2,6-Di-tert-butylpyridine: an unusually weak base in dimethylsulfoxide

Abstract

The ionization constants of the conjugate acids BH+ of pyridine, 2-picoline, 2,6-lutidine, and 2,6-di-tert-butylpyridine (DTBP) have been determined in Me2SO. The partition coefficients of the bases B between Me2SO and water, and the enthalpies of solution and protonation of B in Me2SO have also been obtained. In contrast to its high basicity in the gas phase, DTBP is an abnormally weak base in Me2SO (pK = 0.81). The factors responsible for this very low basicity are analyzed by considering correlations between the gas-phase, Me2SO, and aqueous basicities of B and by comparing the transfer parameters for B and BH+. The solvation of DTBP and DTBPH+ in Me2SO and in water differs. While the solvation of DTBP in Me2SO is normal, the enthalpy of solution of DTBPH+ in Me2SO is abnormally low and close to that of a cation whose solvation is nonspecific. This suggests a much reduced H-bonding between sterically hindered DTBPH+ and the large Me2SO solvent molecule.