Self-association of imidazole and its methyl derivatives in aqueous solution. A study by ultraviolet spectroscopy

Abstract

The self-association of imidazole and its monomethyl derivatives, 1-methyl-, 2-methyl-, and 4(5)-methylimidazole, was studied in aqueous solution, at different pH values, by ultraviolet spectroscopy. The variation in molar absorptivity with the concentration of these compounds was measured for the band near 205 nm. Hypochromic deviations from Beer's law with increasing concentration were detected for all compounds. These observations were interpreted in terms of two processes of self-association: dimerization and polymerization, similarly to other heteroaromatic systems, such as pyridine and pyrimidine, in aqueous solution. From the experimental data, self-association constants for dimerization ( K 2) and polymerization ( K n) were calculated. Hypochromic effects were also found in a band near 195 nm, observed in dilute solutions of these compounds at basic pH. The results obtained are discussed with respect to the structural domains of imidazole and the location of the methyl group on the ring. A model of association based on the stacking of the rings, with the possible reinforcement due to hydrogen bonding between rings, is suggested for these compounds in aqueous solution. Self-association of another compound with the ring partially reduced, 2-methyl-2-imidazoline, was also studied, with the result that the aromatic character of the nucleus is not critical for the occurrence of self-association.