The excess basicity of alkali metal methoxides in methanol

Abstract

The deprotonation equilibria of weak neutral and anionic nitrogen acids (nitro-substituted arylamines and indoles) and methoxide addition to nitro-substituted anisoles have been investigated in methanol solutions of MeOLi, MeONa, and MeOK. The basicity of these media has been determined as excess basicity (X) functions. The available methods for the calculation of X and data processing have been critically compared and tested. Equilibrium constants (pK_a) and slope parameters (m*) have been determined for the above compounds. The basicity of the three media (as probed by X) increases in the series Li < Na < K. Structural effects on m* and pK_a values are discussed in terms of the solvation of the deprotonated acid; m* values in the range 0.7–1.2 (solvation similar to the reference) have been found for all neutral indicators with the exception of indoles, for which m* is ca. 0.2, which indicates higher solvation; anionic acids may have higher m*s (1.2–1.6) if the two negative groups are ortho to each other.