Annular tautomerism in nitrogen heterocycles. Part I. Tetrazoles

Abstract

Values of the macroscopic ionization constants, pK_M, of 5-substituted tetrazoles have been calculated from calculated values of the microconstants which were themselves obtained from the extended Hammett equation by comparison with suitable model systems: Q_X=ασ_I,X+βσ_R,X+h Agreement between calculated and observed pK_M values is (with the exception of those of 5-acetylaminotetrazole for amido–imido-tautomerism is likely) very good. From the calculated values of K_M, K₁, and K₂ values of pK_T(where K_T is the tautomeric equilibrium constant) and of f₁, that fraction of the total tautomer concentration present as the N²-protonated tautomer, were calculated. In all the substituted tetrazoles studied the N²-protonated tautomer appears to be predominant. Further evidence for protonation at N(2) is provided by a comparison of the α and β values obtained from correlation of the observed pK_M values of the 5-substituted tetrazoles with α and β values of 2- and 3-substituted 5-membered aza-arenes.