2-Hydroxynaphthalene-3,6-disulfonic acid [148-75-4]. (3-hydroxynaphthalene-2,7- disulfonic acid), R acid, C10H8O7S2, Mr 304.3, is readily soluble in water and ethanol. Although the sodium salt (R salt) is soluble in water, it can be precipitated by sodium chloride. The barium salt is sparingly soluble in cold water but is 8 % soluble in boiling water. Whereas sulfonation with 20 % oleum yields 2-hydroxynaphthalene3,6,8-trisulfonic acid, the reaction with Na2SO3– MnO2 gives 2-hydroxy-1,3,6-trisulfonic acid. Alkali fusion with sodium hydroxide gives 2,3-dihydroxynaphthalene-6-sulfonic acid and some 2,3-dihydroxynaphthalene. Amination (Bucherer) produces 2-aminonaphthalene-3,6- disulfonic acid. Diazo coupling and nitrosation take place in the 1-position.
2-Naphthol is heated in excess 98 % sulfuric acid to 60℃, and anhydrous sodium sulfate is added. The mixture is then heated for 24 h at 105℃, 12 h at 110 ℃, and 12 h at 120℃ before being diluted with more sulfuric acid and quenched in water. Sodium chloride is added at 60℃, and the mixture is cooled slowly to 30℃ with crystallization of the product as its sodium salt. The isolated yield is 68 %, and the product contains about 3 % of Schaeffer’s salt as the major impurity. Alternatively, R salt may be recovered as a byproduct from the G acid process (see 2-Hydroxynaphthalene-6,8-disulfonic acid). Further purification may be effected via the aniline salt.
