Solvation of Ions. V. Rates of S_N2 Reactions in Mixtures of Protic and Dipolar Aprotic Solvents

Abstract

The rates of reaction of methyl iodide with chloride ion in some protic-dipolar aprotic solvent mixtures have been measured. The rate-retarding effect of protic solvents at M concentration in dimethylacetamide is in the order p-NO₂C₆H₄OH > C₆H₅OH > C₆H₅SH > C₆H₅CO₂H ≈ CH₃OH > C₆H₅NH₂ >H₂O ≈ D₂O. Protic solvents slow the reaction by a general rather than a specific hydrogen bonding interaction with the anion. Both the dipolar aprotic solvent and the protic solvent, as well as the anion, accept hydrogen bonds, and this influences the interaction between protic solvent and anion.