Encyclopædia Britannica, Ninth Edition/Aniline
ANILINE, or Phenylamine (C 6 H.N), is one of the very numerous products of the distillation of coal tar. The substance has been known to chemists since the year 1826, when M. Unverdorben, by the destructive distilla tion of indigo, produced a liquid of an oily consistence, which he termed crystalline. In the year 1835, Runge, a German chemist, isolated from the oil of coal tar a sub stance which, having the property of producing a beautiful blue colour on treatment with chloride of lime, he named kyanol. In 1840, another investigator showed that by treating indigo with caustic potash it yields an oil, and ho- gave to his product the name Aniline, from the specific, name of one of the indigo yielding plants, Indigofent. anil. About the same time it was discovered that on treating nitrobenzol with a reducing agent, an organic base is formed, to which the observer of this reaction gave the name benzidam. Professor Hofmann, while a student in the laboratory of Baron Liebig, investigated these variously prepared substances, and found them to be identical in composition and action, and thenceforth they took their place as one body, under the name Aniline or Phenylamine. Pure aniline is a basic substance of an oily consistence, colourless, boiling at a temperature of 182 C. Dropped on paper aniline produces a greasy stain, which, owing to its volatilising at an ordinary atmospheric temperature, quickly disappears. On exposure to air it absorbs oxygen rapidly, and becomes of a deep brown colour; and it ignites readily, burning with a large smoky flame. It is possessed of a somewhat pleasant vinous odour, and a burning aromatic taste. It is a highly acrid poison, and Dr Letheby has pointed out that the poisonous action of nitrobenzol is due. to a change of that substance into aniline taking place within the stomach.
The readiness with which aniline, in certain of its re actions, produces very brilliant colours, was of course known to chemists from the time Runge produced it from coal tar and gave it the name kyanol. Up to year 1858 it, however, possessed nothing beyond a scientific interest. On the 26th of August 1858, Mr W. H. Perkin obtained a patent for the production of a dye-stuff derived from aniline, which soon became well known as mauve, or Perkin s purple, as well as by various other names. The discovery of Mr Perkin formed the turning-point in the history of aniline, and was indeed the beginning of a great revolution in the arts and manufactures connected with the dyeing of textile fabrics. The manufacture of aniline dyes was really first begun in France, the French manu facturers acting on the information supplied by Mr Perkin s patent specification. It immediately spread to all in dustrial centres, and became one of the most eagerly investigated of all commercial undertakings. A rapid suc cession of patents were applied for and obtained ; new processes and combinations were continually being pro jected, and a great variety of colours were tried with more or less success as commercial substances. The activity of scientific research kept pace with the energy of manufactur ing enterprise, resulting in a rapid improvement of pro cesses, decrease in the cost of the manufacture, and a great increase in the beauty and tinctorial effect of the dyes pro duced. At the present time every colour, and all tints and shades of colours, are produced from aniline ; and, while the processes employed and the combinations formed are very numerous, the names under which the dye-stuffs are sold must be said to be endless.
The method by which Mr Perkin obtains from aniline the violet colour known as mauve, is by mixing a solu tion of sulphate of aniline and of potassic dichromate in equivalent proportions, and leaving them for several hours till the resulting reaction is complete. A black precipitate is formed, which is washed free from the potassic sulphate it contains, and then treated with naphtha to dissolve out the resinous matter contained in the mass. The residue consists of the mauve dye, and may be dissolved in alcohol. It has been found to be the sulphate of a base, to which the name mauveine has been given.
The greater proportion of aniline dyes manufactured are now produced directly or indirectly from another basic body, termed rosauiline. About the time of the French-Austrian war in 1859, a second coal-tar dye was introduced into commerce, which became popularly known as aniline red or Magenta, from the battle fought at the period of its intro duction. Dr Hofmann s investigations into the nature and composition of this dye have done more to place the entire industry on a satisfactory basis than any other undertaking. He found that it consisted of a salt of a basic substance, to which he gave the name rosaniline (C 20 H 19 N 3 H 2 0). For the preparation of rosaniline, Hofmann found that a certain proportion of toluidine must be present in the commercial aniline employed. Rosaniline is now entirely manufactured by treating aniline of a known composition with a strong solution of arsenic acid in an iron retort, heated to a temperature of not more than 180 C. The reaction occupies about eight hours ; and at the conclusion of the operation a crude mass, consisting of rosaniline arsenite and arseniate, is found, which is next dissolved in water acidulated with hydrochloric acid. Common salt is added to this solution, when a double decomposition takes place. Rosaniline hydrochlorate is formed on the one hand, and sodium arseniate and arsenite on the other. For the preparation of the salts of rosaniline, the base is treated directly with acids.
Rosaniline blues are prepared by acting upon commercial aniline with a salt of rosaniline, such as the acetate, under the influence of a heat of about 190 C., kept up for two hours. The reaction which takes place is the substitu tion of three atoms of phenyl for three of hydrogen, and hence these blues are chemically salts of triphenylrosani- line. Violet colours, such as Hofmann s violet, are pre pared by an analogous process to that employed for the blues, by treating rosaniline with the iodides of methyl or ethyl, atoms of these radicals taking the place of hydrogen, and forming ethylic or methylic rosauiline. Aniline green is formed by the action of aldehyd on a solution of aniline red in sulphuric acid, and subsequent boiling in a solution of hyposulphite of soda. Aniline yellow, or yellow fuchsine, is formed from a base named chrysaniline, a minor product of the formation of rosaniline ; but most of what is termed aniline yellow is prepared from picric acid, a different pro duct of the distillation of coal tar. Various shades of brown and maroon, as well as black colours, are regular commercial products ; but they are neither so well under stood, nor of such consequence as the reds, blues, and greens.
Aniline colours are employed in the industrial arts for numerous other purposes besides their great use as dyeing materials. Violet ink and other fancy coloured inks are prepared from them. They are used by paper manu facturers for tinting pulps, and for the superficial staining of finished paper. They are likewise used by paper stainers in the printing of wall papers, in the preparation of lithographic inks, and to some extent for water colours. They are largely employed as colouring materials in per fumery, fancy soaps, and cosmetics, besides having many other minor applications.
Concerning these dyes, Dr Hofmann, to whom the industry is so much indebted, wrote, in 1862, while it was yet in its infancy, " Instead of disbursing her annual millions for these substances, England will, beyond question, at no distant day become herself the greatest colour- producing country in the world; nay, by the very strangest of revolutions, she may erelong send her coal-derived blues to indigo-growing India, her distilled crimson to cochineal- producing Mexico, and her fossil substitutes for quercitron and safflower to China, Japan, and the other countries whence these articles are now derived." It is scarcely needful to say that these bold anticipations have already been fully realised.