Formation of Schiff bases of 5′-deoxypyridoxal and hexylamine in aqueous and non-aqueous media

Abstract

We have studied the formation of Schiff bases of 5′-deoxypyridoxal (DPL) and hexylamine in various non-aqueous solvents (dioxane, pentanol/dioxane mixtures, pentanol, butan-2-ol, propanol, ethanol, methanol and ethane-1,2-diol) at 25 °C. The results are interpreted in terms of the tautomer forms of DPL present in the medium, and the rate constant of formation of the Schiff base has been obtained for the neutral species of DPL and the zwitterionic form (k_1n= 1.7 × 10² dm³ mol^–1 min^–1 and k_1z= 3.6 × 10³ dm³ mol^–1 min^–1, respectively). We also determined the constant of the tautomeric equilibrium between the corresponding forms of the uncharged species of DPL in the different solvents. These results and those obtained in water/dioxane media indicate that the intramolecular acid catalysis involved in the formation of Schiff bases is also governed by the occurrence of a charged transition state.