Band-shape analysis of electronic spectra and study of the hydrolysis of the Schiff bases of 5′-deoxypyridoxal and n-hexylamine in aqueous and non-aqueous media

Abstract

The bands of the absorption spectra of the Schiff bases formed between 5′-deoxypyridoxal (DPL) and n-hexylamine in various aqueous solvents (water–dioxane mixtures) and non-aqueous media (dioxane, pentanol–dioxane mixtures, pentanol, butan-2-ol, propanol, ethanol and methanol) have been analysed at 25°C. We have also calculated the hydrolysis rate constants of the different tautomers of the Schiff bases. According to our results, the zwitterionic and enol tautomers are more stable on account of the formation of very stable hydrogen bonds. On the other hand, the ion–dipole form is the most reactive owing to the absence of stabilizing intramolecular hydrogen bonds and the protonated state of its pyridine nitrogen.

The results obtained in this work also show that the relative perimittivity of the medium does not significantly affect the tautomeric equilibrium of the chemical species of the Schiff bases of DPL and n-hexylamine. Marshall plots reveal that the microscopic pK most markedly influenced by solvation is that of the phenolic oxygen, whereas that of the pyridine nitrogen is absolutely insensitive to solvation.