Part 4 (1/2)

1861. Methyl violet discovered; manufactured in 1866; leading to a new use of copper salts as oxidizing agents, and to the manufacture of dimethylaniline.

1862. Hofmann violets discovered; leading to manufacture of methyl iodide from iodine, phosphorus, and wood spirit.

1862. Phosphine (chrysaniline) discovered in crude magenta.

1863. Aniline black introduced; leading to a new use for pota.s.sium chlorate and copper salts, and to the manufacture of aniline salt.

1863. Aniline yellow introduced, the first azo-colour.

1864. Induline discovered; leading to new use for aniline yellow.

1866. Manchester brown introduced, the second azo-colour; leading to the manufacture of sodium nitrite, and of dinitrobenzene.

1866. Iodine green introduced; leading to further use for methyl iodide.

1866. Diphenylamine blue introduced; leading to manufacture of diphenylamine.

1868. Blue shade of methyl violet introduced; leading to manufacture of benzyl chloride.

1868. Saffranine introduced.

1869. Nitrobenzene process for magenta discovered.

1876. Chrysodine introduced, the third azo-colour.

1876. Methylene blue introduced; leading to manufacture of nitrosodimethylaniline.

1877. Acid magenta discovered.

1878. Methyl green introduced; leading to utilization of waste from beet-sugar manufacture.

1878. Malachite green discovered; leading to manufacture of benzoic aldehyde.

1878. Acid yellow discovered; leading to new use for aniline yellow.

1879. Neutral red and allied azines introduced; leading to a new use for nitrosodimethylaniline.

1883. Phosgene colours of rosaniline group introduced; leading to manufacture of phosgene.

CHAPTER III.

Among the most venerable of natural dye-stuffs is indigo, the substance from which Unverdorben first obtained aniline in 1826. The colouring matter is found in a number of leguminous (see Fig. 7), cruciferous, and other plants, some of which are largely cultivated in India, China, the Malay Archipelago, South America, and the West Indies; while others, such as woad (see Fig. 8), are grown in more temperate European climates. The tinctorial value of these plants was known in India and Egypt long before the Christian era. Egyptian mummy-cloths have been found dyed with indigo.

The dye was known to the Greeks and Romans; its use is described by the younger Pliny in his Natural History. Indigo was introduced into Europe about the sixteenth century, but its use was strongly opposed by the woad cultivators, with whose industry the dye came into compet.i.tion. In France the opposition was strong enough to secure the pa.s.sing of an act in the time of Henry IV. inflicting the penalty of death upon any person found using the dye. The importance of indigo as an article of commerce is sufficiently known at the present time; more than 8000 tons are produced annually, corresponding in money value to about four million pounds. It is of importance to us as rulers of India to remember that the cultivation and manufacture of indigo is one of the staple industries of that country, from which the European markets derive the greater part of their supply.

[Ill.u.s.tration: FIG. 7.--INDIGO PLANT (_Indigofera tinctoria_).]

Imagine the industrial revolution which would be caused by the discovery of a process for obtaining indigo synthetically from a coal-tar hydrocarbon, at a price which would compare favourably with that of the natural product. This has not actually been done as yet, but chemists have attempted to compete with Nature in this direction, and the present state of the compet.i.tion is that the natural product can be cultivated and made more cheaply. Nevertheless the dye can be synthesised from a coal-tar hydrocarbon, and this is one of the greatest achievements of modern chemistry in connection with the tar-products. For more than half a century indigo had been undergoing investigation by chemists, and at length the work culminated in the discovery of a method for producing it artificially. This discovery was the outcome of the labour of Adolf v.

Baeyer, who commenced his researches upon the derivatives of indigo in 1866, and who in 1880 secured the first patents for the manufacture of the colouring-matter. It is to the laborious and brilliant investigations of this chemist that we owe nearly all that is at present known about the chemistry of indigo and allied compounds.

[Ill.u.s.tration: FIG. 8.--WOAD (_Isatis tinctoria_).]

Two methods have been used for the production of artificial indigo--benzal chloride being the starting-point in one of these, and nitrobenzoic aldehyde in the other. The generating hydrocarbon is therefore toluene. By heating benzal chloride with dry sodium acetate there is formed an acid known as cinnamic acid, a fragrant compound which derives its name from cinnamon, because the acid was prepared by the oxidation of oil of cinnamon by Dumas and Peligot in 1834. The acid and its ethers occur also in many balsams, so that we have here another instance of the synthesis of a natural vegetable product from a coal-tar hydrocarbon. The subsequent steps are--(1) the nitration of the acid to produce nitrocinnamic acid; (2) the addition of bromine to form a dibromide of the nitro-acid; (3) the action of alkali on the dibromide to produce what is known as ”propiolic acid.” The latter, under the influence of mild alkaline reducing agents, is transformed into indigo-blue. The process depending on the use of nitrobenzoic aldehyde is much simpler; but the particular nitro-derivative of the aldehyde which is required is at present difficult to make, and therefore expensive. If the production of this compound could be cheapened, the compet.i.tion between artificial and natural indigo would a.s.sume a much more serious aspect.[5]

The light oil of the tar-distiller has now been sufficiently dealt with so far as regards colouring-matters; let us pa.s.s on to the next fraction of the tar, the carbolic oil. The important const.i.tuents of this portion are carbolic acid and naphthalene. The carbolic oil is in the first place separated into two distinct portions by was.h.i.+ng with an alkaline solution.

Carbolic acid or phenol belongs to a cla.s.s of compounds derived from hydrocarbons of the benzene and related series by the subst.i.tution of the residue of water for hydrogen. This water-residue is known to chemists as ”hydroxyl”--it is water less one atom of hydrogen. Carbolic acid or phenol is hydroxybenzene; and all a.n.a.logous compounds are spoken of as ”phenols.”

It will be understood in future that a phenol is a hydroxy-derivative of a benzenoid hydrocarbon. Now these phenols are all more or less acid in character by virtue of the hydroxyl-group which they contain. For this reason they dissolve in aqueous alkaline solutions, and are precipitated therefrom by acids. This will enable us to understand the purification of the carbolic oil.

The two layers into which this oil separates after was.h.i.+ng with alkali are (1) the aqueous alkaline solution of the carbolic acid and other phenols, and (2) the undissolved naphthalene contaminated with oily hydrocarbons and other impurities. Each of these portions has its industrial history.

The alkaline solution, on being drawn off and made acid, yields its mixture of phenols in the form of a dark oil from which carbolic acid is separated by a laborious series of fractional distillations. The undissolved hydrocarbon is similarly purified by fractional distillation, and furnishes the solid crystalline naphthalene. The tar from one ton of Lancas.h.i.+re coal yields about 1-1/2 lbs. of carbolic acid, equal to about 1 per cent. by weight of the tar, and about 6-1/4lbs. of naphthalene, so that this last hydrocarbon is one of the chief const.i.tuents of the tar, of which it forms from 8 to 10 per cent. by weight.

The crude carbolic acid as separated from the alkaline solution is a mixture of several phenolic compounds, and all of these but the carbolic acid itself are gradually removed during the process of purification.

Among the compounds a.s.sociated with the carbolic acid are certain phenols of higher boiling-point, which bear the same relations.h.i.+p to carbolic acid that toluene bears to benzene. That is to say, that while phenol itself is hydroxybenzene, these other compounds, which are called ”cresols,” are hydroxytoluenes. The cresols form an oily liquid largely used for disinfecting purposes under the designation of ”liquid carbolic acid,” or ”cresylic acid.” Carbolic acid is a white crystalline solid possessing strongly antiseptic properties, and is therefore of immense value in all cases where putrefaction or decay has to be arrested. It was discovered in coal-tar by Runge in 1834, and was obtained pure by Laurent in 1840.