Acidity functions and the protonation of weak bases. Part VII. The protonation behaviour of dimethylaminopyridines and their N-oxides

Abstract

The first and second pK_a values for the title compounds and some nitro-substituted derivatives are reported. First protonation occurs in all cases on the ring nitrogen (pyridines) or at the N-oxide oxygen (N-oxides), and the pK_a values are in line with predictions from the Hammett equation. The second protonation at the NMe₂ group follows the H₀‴ acidity function, and thermodynamic second pK_a values calculated using the slope m of [BH⁺]/[B]vs. H₀ are also well correlated by the Hammett equation.