Annular tautomerism in nitrogen heterocycles. Part I. Tetrazoles

Values of the macroscopic ionization constants, pKM, 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: QX=ασI,X+βσR,X+h Agreement between calculated and observed pKM values is (with the exception of those of 5-acetylaminotetrazole for amido–imido-tautomerism is likely) very good. From the calculated values of KM, K1, and K2 values of pKT(where KT 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 pKM values of the 5-substituted tetrazoles with α and β values of 2- and 3-substituted 5-membered aza-arenes.