Indoxyl fusion

In order to prepare the potassium salt, 20 g. of phenylglycine are made exactly neutral to phenolphthalein with 2 A-potassium hydroxide solution (about 70 c.c. are required), and the clear solution is then evaporated to dryness on the water bath. Fot the indoxyl fusion the residue of salt must be dried in an oven at 100° for several hours. 

Indoxyl Fusion.2—A mixture of 15 g. of sodium hydroxide and 20 g. of potassium hydroxide is fused and carefully dehydrated by heating to about 500° in a nickel crucible. When the mass has barely solidified it is just remelted by gentle heating and poured into a Jena glass conical flask (capacity 100 c.c.) which is at a temperature of 220° in an oil bath. If this procedure is adopted there need be no fear that the glass will crack. 

Sodamide (10 g.) is added to the melt in the flask and dissolves with slight evolution of ammonia. The pure potassium phenylglycine (20 g.) which has been completely dried at 100° in an oven is 

After oxidation has thus been completed the indigo is collected at the pump, washed with hot water, transferred to a beaker by means of a jet of water, boiled with 10 per cent hydrochloric acid, again collected at the pump, washed with hot water, and dried. The yield reaches 60 to 70 per cent of the theoretical. 

A simple qualitative test of the purity of the indigo obtained can be carried out as follows A little of the material is heated for some time (with shaking) in a test tube with pyridine and some drops of the liquid are then poured on to a filter paper. If the indigotin is pure the pyridine is not coloured, whereas impurities which may be formed when working on a small scale confer on it a more or less dirty brown colour, as shown by a spot test. If it is desired to purify the whole of the indigo with pyridine, the dye is collected at the pump after boiling with the liquid, washed with pure hot pyridine, boiled once more with hydrochloric acid, collected at the pump, washed with hot water, and dried.2 

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The greatest improvement in the manufacture of indigotin came when sodamide was used with alkali in the conversion of phenylglycine to indoxyl (125). Not only was the fusion temperature lowered from about 300°C to 200°C, but also the reaction was made practically anhydrous by the sodamide... 

In the synthesis due to Heumann and Pfleger the fusion yields the potassium derivative of indoxyl, which is dehydrogenated to indigo even by atmospheric oxygen at the same time hydrogen peroxide is produced (see p. 175). 

There are numerous additional studies based on the properties of enam-ines and the synthesis of the pyrrole ring. Thus, fusion of dienetriamines formed by a reaction of aminals of a,/3-unsaturated /3-dimethylaminoalde-hydes (41) with /V-acetylindoxyl results in cyclization to pyrroloindoles (42) (87IZV821). Dienetriamino derivatives of a different type (43) obtained from indoxyl are also transformed into pyrrolo[l, 2-a]indoles (44) upon... 

Nearly all indigo is produced from N-phenylglycine (see Fig. 8.18) via fusion with potassium and sodium hydroxide, followed by treatment with sodamide [103]. The melt containing the dialkalimetal salt of indoxyl is subsequently dissolved in water, and indigo is formed by aerobic oxidation. Filtration and wash-... 

Heumann s Synthesis, Phenyl Glycine Ortho-carboxylic Acid.—The synthesis that has resulted in placing synthetic indigo on the market is that of Heumann by the fusion of phenyl glycine ortho-carboxylic acid with caustic potash. The product of this fusion is indoxyl which by atmospheric oxygen is oxidized to indigo. The industrial success of this synthesis was achieved only when the preparation of the-phenyl glycine ortho-carboxylic acid from a cheap source was accomplished. 

Fusion with sodium hydroxide converts the acid into indoxyl,... 

Among later syntheses of indigo was that from phenylglycine carboxylic acid by Heumann. This was first made from aniline, but its preparation from naphthalene made the process technically workable. The research was carried out for the Badische Co. by their chemist E. Sapper (1891-7) and depended on the oxidation of naphthalene to phthalic acid by heating with concentrated sulphuric acid in presence of mercury as a catalyst (said to have been discovered accidentally by the breaking of a thermometer bulb). Phthalic acid was converted into phthalic anhydride, phthalimide, and anthranilic acid, and phenylglycine carboxylic acid by condensation of this with chloracetic acid. On fusion with caustic potash, or better sodamide, this formed indoxylic acid, and indoxyl, which was easily oxidised to indigo. 

After the fusion step, water is added, and air is blown into the alkaline solution of indoxyl. Indigo forms and separates from solution. It is filtered, washed, and standardized as a paste of about 20 percent solids. 

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