Aniline. Cells.N132.—This substance was discovered in 1826, by Unverdorben, who obtained it from indigo, henoe its name, and, being the Portuguese for indigo. It exists in the heavier tar oils ; but its extraction front them is a matter of scientific curiosity rather than of comrnercial importance, and its quantity is too small to compete with nitro-benzol. By the action of reducing agents, nitro-benzol is converted into "aniline," or " amido-benzol." The process generally used for the commercial production of aniline, is, with slight modifications, that known as Bechamp's. In a capacious retort are mixed 1 part by weight of nitro-benzol, 1.2 part of iron-filings, and acetic acid (1.068 sp. gr.) equal in volume to the nitro-benzol. A reaction immediately com mences, with active effervescence, accompanied by a rise of temperature. To prevent the passing over of the acetic acid, it is well to cool the retort. Whatever may have passed over should be retu;.ned, and, after a little time, the distillation may be begun. The product will consist of aniline, acetate of aniline, and acetaniline, the last appearing towards the end of the operation. A little milk of lime, or potash, added before distilling, will greatly prevent the passing over of the two last. By using acetate of protoxide of iron, the action has been rendered less energetic, and more controllable. The yield of aniline will be 60-65 per cant. on the nitro-benzel used.
The main points in the commercial production of aniline are practically the same ; but scarcely two manufacturers adopt the same details. The superiority of many of the aniline colours depends upon the quality of the aniline employed, which is generally subjected to one or more rectifications. This is performed by distilling, and e,ollectinethe dietillates separately, the portien richest in benzol aniline being collected between 182° and 199° (360° and 390°F.). It is difficult, by fractionizing, to free aniline from toluidine, consequently high-class benzols should be used for anilines.
Some manufacturers reduee the nitro-benzol to crude anilioe in separate vessels, and distil in special retorts ; this is not necessary, and the two operations are usually performed in the same vessel. A great advantage in this consists in obtaining the aniline at one operation ; the unaltered benzol is collected, and returned to the still, until the reduction is complete, when direct heat may be applied, and the aniline may be drawn over sufficiently pure without further rectification by distilling, unless for the very finest class anilines. In this case, a little lime is thrown into the still before drawing over the aniline. Much depends upon the extent of the operations : a method which would be far from economical on a small scale, may be the only one practicable on an extensive scale. On a.small scale, it is better to work with nitro-benzol suited to the special description of
aniline required, and to work it off in one still, as shown in Fig. 486. A cylindrical still A is fitted into masonry, with a furoace so constructed as to avoid direct' action of the fire. Through the cover, which is bolted on, passes the spindle a of the agitator b, which rotates during the whole time that the conversion is going on ; a bent tube or still head 0, fitted to the cover, conveys the vapour to a properly cooled condenser. Be fore the cover is bolted on, it is usual to put into the still the required quantity of iron turnings, otherwise a suitable opening d must be left, especially for continnons worldng, to avoid removing the cover every time the still is filled.
A small opening e, provided with a stop-cock, and communicating with the nitro-benzol reservoir, enables this liquid to be delivered as required, at or near the bottom of the vessel, so as to avoid its evaporation before coming into contact with the iron-turnings. A quantity of acetic aeid is poured over the turnings, either alI at once, or gradually. It is sometimes replaced by hydro chloric acid, whose more energetic action requires it to be added more cautiously. The heat gene rated will drive some unaltered nitro-benzol into the condenser ; this should be poured back again. The agitator should be worked gently, and at intervals, as required. As the nitro-benzol is added, the heat increases, and when no further augmentation of heat is perceived, the reaction may be considered almost complete ; this, how ever, will depend upon the way in which the operation has been conducted. The main pre caution is the prevention of volatilization, until the whole of the nitro-benzol is reduced ; in older to secure this more effectually, it is better to add the liquids alternately at intervals, and to keep them well cooled and agitated.
When the liquids which pass into the condenser remain clear on the addition of hydrochloric . acid, it may be assumed that all the nitro-benzol is reduced ; and the distillation may be com menced, by lighting the fire under the still. The spindle a is hollow, so as to admit the steam from the pipe f g for distilling. A little care will remove most of the difficulties of the process ; and, by giving a little extra time for working off the charge, a product can be obtained, which, by one recti fication, will yield an aniline of sufficient purity for almost any purpose. The aniline can be freed from the mass hy occasionally stirring with the agitator.
Crude aniline is generally fractionized by the colour manufacturer, who is often in a position to supply himself with anilines which he could purchase only at a highly enhanced cost.