No abstract

The acid dissociation constants (pK a ) of eight biologically active pyridyl Shiff bases were determined using a UV-vis spectroscopic technique at 25 ( 0.1 °C. The first acidity constants (pK a1 ) of the 2- 7), and 2-[2-aza-2-(5-methyl-(2-pyridly))vinyl]-4,6-dinitrophenol (8) are found to be associated with the protonation of phenolate oxygen. The second acidity constants (pK a2 ) are found to correspond to protonation of pyridine nitrogen for compounds 5, 6, and 7 and oxoprotonation for molecules 1, 2, 3, 4, and 8. The third acidity constants (pK a3 ) are found to be associated with the protonation of pyridine nitrogen for compounds 1, 3, 7, and 8 and amino protonation for molecules 2 and 5. For molecules 4 and 6, it is associated with oxo protonation. The contribution of the ketoamino tautomeric form was found to be considerably important.