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 S_N1 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 S_N2 substrates, ethyl bromide and methyl tosylate, were much decreased by the addition of concentrated LiClO₄. The log k values of borderline S_N1–S_N2 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 (k_x/k_H) values themselves. We propose here an expansion of the use of the Hammett equation for the concentrated salt effect.