The history of another important group of colouring-matters dependent on naphthalene begins with A. v. Baeyer in 1871 and with Caro in 1874. Two products formerly known only as laboratory preparations were called into requisition by this discovery. One of these compounds, phthalic acid, is obtained from naphthalene, and the other, resorcin or resorcinol, is prepared from benzene. Phthalic acid, which was discovered in 1836 by Laurent, is a product of the oxidation of many benzenoid compounds. Chemically considered it is a di-derivative of benzene, i.e. two of the hydrogen atoms of benzene are replaced by certain groups of carbon, oxygen, and hydrogen atoms. We have seen how the replacement of hydrogen by an ammonia-residue, amidogen, gives rise to bases such as amidobenzene (aniline), or diamidobenzene. Similarly, the replacement of hydrogen by a water-residue, hydroxyl, gives rise to a phenol. The group of carbon, oxygen, and hydrogen atoms which confers the property of acidity upon an organic compound is a half-molecule of oxalic acid—it is known as the carboxyl group. Thus benzoic acid is the carboxyl-derivative of benzene, and the phthalic acid with which we are now concerned is a dicarboxyl-derivative of benzene. It is related to benzoic acid in the same way that diamidobenzene is related to aniline. Three isomeric phthalic acids are known, but only one of these is of use in the present branch of manufacture. The acid in question, although a derivative of benzene, is most economically prepared by the oxidation of certain derivatives of naphthalene which, when completely broken down by energetic oxidizing agents, furnish the acid. Thus the dinitronaphthol described as Manchester yellow, if heated for some time with dilute nitric acid, furnishes phthalic acid. The latter is made on a large scale by the oxidation of a compound which naphthalene forms with chlorine, and known as naphthalene tetrachloride, because it contains four atoms of chlorine.
The other compound, resorcinol, was known to chemistry ten years before it was utilized as a source of colouring-matters. It was originally prepared by fusing certain resins, such as galbanum, asafœtida, &c., with caustic alkali. Soon after its discovery, viz. in 1866, it was shown by Körner to be a derivative of benzene, and from this hint the technical process for the preparation of the compound on a large scale has been developed. Resorcinol is a phenolic derivative of benzene containing two hydroxyl groups; it is therefore related to phenol in the same way that diamidobenzene is related to aniline or phthalic acid to benzoic acid. The relationships can be expressed in a tabular form thus—
| Amidobenzene or Aniline. | Benzoic acid. | Carbolic acid or Phenol. | ||
| Diamidobenzene. | Phthalic acid. | Resorcinol. |
Resorcinol is now made by heating benzene with very strong sulphuric acid so as to convert it into a disulpho-acid, and the sodium salt of the latter is then fused with alkali. As a technical operation it is one of great delicacy and skill, and the manufacture is confined to a few Continental factories.
When phthalic acid is heated it loses water, and is transformed into a white, magnificently crystalline substance known as phthalic anhydride, i.e. the acid deprived of water. In 1871, A. v. Baeyer, the eminent chemist who subsequently synthesised indigo, published the first of a series of investigations describing the compounds produced by heating phthalic anhydride with phenols. To these compounds he gave the name of “phthaleïns.” Baeyer’s work, like that of so many other chemists who have contributed to the advancement of the coal-tar colour industry, was of a purely scientific character at first, but it soon led to technological developments. The phthaleïns are all acid compounds possessing more or less tinctorial power. One of the first discovered was produced by heating phthalic anhydride with an acid known as gallic acid, which occurs in vegetable galls, and in the form of tannin in many vegetable extracts which are used by the tanner. The acid is a phenolic derivative of benzoic acid, viz. trihydroxybenzoic acid, and on heating it readily passes into trihydroxybenzene, which is the “pyrogallic acid” or pyrogallol familiar as a photographic developer. The phthaleïn formed from gallic acid and phthalic anhydride really results from the union of the latter with pyrogallol. It is now manufactured under the name of “galleïn,” and is largely used for imparting a bluish grey shade to cotton fabrics. By heating galleïn with strong sulphuric acid, it is transformed into another colouring-matter which gives remarkably fast olive-green shades when dyed on cotton fibre with a suitable mordant. This derivative of galleïn is used to a considerable extent under the name of “cœruleïn.” These two colouring-matters were the first practical outcome of v. Baeyer’s researches. There is another possible development in this direction which chemistry may yet accomplish, and another natural colouring-matter may be threatened, even as the indigo culture was threatened by the later work of the same chemist. There is reason for believing that the colouring-matter of logwood, known to chemists as hæmateïn, is related to or derived in some way from the phthaleïns, and the synthesis of this compound may ultimately be effected.
The dye introduced by Caro in 1874 is the brominated phthaleïn of resorcinol. The phthaleïn itself is a yellow dye, and the solutions of its salts show a splendid and most intense greenish yellow fluorescence, for which reason it is called “fluoresceïn.” When brominated, the latter furnishes a beautiful red colouring-matter known as “eosin” (Gr. ἕως, dawn), and the introduction of this gave an industrial impetus to the phthaleïns which led to the discovery of many other related colouring-matters now largely used under various trade designations. About a dozen distinct compounds producing different shades of pink, crimson and red, and all derived from fluoresceïn, are at present in the market, and a few other phthaleïns formed by heating phthalic anhydride with other phenolic compounds instead of resorcinol or pyrogallol (e.g. diethylamidophenol), are also of industrial importance. By converting nitrobenzene into a sulpho-acid, reducing to an amido-sulpho-acid, and then fusing with alkali, an amido-phenol is produced, the ethers of which, when heated with phthalic anhydride, give rise to red phthaleïns of most intense colouring power introduced by the Baden Aniline Company as “rhodamines.”
It remains to point out that the scientific spirit which prompted the investigation of the phthaleïns in the first instance has followed these compounds throughout their technological career. The researches started by v. Baeyer were taken up by various chemists, whose work together with that of the original discoverer has led to the elucidation of the constitution of these colouring-matters. The phthaleïns are members of the triphenylmethane group, and are therefore related to magenta, corallin, malachite green, methyl violet, and the phosgene dyes.
It has been said that the phenolic and amidic derivatives of naphthalene, i.e. the naphthols and naphthylamines, are of the greatest importance to the colour industry. One of the first uses of alpha-naphthylamine has already been mentioned, viz. for the production of the Manchester yellow, which was afterwards made more advantageously from alpha-naphthol. A red colouring-matter possessing a beautiful fluorescence was afterwards (1869) made from this naphthylamine and introduced as “Magdala red.” The latter was discovered by Schiendl of Vienna in 1867. It was prepared in precisely the same way as induline was prepared from aniline yellow. The latter, which is amido-azobenzene, and which is prepared, broadly speaking, by the action of nitrous acid on aniline, has its analogue in amido-azonaphthalene, which is similarly prepared by the action of nitrous acid on naphthylamine. Just as aniline yellow when heated with aniline and an aniline salt gives induline, so amido-azonaphthalene when heated with naphthylamine and a salt of this base gives Magdala red. The latter is, therefore, a naphthalene analogue of induline, as was shown by Hofmann in 1869, and the knowledge of the constitution of the azines which has been gained of late years, enables us to relegate the colouring-matter to this group. This knowledge has also enabled the manufacture to be conducted on more rational principles, viz. by the method employed for the production of the saffranines, as previously sketched.
The introduction of azo-dyes, formed by the action of a diazotised amido-compound on a phenol or another amido-compound, marks the period from which the naphthols and naphthylamines rose to the first rank of importance as raw materials for the colour manufacturer. The introduction of chrysoïdine in 1876 was immediately followed by the manufacture of acid azo-dyes obtained by combining diazotised amido-sulpho-acids with phenols of various kinds, or with bases, such as dimethylaniline and diphenylamine. From what has been said in the foregoing portion of this volume, it is evident that all such azo-compounds result from the combination of two things, viz. (1) a diazotised amido-compound, and (2) a phenolic or amidic compound. Either (1) or (2) or both may be a sulpho-acid, and the resulting dye will then also be a sulpho-acid.
The first of these colouring-matters derived from the naphthols, was introduced in 1876-77 by Roussin and Poirrier, and by O. N. Witt. They were prepared by converting aniline into a sulpho-acid (sulphanilic acid), diazotising this and combining the diazo-compound with alpha- or beta-naphthol. The compounds formed are brilliant orange dyes, the latter being still largely consumed as “naphthol orange.” Other dye-stuffs of a similar nature were introduced by Caro about the same time, and were prepared from the diazotised sulpho-acid of alpha-naphthylamine combined with the naphthols. By this means alpha-naphthol gives what is known as “acid brown,” or “fast brown,” and beta-naphthol a fine crimson, known as “fast red,” or “roccellin.” Diazotised compounds combine also with this same sulpho-acid of alpha-naphthylamine (known as naphthionic acid), the first colouring-matter formed in this way having been introduced by Roussin and Poirrier in 1878. It was prepared by diazotising a nitro-derivative of aniline, and acting with the diazo-salt on napthionic acid, and this dye is still used to some extent under the name of “archil substitute.” In 1878, the firm of Meister, Lucius & Brüning of Höchst-on-the-Main gave a further impetus to the utilization of naphthalene by discovering two isomeric disulpho-acids of beta-naphthol formed by heating that phenol with sulphuric acid. By combining various diazotised bases with these sulpho-acids, a splendid series of acid azo-dyes ranging in shade from bright orange to claret-red, and to scarlets rivalling cochineal in brilliancy were given to the tinctorial industry.