1-Amino-2-naphthol-6-sulfonic acid

1-Amino-2-hydroxynaphthalene-6-sulfonic acid [5639-34-9]. (5-amino-6-hydroxynaphthalene-2-sulfonic acid), amino-Schaeffer acid, C10H9NO4S, Mr 239.2, is obtained as needles or prisms that are only slightly soluble in boiling water. The sodium salt is soluble in water but can be isolated as the hemipentahydrate. Diazotization does not occur in mineral acid medium. The diazo oxide is formed by reaction with sodium nitrite in the presence of copper(II) sulfate.

2-Hydroxynaphthalene-6-sulfonic acid is nitrosated in aqueous solution, and the resulting nitroso derivative is reduced in situ by addition of zinc dust to the acidified batch, with the temperature allowed to rise to 40 ℃. The precipitated product is isolated in 70 % yield.