Coal, and What We Get from It

by Raphael Meldola

Other transformations of rosaniline have yet to be chronicled. In 1862 Hofmann found that by acting upon this base—the base of magenta—with the iodide of methyl, violet colouring-matters were produced, and these were for some years extensively employed under the name of Hofmann’s violets. And still more remarkable, by the prolonged action of an excess of methyl iodide upon rosaniline, Keisser found that a green colouring-matter was formed. The latter was patented in 1866, and the dye was for some time in use under the name of “iodine green.” The statement that technology profited by the introduction of magenta has therefore been justified.

It remains to add, that the tar obtained from one ton of Lancashire coal furnishes an amount of aniline capable of giving a little over half a pound of magenta. The colouring power of the latter will be inferred from the fact, that this quantity would dye 375 square yards of white flannel of a full red colour, and if converted into Hofmann violet by methylation, would give enough colour to dye double this surface of flannel of a deep violet shade. It should be stated also, that during the formation of magenta by the arsenic acid process, there are formed small quantities of other colouring-matters which are utilized by the manufacturer. Among these by-products is a basic orange dye, which was isolated by Nicholson, and investigated by Hofmann in 1862. Under the name of “phosphine” this colouring-matter is still used, especially for the dyeing of leather. Even the spent arsenic acid of the magenta-still has its use. The arsenious acid resulting from the reduction of this arsenic acid is generally obtained in the form of a lime salt after the removal of the magenta by the purifying processes to which the crude product is submitted. From the arsenical waste arsenious acid can be recovered, and converted back into arsenic acid by the action of nitric acid. Quite recently the arsenical residue has been used with considerable success in America as an insecticide for the destruction of pests injurious to agricultural crops.

Concurrently with these technical developments of coal-tar products, the scientific chemist was carrying on his investigations. The compounds which science had given to commerce were made on a scale that enabled the investigator to obtain his materials in quantities that appeared fabulous in the early days when aniline was regarded as a laboratory curiosity, and magenta had been seen by only a few chemists.

The fundamental problem which the modern chemist seeks to solve is in the first place the composition of a compound, i.e. the number of the atoms of the different elements which form the molecule, and in the next place the way in which these atoms are combined in the molecule. Reverting to our former analogy, the first thing to be found is how many different blocks enter into the composition of the structure, and the next thing is to ascertain how the blocks are arranged. When this is done, we are said to know the “constitution” or “structure” of the molecule, and in many cases when this is known we can build up or synthesise the compound by combining its different groups of atoms by suitable methods. The coal-tar industry abounds with such triumphs of chemical synthesis; a few of these achievements will be brought to light in the course of the remaining portions of this work.

The chemical investigation of magenta was commenced by Hofmann, whose name is inseparably connected with the scientific development of the coal-tar colour industry. In 1862 he showed that magenta was the salt of a base which he isolated, analysed, and named rosaniline. He established the composition of this base and of the violet and blue colouring-matters obtained from it by the processes already described. In 1864 he made the interesting discovery that magenta is not formed by the oxidation of pure aniline, but that a mixture of aniline and toluidine is essential for the production of this colouring-matter. In fact, the aniline oil used by the manufacturer had from the beginning consisted of a mixture of aniline and toluidine, and at the present time “aniline for red” is made by nitrating a mixture of benzene and toluene and reducing the nitro-compounds.

From this work of Hofmann’s suggestions naturally arose concerning the “constitution” of rosaniline, and new and fruitful lines of work were opened up. Large numbers of chemists of the greatest eminence pursued the inquiry, but the details of their work, although of absorbing interest to the chemist, cannot be discussed in the present volume. The final touch to a long series of investigations was given by two German chemists, Emil and Otto Fischer, who in 1878 proved the constitution of rosaniline by obtaining from it a hydrocarbon, the parent hydrocarbon from which the colouring-matter is derived. The purely scientific discovery of the Fischers threw a flood of light on the chemistry of magenta, and enabled a large number of colouring-matters related to the latter to be classed under one group, having the parent hydrocarbon as a central type. This hydrocarbon, it may be remarked, is known as triphenylmethane, as it is a derivative of methane, or marsh gas. The blues and violets obtained from rosaniline belong to this group, and so also do certain other colouring-matters which had been manufactured before the Fischers’ discovery. In order to carry on the story of the utilisation of aniline, it is necessary to know something about these other colouring-matters which are obtained from it.

It has been explained that by the methylation of rosaniline Hofmann obtained violet colouring-matters. Now as rosaniline is obtained by the oxidation of a mixture of aniline and toluidine, it seems but natural that if these bases were methylated first and then oxidized a violet dye would be produced. The French chemist Lauth first obtained a violet colouring-matter by this method in 1861. In 1866 this violet dye was manufactured in France by Poirrier, and it is still made in large quantities, being known under the name of “methyl violet.” This colouring-matter, and a bluer derivative of it discovered in 1868, gradually displaced the Hofmann violets, chiefly owing to their greater cheapness of production. We are thus introduced to methylated aniline as a source of colouring-matters, and as the compound in question has many different uses in the coal-tar industry, a few words must be devoted to its technology.

Aniline, toluidine, and similar bases can be methylated by the action of methyl iodide, but the cost of iodine is too great to enable this process to be used by the manufacturer. Methyl chloride, however, answers equally well, and this compound, which is a liquid of very low boiling point (-23° C.), is prepared on a large scale from the waste material of another industry, viz. the beet-sugar manufacture. It is interesting to see how distinct industries by chemical skill are made to act and react upon one another. Thus the cultivation of the beet, as already explained, is largely dependent on the supply of ammonia from gas-liquor. During the refining of the beet-sugar, a large quantity of uncrystallisable treacle is separated, and this is fermented for the manufacture of alcohol. When the latter is distilled off there remains a spent liquor containing among other things potassium salts and nitrogenous compounds. This waste liquor, called “vinasse,” is evaporated down and ignited in order to recover the potash, and during the ignition, ammonia, tar, gas, and other volatile products are given off. Among the volatile products is a base called trimethylamine, which is a derivative of ammonia; the salt formed by combining trimethylamine with hydrochloric acid when heated gives off methyl chloride as a gas which can be condensed by pressure.

Here we have a very pretty cycle of chemical transmigration. The nitrogen of the coal plants, stored up in the earth for ages, is restored in the form of ammonia to the crops of growing beet; the nitrogen is made to enter into the composition of the latter plant by the chemico-physiological process going on, and the nitrogenous compounds removed from the plant and heated to the point of decomposition in presence of the potash (which also entered into the composition of the plant), give back their nitrogen partly in the form of a base from which methyl chloride can be obtained. The latter is then made to methylate a product, aniline, derived indirectly from coal-tar. The utilisation of the “vinasse” for this purpose was made known by Camille Vincent of Paris in 1878.

The methylation of aniline can obviously be carried out by the foregoing process only when beet-sugar residues are available. There is another method which is more generally used, and which is interesting as bringing in a distinct branch of industry. The same result can, in fact, be arrived at by heating dry aniline hydrochloride, i.e. the hydrochloric acid salt of aniline, with methyl alcohol or wood-spirit in strong metallic boilers under great pressure. This is the process carried on in most factories, and it involves the use of pure methyl alcohol, a branch of manufacture which has been called into existence to meet the requirements of the coal-tar colour maker.[4] This alcohol or wood-spirit is obtained by the destructive distillation of wood, and is purified by a series of operations which do not at present concern us. It must be mentioned that the product of the methylation of aniline, which it is the object of the manufacturer to obtain, is an oily liquid called dimethylaniline, which, by virtue of the chemical transformation, is quite different in its properties to the aniline from which it is derived. By a similar operation, using ethyl alcohol, or spirit of wine, diethylaniline can be obtained, and by heating dry aniline hydrochloride with aniline under similar conditions a crystalline base called diphenylamine is also prepared.