2-Hydroxynaphthalene-6,8-disulfonic acid [118-32-1]. (7-hydroxynaphthalene-1,3- disulfonic acid), G acid, C10H8O7S2, Mr 304.3: most salts of 55 are more readily soluble in water than those of R acid, with the notable exception of the potassium salt. Sulfonation with 20 % oleum at 100℃ yields 2-hydroxynaphthalene3,6,8-trisulfonic acid. Diazo coupling takes place in the 1-position, although the reaction is sluggish, as in the case of 2-hydroxynaphthalene8-sulfonic acid.
The G acid is aminated (Bucherer reaction) to give 2-aminonaphthalene-6,8- disulfonic acid and subjected to caustic fusion at 200℃ to yield 2,8-dihydroxynaphthalene-6-sulfonic acid (a precursor of g acid). It is also used as a coupling component for a wide range of azo colorants, e.g., C.I. Acid Orange 10, C.I. Acid Red 187, and C.I. Food Red 7.
2-Naphthol is added to 98 % sulfuric acid at 40℃. Then, 20 % oleum is added over 6 h and the temperature is allowed to rise to 60℃. The reaction is completed by heating for a further 16 h at 60℃ followed by 15 h at 80℃. After the reaction mixture has been poured into water, potassium chloride is added to the hot solution and the batch is cooled slowly to 35℃ before filtering off the crystalline dipotassium salt of G acid and carefully washing out any residual R salt; the yield is 60 %. In addition, R salt can be recovered from the filtrate in 12 % yield after conversion to its sodium salt.
