An Azo Dye Zoo

This year we performed the “Combinatorial Synthesis of an Azo Dye” experiment with new aniline derivatives. The aromatic nucleophiles were the “traditional” phenolic 1-naphthol, 2-naphthol , and salicylic acid. The aniline derivatives were the three nitroaniline positional isomers. The dye colors were the usual assortment of earthy reds, oranges, and yellows that we have been producing with sulfanilic acids. Understandably, the solubility of nitroaniline and its derivative in water was different than the sulfonic acids we had done previously. A few of the nine different combinations represent commercial dye structures. For example, Para Red, 1-[(E)-(4-nitrophenyl)diazenyl]-2-naphthol, is the likely product of 4-nitroaniline and 2-naphthol. Apparently this was one of the first commercial azo dyes produced. The dyes were analyzed by dying a multifiber swatch and observing the dying pattern with six different kinds of cloth. Also, the visible spectrum of the dyes in water was recorded. The figure shows the aromatic hydrogens of 2-hydroxy-5-[(3-nitrophenyl)diazenyl]benzoic acid. The impurity is likely unreacted 3-nitroaniline.



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