Naphthalenesulphonic Acids.—Naphthalene-a-sulphonic acid, predominates when the hydrocarbon is sulphonated below 8o° C, the 0-isomeride when the temperature of sulphona tion is 16o-18o° C. Further sulphonation of the former yields I :5- and :6-, and of the latter 2:7- and 2:6-disulphonic acids. Other disulphonic acids are obtainable by synthetic methods, and numerous tri- and tetra-sulphonic acids are also known. All these acids liberate hydrochloric acid from sodium chloride. They and their alkali salts are soluble in water, but the lead, barium and calcium salts often differ widely in solubility, and can occasionally be utilized in the separation of mixtures. The value of these polysulphonic acids depends largely on the ease with which they lose sulphonic groups or exchange them for hydroxyl groups, both changes taking place more readily from a- than from f3-positions.
a-Naphthylamine, C10H7.NH2, is made in large quantities by reducing a-nitronaphthalene with soft iron borings ("swarf") and dilute hydrochloric acid at 50-7o° C; milk of lime is then added to neutralize the acid, and the liberated amine distilled, either with superheated steam, or preferably in a vacuum. Further distilla tion and crystallization are necessary to obtain the pure product. If naphthalene is heated at C with sodamide in presence of phenol, a-naphthylamine is formed, hydrogen being evolved. It may also be prepared by heating a-naphthol with ammonium bisulphite under pressure or with ammonia-calcium chloride at 270° C. a-Naphthylamine is soluble in most solvents, and crystal lizes when quite pure in colourless flat needles or scales, which melt at 5i° C, and boil at 300° C. The technical product gradually acquires a greyish-violet colour when exposed to air, owing prob ably to the presence of easily oxidized :8-diaminonaphthalene.
a-Naphthylamine can be readily distinguished from the f3-iso meride by its faecal odour, by its slight volatility in steam, and by the azure-blue precipitate, soluble in chloroform, which aque ous solutions of its salts give when oxidized with ferric chloride. Diazo-solutions of amines couple with a-naphthylamine in the 4 position, or if this is occupied in the 2-position, giving aminoazo compounds. If both positions are blocked, either one of the intruding substituents is removed, or azo-coupling does not take place.
a-Naphthylaminesulphonic acids can be prepared in several ways, of which the most important technically are (a) sulphona tion of a-naphthylamine, (b) amination of a-naphtholsulphonic acids, using ammonia and ammonium sulphite, and (c) nitration of naphthalenesulphonic acids, with subsequent reduction. Of the seven possible monosulphonic acids of a-naphthylamine, all except the 3- acid are in common use; the most important is i-naph thylamine-4-sulphonic acid (naphthionic acid), produced by "bak ing" a-naphthylamine acid sulphate at 18o-200° C in a vacuum, or by heating the amine with sulphuric acid at i7o° C in presence of 3% of oxalic acid. When diazotized it constitutes the first component of many azo-dyes of the fast red series. It also couples with diazotized bases forming ortho-azo-dyes, but in these circumstances the still intact amino-group in the 1-position cannot be further diazotized owing to the proximity of the diazo-complex; one of the best known dyes in this class is Congo red, from diazo tized benzidine (q.v.) and two molecules of naphthionic acid.
f3-Naphthylamine is manufactured in almost quantitative yield by heating in an autoclave under the pressure of 6 atmos. j3-naph thol, 4o% ammonium sulphite solution, and 20% ammonia in suitable proportions at C for several hours. (This reaction, named after Bucherer, has an extensive application throughout the naphthalene series.) The crude naphthylamine is removed by filtration, washed with caustic soda to remove unchanged naphthol, and finally distilled in a vacuum to remove which is always produced in small quantity. When pure, j3-naph thylamine crystallizes from alcohol in colourless, lustrous scales, melts at 112° C, and boils at C. Unlike the a-isomeride, it is odourless, moderately volatile in steam, and appreciably soluble in hot water, and solutions of its salts give no colour with ferric chloride or other mild oxidizing agents.