Alternative Mechanistic Scheme for Salt Effects on Solvolysis Reactions of Haloalkanes and Related Compounds in Binary DMSO/H2O Solvent Mixture

Abstract In 75% (v/v) DMSO/H2O solvent mixture, salt effects on the solvolysis reaction rates of haloalkanes and related compounds (RX) have been examined. In spite of the decreased water activity in the solvent mixture, the log(k/s−1) values of typical SN1 substrates, such as 1-bromoadamantane, increase with increasing concentration of added metal perchlorates (the order: Li+ < Na+ < Mg2+ < Ba2+), which is attributed to the direct chemical interaction between the leaving-group anion and the metal cation in the “modified” solution. Contrastingly, the log(k/s−1) value of an SN2 substrate decreases with increasing concentration of the metal perchlorates. When nonmetallic salts containing anions (Y− = Cl− or Br−) different from RX (X− = Cl−, Br−, or TsO−) are present, solvolyses of SN2, such as 1-bromohexane, are subjected to an anion-exchange reaction. By the detailed examination of Δlog(k/s−1)/Δ[Mg(ClO4)2] for typical SN1, SN1–SN2 borderline, and SN2 substrates, we were able to demonstrate a linearity between the Mg(ClO4)2 effects in the solvolysis rates and the carbocation stabilities expressed by the Gibbs free energy values (ΔG) of RX in the gas phase. The salt effects on the solvolyses of SN1 to SN2 substrates are accounted for without relying on Winstein’s reaction scheme or the arbitrary function of ion pairs of two types.