While analyzing the dyes found in a fragment of a reportedly 15th century Persian carpet, Chen et al. identified Congo red, a symmetrical diazo dye invented in 1883, as well as unknown compounds that derived from Congo red reacting with formaldehyde. One of these compounds, simply referred to by the researchers as the “yellow compound,” was believed to be an oxidized form of the benzimidazole formed by reaction of formaldehyde with the dye. This species was unable to be isolated at that time, and related compounds resisted NMR characterization due to the formation of aggregates. In dyes related to Congo red, the aminobiphenyl moiety engages in significant pi-stacking, resulting in aggregation of the dye during NMR analysis and broad bands in liquid chromatography. Furthermore, the reaction of the dyes with formaldehyde without catalysis occurs on a very slow timescale. In order to overcome these obstacles, we synthesized analogs, equivalent to half of the symmetrical dye molecule, of Congo red and similar azo dyes. Bisecting the structure minimized aggregation and increased solubility, enabling better purification options for subsequent reactions. Additionally, the use of various catalysts was explored to help increase reaction rate of the synthesized dyes with formaldehyde. Phosphine catalysis provided faster reaction times, increasing the yield of the half dye analog of the “yellow compound.” Efforts related to the synthesis and characterization of formaldehyde - dye analogs will be presented, together with our analysis of the effects of conformational effects of substitution on the reaction mechanism.

Reactions of Congo red and azo dye congeners with formaldehyde

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