Formation mechanisms and yields of small imidazoles from reactions of glyoxal with NH4+ in water at neutral pH

Abstract

Imidazoles have numerous applications in pharmacology, chemistry, optics and electronics, making the development of their environmentally-friendly synthetic procedures worthwhile. In this work, the formation of imidazole, imidazole-2-carboxaldehyde, and 2,2-bis-1H-imidazole was investigated in the self-reaction of glyoxal and its cross-reactions with each of these compounds in aqueous solutions of inorganic ammonium salts at pH =7. Such conditions are relevant both as cheap and environmentally-friendly synthetic procedures and for the chemistry of natural environments where NH₄⁺ is abundant, such as in atmospheric aerosols. These reactions were investigated both by ¹H-NMR and UV-Vis absorption spectroscopy at room temperature with the objective to determine the formation pathways of the three imidazoles and the parameters affecting their yields, to identify the optimal conditions for their synthesis. The results show that only the simplest imidazole is produced by the self-reaction of glyoxal and that imidazole-2-carboxaldehyde and 2,2-bis-1H-imidazole are produced by cross-reactions of glyoxal with imidazole and imidazole-2-carboxaldehyde, respectively. The yields of imidazole-2-carboxaldehyde and 2,2-bis-1H-imidazole formed by the cross-reactions were close to unity, but the yield of imidazole formed by the self-reaction of glyoxal, Y_Im, was small and varied inversely with the initial glyoxal concentration, [G]₀: Y_Im > 10% only for [G]₀ < 0.1 M. The latter result was attributed to the kinetic competition between the imidazole-forming condensation pathway and the acetal/oligomer formation pathway of the glyoxal self-reaction and constitutes a bottleneck for the formation of higher imidazoles. Other parameters such as pH and the NH₄⁺ concentration did not affect the yields. Thus, by maintaining small glyoxal concentrations, high imidazole yields can be achieved in environmentally-friendly aqueous ammonium solutions at neutral pH. Under the same conditions, higher yields are expected expected from substituted carbonyl compounds, regardless of their concentration, as they produce less acetals.