Spectroscopic Determination of Acid Dissociation Constants of Some Pyridyl Shiff Bases

Abstract

The acid dissociation constants (p K 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 (p K a1 ) of the 2-[2-aza-2-(3-methyl-(2-pyridly))vinyl]phenol ( 1), 2-[2-aza-2-(3-methyl-(2-pyridly))vinyl]-6-nitrophenol ( 2 ), 2-[2-aza-2-(3-methyl-(2-pyridly))vinyl]-4-nitrophenol ( 3 ), 2-[2-aza-2-(3-methyl-(2-pyridly))vinyl]-4,6-dinitrophenol ( 4 ), 2-[2-aza-2-(5-methyl-(2-pyridly))vinyl]phenol ( 5 ), 2-[2-aza-2-(5-methyl-(2-pyridly))vinyl]-6-nitrophenol ( 6 ), 2-[2-aza-2-(5-methyl-(2-pyridly))vinyl]-4-nitrophenol ( 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 (p K 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 (p K 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.