STUDIES OF AZOXY DYESTUFFS-16 INVESTIGATIONS OF THE PROTONATION AND TAUTOMERIC EQILIBRIA OF 4-(p'-HYDROXYPHENYLAZO)PYRIDINE AND RELATED SUBSTRATES

Article abstract of DOI:10.1016/S0040-4020(01)91871-X

A study of the protolytic equlibria undergone by the title compound 1 and the related substrates 2-4 has been carried out by spectrophotometric methods.Compound 1 is associated with two macroscopic protolytic equilibrium constants (pK_a1=7.73, pK_a1=4.65) while compounds 2, 3 and 4 are associated with a single protolytic equilibrium in each case (pK_a2=6.31, pk_a3=6.31, pk_a4=4.53).Compounds 2 and 3 exhibit identical spectral behaviour as function of pH, which provides clues to interpretation of the equilibria involved in this series.Further consideration of possible protolytic and tautomeric equilibria (Schemes 1-3) has allowed the various microscopic equilibrium constants to be assigned, the first intance in the hydroxyphenylazopyridine series.It is concluded that hydrazone tautomeric forms are not important in these systems.The equilibrium between the neutral (HA) and zwitterionic (AH^+/-) forms of 1 is predominantly on the former side, whereas the equlibrium between 4-hydroxypiridine and 4-pyridone greatly favours the latter form.Discussion of various eqilibrium contants is given in terms of structural and electronic effects.