Ultraviolet absorption and protonation equilibria of amino- and nitro-substituted pyridines

Solvent effects on the bathochromic shift in the first π–π* band of aminonitropyridines were examined. They were interpreted in terms of inter- and intra-molecular hydrogen bonding, as well as of transition polarizability which determines solvatochromism. All possible dissociation constants of conjugate acids of amino-, diamino-, nitro-, and aminonitro-pyridines have also been measured at 25°C by spectrophotometric determination of acid–base concentration ratios. The influence of amino- and nitro-groups in the pyridine ring on pKa1 and pKa2 was discussed on the basis of electrical characteristics of substituents and electron availability of reaction site.