[Illustration: 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 competition 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.
[Illustration: 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 competition between artificial and natural indigo would assume a much more serious aspect.[5]