Concentrated salt effects on the rates of solvolyses involving carbocations as reaction intermediates in acetone–water mixed solvents

Abstract

Extensive studies have been carried out on the concentrated salt effects on the solvolysis reaction rates of aliphatic halides and related compounds (RX) in acetone–water mixed solvents. In 90 vol% acetone– 10 vol% water solution, the pseudo-first-order rate constant (k/s^–1) of a typical S_N1 substrate, tert-butyl chloride, at 50 °C was increased exponentially by the addition of M⁺ClO₄^– (M⁺ = Li⁺, Na⁺: up to 4.0 mol dm^–3) and M²⁺(ClO₄^–)₂ (M²⁺ = Mg²⁺, Ba²⁺: up to 2.0 mol dm^–3); the extent of the cation effects increased as Na⁺ ⩽ ⩽ ⩽ Li⁺ < Mg²⁺ ⩽ ⩽ ⩽ Ba²⁺. However, the addition of Et₄NClO₄ (up to 1.0 mol dm^–3) decreased the solvolysis rate substantially. In 50 vol% acetone–water solution, the effects of the metal perchlorates on the solvolysis rates of 1-adamantyl chloride at 50 °C increased as Na⁺ < Li⁺ < Ba²⁺ < Mg²⁺. Addition of >1.0 mol dm^–3 Et₄NBr decreased the solvolysis rate markedly, whereas it was increased slightly by lower Et₄NBr concentrations. The positive effects of metal ions for typical S_N1 substrates were explained by the change of solvent structure and by a “chemical” interaction between the anions from the substrates (R⁺–X^–) and M⁺ or M²⁺ in the presence of very concentrated salts; the negative effects of nonmetallic salts should have been brought about by the decrease in activity of H₂O. The solvolysis rate of 2-adamantyl tosylate (C₁₀H₁₅OTs) in 50 vol% acetone–water solution at 50 °C was also increased exponentially by the addition of LiClO₄, whereas those of typical S_N2 substrates, methyl tosylate (CH₃OTs) and ethyl bromide, were decreased by the addition of LiClO₄. On the other hand, for isopropyl bromide and benzyl chloride, the solvolysis rates were not changed by the addition of LiClO₄. A good linearity was observed between the increase in log (k/s^–1) in the presence of 1.0 mol dm^–3 LiClO₄ and the m-values of the substrates (by Grunwald–Winstein). It is proposed that one could simply distinguish S_N1 from S_N2 reactions merely by observing a substantial increase in the solvolysis rate constant at 1.0 mol dm^–3 LiClO₄ in aqueous mixed solvents. The salt effects on the solvolysis rates of sulfonyl chlorides in 50% acetone–water at 35 °C were very different from those for substrates with carbocations as reaction intermediates.

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