4 декабря, 2015 
Pokraskin 100 grams of p-nitrotoluene is added to 280 grams of 25 per cent oleum, and the mixture is heated to 100°C. while being stirred with a glass or iron stirrer. The sulfonation should be complete within 20 minutes; if necessary, additional fuming sulfuric acid is added. A test sample, mixed with water, should have no odor of p-nitrotoluene and should form a perfectly clear solution.
When the sulfonation is complete, the mixture is cooled to 40° and poured into a mixture of 300 cc. water and 300 grams of ice. The sulfonic acid is salted out in the form of the sodium salt by adding 250 grams of salt. The sulfonate is filtered off, after cooling the mixture, and pressed out well. It is advisable to use continuous mechanical stirring during the precipitation and to wash the product on the filter with 15 per cent salt solution to remove most of the sulfuric acid. The press cake weighs about 220 grams.
Condensation to Sun Yellow. The press cake is dissolved in about 400 cc. water, and enough soda is added to neutralize the residual acid. The volume is made up to 1 liter, and the solution is heated to 65°C. With continuous stirring, 200 grams of 35 per cent sodium hydroxide is added over a period of 15 minutes, and the temperature is raised carefully to 69° at the same time. The caustic solution should not be added too rapidly, or thick clumps of the insoluble sodium salt are formed which retard the condensation. Stirring is continued for 1 hour at 70°, then for 1 hour at 74°, and finally for 1 hour at 76°, taking care that the temperature does not exceed this value locally. The heating is done most satisfactorily on a water bath.
When the condensation is completed, the suspension of the dye is neutralized with about 220 grams of concentrated hydrochloric acid in such a way that the dye does not change color to a brown black. The mixture is thoroughly cooled and the dye is filtered off and dried at 80°C. The yield of pure dye is about 170 grams.
Remarks. The condensation of the sulfo derivative of p-nitrotoluene was first observed accidentally by Johann Walther, and further investigation of the reaction has shown that different dyes, varying in color from yellow to red, are formed depending on the nature of the reaction. If the condensation is carried out under somewhat more drastic conditions, for example, with more sodium hydroxide and at a liigher temperature, the color of the dye is more greenish. Still more greenish dyes can be obtained by careful oxidation with sodium hypochlorite (sun yellow 3G, polyphenyl yellow 3G).
Reduction of these dyes leads to tints which are appreciably more reddish, going as far as reddish oranges. If, for example, the dye prepared above is only partly neutralized and reduced directly after the condensation with 40 grams of crystalline sodium sulfide (1 hour at 75°C.), followed by acidification as described above, Mikado orange R is obtained. This is a very good, light-fast cotton dye.
All of these dyes are characterized by the fact that they do not dye wool and silk from neutral baths, in contrast to the behavior of chrysophenine, but on vegetable fibers they exhibit remarkable fastness to light and washing. They are also used in coloring paper. The structures of the dyes are not known with certainty. They all give a mixture of diaminostilbene — and diaminodibenzyldisulfonic acids on vigorous reduction, for example, with zinc dust and sodium hydroxide or with am — uioniacal ammonium sulfide. These dyes are prepared in the plant in small concrete vats equipped with concrete stirrers. The equipment lasts many years. Copper is also very suitable, but is usually too costly.
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