Hydrogen bonding and tautomeric equilibria in Schiff bases derived from 2-amino pyridines: electronic spectral evidence for substituent effects

Abstract

Schiff bases of the type C₆H₃(R)(OH) CH = NC₅H₃N(Y)[where R = H, 3-OCH₃, 5,6-benzo, 3-CHO-5-CH₃,5-CI, 5-Br, 5-CH₃, 4-CH₃, 3-nitro and 5-nitro and Y=H, 3-CH₃ 4-CH₃, 5-CH₃, 6-CH₃, 4,6-demethyl and 5-CI) have now been, synthesised and evidence sought for the electronic interaction between the substituents R (in the benzene ring) and Y (in the pyridine ring) in the molecule. The effects of substituents R and Y on the strength of hydrogen bond between OH and CH = N, is coupled to keloenamine ⇌ phenoliminc equilibrium. The groups which increase the acidity of the phenolic hydroxyl and the, basicity of the azomethine nitrogen facilitate quinonoid structures with characteristic visible absorption bands in the region 400-600 nm. In those compounds where R = 5,6-benzo, the intensity and the energy of band ~400 nm show a correlation with the electron releasing tendency of Y. Thus there appears an electronic communication between the substituents R and Y causing deviations from the generally accepted model for schiff base structures.