We are Dyeing to Know More About EAS

I have doing a variation of the “methyl orange” azo dye synthesis in organic lab for the past ten years. Lately, we have performed a combinatorial scheme involving the three aminobenzenesulfonic acids and 1- or 2-naphthol. This was inspired by a 2004 article in J. Chem. Ed. by Gung and Taylor. This year, for the first time, I took an NMR (in DMSO) of selected student products. I was expecting a messy mixture of products resulting from the various regioisomers that can be produced from the electrophylic aromatic substitution (EAS) of naphthol. There are 7 possible substitution positions on naphthol with 4 of them electronically favored. I was surprised to see that the NMR aromatic regions were relatively clean and well-resolved for some naphthol/ aminobenzenesulfonic acid combinations. For example, 2-napthol and para-aminobenzenesulfonic acid gave six signals with relative integrations of 1,1,1,5,1, & 1 between 6.8 and 8.6 ppm. Apparently, this is 4-((2-hydroxy-1-naphthalenyl) azo)-benzenesulfonic acid – a popular dye called “acid orange 7.” It is not obvious to me why this particular regioisomer is so heavily favored. Other combinations gave much more messy spectra more indicative of a mixture of several regioisomers.

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  1. […] third project started with the synthesis of an azo dye that we had performed earlier in the semester. The combination of 4-aminosulfonic acid and 2-naphthol yields […]

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