Indigo reaction with phenyl acetyl chloride
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Preparation of Indigo
Other names: 2,2′-Bis(2,3-dihydro-3- oxoindolyliden), Indigotin. Many ways exist to prepare the important dye indigo. Here, two methods are presented. The first one is used industrially on a large scale, the second one is useful for the demonstrative preparation of indigo or if a high purity is desired.

Preparation of Indigo
A mixture of 15 g of sodium hydroxide and 20 g of potassium hydroxide is fused and carefully dehydrated by heating to about 500° C 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 N-phenylglycine (20 g.) which has been completely dried at 100° in an oven is then introduced in small portions during the course of five to ten minutes with cautious stirring by means of a glass rod. (Eyes and hands must be protected.) The temperature of the bath is maintained at 200°-220°. Two minutes after the last portion of potassium salt has been added, the flask, which has been loosely stoppered with a cork, is removed from the bath and left to cool. When quite cold the flask is broken up and the melt, in small pieces, is dissolved in 500 c.c. of water in a beaker (capacity 11.). The liquid is poured rapidly through a large folded filter into a round-bottomed flask or filter flask (capacity 1.5 l), and air is drawn through the solution by means of a water pump until a drop of the aqueous suspension of indigo, placed on filter paper, produces a sharply defined ring of precipitated indigo, outside which the liquid no longer becomes blue on exposure to the air. 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.
Other names: 2,2′-Bis(2,3-dihydro-3- oxoindolyliden), Indigotin. Many ways exist to prepare the important dye indigo. Here, two methods are presented. The first one is used industrially on a large scale, the second one is useful for the demonstrative preparation of indigo or if a high purity is desired.

Preparation of Indigo
A mixture of 15 g of sodium hydroxide and 20 g of potassium hydroxide is fused and carefully dehydrated by heating to about 500° C 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 N-phenylglycine (20 g.) which has been completely dried at 100° in an oven is then introduced in small portions during the course of five to ten minutes with cautious stirring by means of a glass rod. (Eyes and hands must be protected.) The temperature of the bath is maintained at 200°-220°. Two minutes after the last portion of potassium salt has been added, the flask, which has been loosely stoppered with a cork, is removed from the bath and left to cool. When quite cold the flask is broken up and the melt, in small pieces, is dissolved in 500 c.c. of water in a beaker (capacity 11.). The liquid is poured rapidly through a large folded filter into a round-bottomed flask or filter flask (capacity 1.5 l), and air is drawn through the solution by means of a water pump until a drop of the aqueous suspension of indigo, placed on filter paper, produces a sharply defined ring of precipitated indigo, outside which the liquid no longer becomes blue on exposure to the air. 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.
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