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 SN1 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+ 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 SN1 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 SN2 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 SN1 from SN2 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.