Six hundred and eighty-five grams (4.75 moles) of technical anthranilic acid (generally about 95 per cent pure) (Note 1) is dissolved in 1.5 l. of water and 500 cc. of concentrated hydrochloric acid (sp. gr. 1.17) (Note 2), by heating. The insoluble dark impurity present in small amounts is filtered off, and the filtrate is transferred to a 16-l. crock and chilled with stirring. It is then mixed with a mush of 2.5 kg. of ice and 750 cc. of concentrated hydrochloric acid. The crock is cooled externally with ice, and the contents stirred continuously. When the temperature reaches about 3°, a filtered solution of 360 g. (5 moles) of sodium nitrite in 700 cc. of water is dropped in slowly, through a long capillary tube reaching below the surface of the liquid (Note 3), until a faint but permanent reaction to starch-potassium iodide paper is obtained; the temperature is kept between 3 and 5° (Note 4). This operation requires all but about 30 cc. of the nitrite solution and occupies one and one-half to two hours.

To the solution of the diazonium salt is now added 848 g. (885 cc., 7 moles) of dimethylaniline; this may be done rapidly, as the temperature does not rise appreciably. Stirring is continued for one hour, the temperature being kept at 5°. Five hundred cubic centimeters of a filtered solution of 680 g. (8.3 moles) of crystallized sodium acetate diluted to 1200 cc. is then added, and the stirring continued for four hours. If a foamy solid rises to the surface during this time and refuses to become incorporated by the stirrer, a few drops of ethyl acetate may be added to reduce the foam. The mixture is allowed to stand overnight in an ice bath which is well insulated by several thicknesses of burlap; the temperature must be kept below 7° to get a good yield of product. The remainder of the sodium acetate solution is then added with stirring, and, after the mixture has been stirred for an additional period of one to three hours, the temperature is allowed to rise slowly to 20–25° in the course of twenty-four hours. Just enough sodium hydroxide solution is then added, with stirring, to cause the mixture to have a distinct odor of dimethylaniline (about 240 cc. of a 40 per cent solution is generally required), and the mixture is allowed to stand for forty-eight hours or longer at room temperature (20–25°) (Note 5).

The solid is then filtered off, washed first with water, then with 400 cc. of 10 per cent acetic acid (to remove the dimethylaniline), and finally with distilled water. The last filtrate is generally pale pink. The solid is sucked as dry as possible, spread out on a tray in order to allow most of the water to evaporate (fifteen to twenty hours), and then suspended in 4 l. of methyl alcohol in a 12-l. flask. This mixture is stirred on the steam bath under a reflux condenser for one to two hours, allowed to cool slowly, and then chilled in an ice bath and filtered. The solid product is washed with a second 4 l. of cold methyl alcohol (Note 6). After being dried in air, it varies in weight from 820 to 870 g.

The product is extracted with boiling toluene in the following manner: 150 g. is placed in a fluted filter paper of 29-cm. diameter in a 25-cm. glass funnel which passes through the cork of a 2-l. flat-bottomed conical flask containing 1250 cc. of toluene (Fig. 21).

Fig. 21.

The flask is heated on an electric stove, and a 12-l. round-bottomed flask, clamped to a ring stand, is placed on the funnel to act as a condenser, cold water being run through the flask. The toluene is boiled until the condensed liquid runs through almost colorless (this requires from four to ten hours). The heating is then discontinued, and, as soon as the liquid ceases to boil, the flask is removed to a bath containing water at 90–100°; the level of the water should be slightly above the level of the liquid in the flask. This arrangement permits the temperature to fall slowly so that large crystals are obtained. In the meantime a second conical flask containing 1250 cc. of toluene is attached to the funnel, and a new charge of 150 g. of crude methyl red is placed in the paper. The process is repeated until all the crude material has been so treated. When extraction is complete it is found that a certain amount of black amorphous insoluble matter remains on the filter; this is discarded. The crystals of methyl red (Note 7) are filtered off and washed with a little toluene. The weight of pure material is 755–805 g. The mother liquors are concentrated to one-fourth of their volume, and the crystals which separate on cooling are recrystallized from fresh toluene. The recovered toluene can, of course, be employed again. The total yield of pure methyl red is 790–840 g. (62–66 per cent of the theoretical amount) (Note 8). It melts at 181–182°.

The watery mother liquors from the crude methyl red are rendered alkaline with sodium hydroxide and distilled until no more dimethylaniline passes over. In this way 250 to 400 g. of moist dimethylaniline is recovered.