C6H6+NO2OH=C6H5(NO2)+HOH
Benzol. Nitric acid. Nitro-benzol. Water.

Fig. 353.—Iron Pots for making Nitro-Benzol.

Fig. 354.—Section of Apparatus for making Nitro-Benzol.

Nitro-benzol has a sweet taste and a fragrant odour. It is known in commerce under the names of artificial oil of bitter almonds and essence of mirbane, and it has been used for perfuming soap. The chemical action between benzol and concentrated nitric acid is so violent that, when nitro-benzol first had to be manufactured on the large scale, great difficulty was experienced on account of the serious explosions which occurred. The apparatus now used in making nitro-benzol on the large scale is represented in Fig. [353], which shows some of the cast-iron pots, of which there is usually a long row. These pots are about 4½ ft. in diameter, and the same in depth. Each is provided with a stirrer, which is made to revolve by a bevil-wheel, c, on its spindle, working with a pinion on a shaft, b, driven by a steam engine. A layer of water is kept on the tops of the lids, the water being constantly passed in and drawn off through the pipes, d, in order to keep it cool. For the chemical action is, as usual, attended with heat, which vaporizes some of the benzol, but the cold lid re-condenses the vapour, which would otherwise escape with the nitrous fumes that pass off by the pipe, a. There is at e an opening, through which the material may be introduced, and in the bottom of the vessel is an aperture through which the products may be drawn off. Fig. [354] shows a section of one of the cast-iron vessels, and exhibits the mode in which the spindle of the stirrer passes through the lid. In the cup, a, filled with a liquid, a kind of inverted cup, which is attached to the spindle, turns round freely. It would not do to choose water for the liquid in this cup, for water would, by absorbing the nitrous fumes, form an acid capable of attacking and destroying the spindle. Nothing has been found to answer better for this purpose than nitro-benzol itself. The charge introduced into these vessels is a mixture of nitric and sulphuric acids together with the benzol. During the action, which may last twelve or fourteen days, no heat is applied, for the mixture becomes hot spontaneously, and in fact care must be taken that it does not become too hot. The nitro-benzol thus obtained is purified by washing with water and solution of soda.

Fig. 355.—Apparatus for making Aniline.

If nitro-benzol were brought into contact with ordinary hydrogen gas, no action whatever would take place. But it is well known to chemists that gases which are just being liberated from a compound have at the instant of their generation much more powerful chemical properties than they possess afterwards. Gases in this condition are said to be in the nascent state. If we submit nitro-benzol to the action of nascent hydrogen we find a remarkable change is produced. This change consists, first, in the hydrogen robbing the nitro-benzol of all its oxygen atoms; second, in the addition of hydrogen to the remainder; third, in some re-arrangement of the atoms, by which a new body is formed. Not that these changes are successive, or that we actually know the movement of atoms, but we are thus able to form ideas which correspond with the final result. The new substance is named aniline. It is regarded by chemists as a base; that is, a substance capable of neutralizing and combining with an acid to form a salt. Its composition is represented by the symbols C6H5 H2N. Aniline was found in coal-tar in 1834, and even its colour-producing power was noticed, for its discoverer named it kyanol, in allusion to the blue colour it produced with chloride of lime. Later it was obtained by distilling indigo with potash, and hence received its present name from anil, the Portuguese for indigo. The quantity of aniline contained in the tar is quite insignificant.

Aniline is prepared from nitro-benzol on the large scale by heating it with acetic acid and iron filings or iron borings, a process which rapidly changes the nitro-benzol into aniline. The equation representing the change is—