Aromaticity and tautomerism. Part I. The aromatic resonance energy of 2-pyridone and the related thione, methide, and imine

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Abstract
Tautomeric equilibria in the heteroaromatic and the analogous saturated series are compared. Available methods of conversion of free energy ΔG° into enthalpy ΔH° terms are applied and discussed. The ΔH° values obtained are used to demonstrate that 2-pyridone and 2-pyridinethione retain most of the aromatic resonance energy of pyridine, that 2-pyridone imine is also strongly aromatic, but that 2-pyridone methide is far less so. The work is compared with other estimates of the aromaticity of these compounds.