Aromaticity and tautomerism. Part IV. Free energy: enthalpy correlations for protonation of pyridine bases and azine N-oxides and temperature variation of the H0 and HA acidity functions

Abstract

Thermodynamic parameters for the protonation of nine substituted pyridines and nine azine N-oxides are obtained from basicity measurements of the bases at 25, 40, 60, 80, and 90° using the spectrophotometric method. The protonations of both series of bases follow linear ΔH⁰–pK_a correlations. The spectrophotometric measurements are also used to assess the temperature variation of the H₀ and H_A acidity functions and the relationship H_A=mH₀+C is found to be approximately temperature independent.