Solvolysis in Mixed Solvents with Complementary Electrophilic and Nucleophilic Properties. Hexafluoro-2-propanol and 1,3-Propanedithiol

摘要

Solvolytic rates for alkyl tosylates and brosylates were determined for various binary compositions of hexafluoro-2-propanol (HFIP) and 1,3-propanedithiol (PDT). N and Y values were determined by using the Bentley-Schleyer approach. Solvent ionizing power dropped off sharply as PDT replaced HFIP in the mixtures, and a Y value of -4.7 was estimated for pure PDT. A plot of N vs. molar composition of HFIP was nonlinear with solvent nucleophilicity dropping off at high HFIP concentrations. Nevertheless, HFIP-PDT mixtures were, surprisingly, 3 to 4 orders of magnitude more nucleophilic than HFIP water mixtures although pure PDT is estimated to be only 2.5 orders of magnitude more nucleophilic than pure water. To explain these observations, it is suggested that, at high concentrations of electrophilic solvents, hydrogen bonding to the oxygen or sulfur atom in the more nucleophilic solvent molecules reduces the nucleophilicity of these solvents. The good correlation between rates of a wide variety of substrates in HFIP-PDT and 97% aqueous HFIP suggests that these substrates react by a kc (or kdelta) mechanism in these solvents.