Reactions of Congo Red and Azo Dye Congeners With Formaldehyde

Abstract

Analysis of samples of Congo Red dye exposed to formaldehyde were found to form multiple products, including a colorless benzimidazole form previously characterized and an unidentified yellow compound. Congeners equivalent to half of the symmetrical Congo red molecule and related structures were synthesized to simplify reaction separation and analysis. Because the yellow product was expected to be the N-formylated dye, N-formylation was attempted successfully on all congeners. When comparing the retention time of the N-formyl product formed by Congo red and the half Congo red congener to the respective products formed when exposed to formaldehyde, retention times and UV-Vis spectra were consistent with the identification of the unknown yellow product as the N-formyl product. Isolation of the yellow product from the reaction mixture with formaldehyde was successfully achieved with the half-dye analog of Congo red through trituration in a series of successively polar solvents, and NMR experiments confirmed the yellow product as the N-formylated dye. Formation of the N-formyl product was found to be related to the concentration of formaldehyde present, with lower concentrations of formaldehyde forming the benzimidazole product in higher percentages, and higher concentrations forming the N-formyl product. Formation of the N-formyl product was low or nonexistent when sterically-hindered congeners were exposed to formaldehyde. Computational analysis of the structures of all of the dyes studied was used to identify optimized geometry.