The most important technical methods for the manufacture of a-naphtholsulphonic acids are (a) sulphonation of a-naphthol, (b) replacement of the amino-group of a-naphthylaminesulphonic acids by hydroxyl, through the diazo-reaction, (c) conversion of a-naphthylaminesulphonic acids by heating with sodium bisulphite solution (i.e., the reverse of Bucherer's reaction for the intro duction for an amino-group), (d) exchange of one of the a-sul phonic groups in naphthalenepolysulphonic acids for hydroxyl by fusion with caustic alkali, and (e) similar exchange of the chlorine atom in a-chloronaphthalenesulphonic acids. The best known is I-naphthol-4-sulphonic acid (Neville and Winther's acid) which is used for making a large number of azo-dyes.
-Naphthol is invariably made technically by fusion of the sodium salt of naphthalene-f3-sulphonic acid with caustic alkali, and is therefore liable to contain a small amount of the a-iso meride unless carefully purified. When pure it crystallizes in plates, melting point 122° C, boiling point 285°-286° C. Its solubility in various solvents is similar to that of a-naphthol, but it is odourless, and appreciably volatile only in superheated steam. It has marked antiseptic properties, and its salicylate naphtholsalol, is useful medicinally. When ortho-aminoazotoluene is diazotized and coupled with 0-naphthol, a dark reddish-brown powder, called scarlet red, is formed. This possesses valuable therapeutic properties, and is also used for stains in microscopy. The alkyl ethers of 0-naphthol find scope as cheap perfumes. With nitrous acid, 0-naphthol yields the i-nitroso-derivative, which forms insoluble precipitates with certain metallic salts, thus enabling cobalt to be separated from nickel, iron from alu minium, and copper from cadmium, magnesium and zinc. These precipitates are easily converted into metallic oxides by ignition, and so afford a means of estimating cobalt, iron and copper. Chlorination of 13-naphthol gives the I-chloro-derivative, but on nitration I :6-dinitro-0-naphthol is obtained. If 0-naphthol is condensed in alcoholic solution with para-nitrosodimethylaniline hydrochloride, an oxazine, Meldola's blue (dimethylaminonaph thophenoxazonium chloride), is formed (see DYES, SYNTHETIC).
The general methods available for the production of (3-naph tholsulphonic acids are somewhat limited by the difference in reactivity between a- and 3-sulphonic groups in the naphthalene nucleus. For instance, only from those disulphonic acids (and there are but two available, the 2 :6- and 2: 7-) containing both sulphonic groups in 0-positions can a 0-naphthol derivative be obtained by fusion with alkali, for if an a-sulphonic group be present it will be replaced preferentially. Furthermore, replace
ment of the 0-amino-group by hydroxyl is usually impracticable, as the most convenient method of preparing the required 13-naph thylaminesulphonic acids is from the j3-naphtholsulphonic acids themselves. Fortunately a greater number of j3-naphtholsulphonic acids can be obtained by direct sulphonation than in the a-series. Of these, 0-naphthol-I-sulphonic acid (oxy-Tobias acid) is of little value for dyestuff purposes, but it is unique among the acids of the naphthalene series in that it undergoes a curious trans formation when successively condensed with diazotized para nitraniline and treated with an excess of sodium hydroxide, a complex phthalazine I. being formed, instead of the usual para red ILO -Naphthol-6-sulphonic acid (Schaeffer acid), -8-sulphonic acid (Croceine acid), -3 :6-disulphonic acid (R-acid), -6 :8-di sulphonic acid (G-acid), and -3 :6 :8-trisulphonic acid, all pro duced by sulphonation of 0-naphthol under varying conditions, are very important. G-Acid furnishes a series of valuable yellow or orange azo-dyes, hence its name (from Ger. Gelbsiiure = yellow acid), in contradistinction to R-acid (from Rotsaure=red acid) which gives the more common red shades.
The dihydroxynaphthalenes and their sulphonic acids are of greater technical importance than the diaminonaphthalenes, espe cially i :5-dihydroxynaphthalene, i :8-dihydroxy-4-sulphonic acid, and i :8-dihydroxy-3 :6-disulphonic acid (chromotropic acid). When coupled with diazotized amines they furnish dyes which form insoluble lakes with iron, aluminium or chromium mordants.
Among the more important aminonaphtholsulphonic acids are 2-amino-5-naphthol-7-sulphonic acid (J-acid) (which, unlike its isomerides, endows many dyes of which it is a component with the property of dyeing unmordanted cotton), 2-amino-8-naphthol 6-sulphonic acid (7-acid), and I-amino-8-naphthol-3 :6-disulphonic acid (H-acid), the last being one of the most valuable intermedi ates that the dyestuff industry possesses. H-Acid, as well as other 3 :6-naphthalenedisulphonic acids, has also been employed in the synthesis of trypanocides for use in combating sleeping sickness. Several of the aminonaphtholsulphonic acids, e.g., "Eikonogen" and "Diogen," are used in photography.