mixed solvent and the expansion of the use of the Hammett equation for salt effects

Abstract

In the mixed solvent system 50 vol% 1,4-dioxane and H2O, the concentrated salt effects of alkali metal (M+) and alkaline earth metal (M2+) perchlorates on the solvolysis rates of aliphatic halides (RX) and related compounds were investigated. The “pseudo” first-order reaction rates (k/s−1) of typical SN1 substrates, 1-adamantyl chloride and bromide and tert-butyl chloride, increased exponentially with increasing concentration of metal salt. The effects of M2+ on the reaction rates were larger than those of M+: Na+, Li+ < Ba2+ < Mg2+. A non-metallic salt, Et4NBr, at concentrations of ≥1.0 mol dm−3 caused the log k value to decrease, although the log k was slightly increased by lower concentrations of the salt (<1.0 mol dm−3). Raman spectra of D2O containing high concentrations of Et4NBr suggested a complete distortion of the solvent structure. The increase in log k values in the presence of metal ions was explained by direct “chemical” interaction between halide ions and metal ions with the concentrated salts in the modified media: X−⋯M+ or X−⋯M2+. The solvolysis rates of typical SN2 substrates, ethyl bromide and methyl tosylate, were much decreased by the addition of concentrated LiClO4. The log k values of borderline SN1–SN2 reacting substrates, isopropyl bromide and benzyl halides, remained almost constant, however, more positive slopes in log k in the presence of LiClO4 were caused by the introduction of methyl substituents on benzyl halides; negative slopes were observed upon the introduction of strong electron-withdrawing substituents, such as NO2 and CN. We observed a correlation (of two different slopes) between the σ+ values in the Hammett equation and the Δlog k for substituted benzyl halides upon the addition of 1.0 mol dm−3 LiClO4, as well as between the log (kx/kH) values themselves. We propose here an expansion of the use of the Hammett equation for the concentrated salt effect.