Fundamental processes of dye chemistry. 1949

312	DYES
and a 20 per	cent solution containing 14.7 grams of sodium nitrite is

added over a period of 1 hour. Nitrosation is complete in 4 hours. To the solution are added 110 grams of 30 per cent hydrochloric acid and 200 grams of ice, and then, over a 15-minute period with thorough mechanical stirring, 35 grams of high quality zinc dust. The temperature may rise to 25°C. without causing damage. The solution, which becomes colorless and neutral, is filtered and the residue of zinc dust is washed with a small amount of water.

(b)Thiosul/onic Acid of Bindschedler Green

This oxidation must be carried out in the presence of a zinc chloride solution which has no reducing action. A suitable solution can beprepared by dissolving sheet zinc in concentrated hydrochloric acid. (In the industry, the technical zinc chloride liquor is treated with enough sodium bichromate to destroy the reducing action. Frequently, this requires 100to 250 grams of bichromate for 100kilogramsof the liquor.) The thiosulfuric acid is supplied by aluminum thiosulfate which is so highly dissociated that it reacts as free thiosulfuric acid.

Before the preparation of methylene blue is started, solutions of the necessary reagents are prepared so that the materials can be added rapidly and at the correct temperature.

Solution I. 38 grams of pure aluminum sulfate in 60 cc. water. Solution II. 52.5 grams of crystalline sodium thiosulfate in 50 cc. water. Solution III. 57 grams of sodium bichromate in 90 cc. solution.

Solution IV. 20 grams of dimethylaniline in 27 grams of concentrated hydro-

chloric acid.

Solution V. 25 grams of very finely ground manganese dioxide slurried with 30 cc. water.

The clear, neutral solution of p-aminodimethylaniline is made acid by the addition of 4 grams of concentrated sulfuric acid, and 100 grams of a 50 per cent, nonreducing solution of zinc chloride is added.

The beaker is placed on a felt pad, and a tube is arranged for blowing in steam. With thorough stirring, Solution I is added at room temperature, followed by Solution II, and 2 seconds later by one-third of Solution III (equivalent to 19 grams of sodium bichromate). The temperature of the mixture is raised to 40°C. in the course of 1 minute by the introduction of dry steam, and then Solution IV is added followed by the remainder of Solution III.The mixture is then heated rapidly to 70°. The solution becomes dark greenish blue in color due to the formation of the thiosulfonic acid of Bindschedler green. When the temperature reaches 70°, the slurry (V) is added and the temperature is raised to 85°.

OXAZINE AND THIAZINE DYES

313

The manganese dioxide is added to convert the sulfurous acid formed in the ring closure reaction to dithionate which is harmless. Equally good results can be

obtained by using 40 grams of copper sulfate which is converted in the reaction to insoluble CuoO.

The solution at 85° has a lustrous bronzy appearance as the dye precipitates from the concentrated zinc chloride solution. After 30 minutes, the mixture is allowed to cool to 50°C., and 70 grams of concentrated sulfuric acid is added to dissolve the manganese salts, aluminum hydroxide, and chromium oxide. The dye is filtered off at 20° and washed with a small volume of 10 per cent salt solution. This crude product is dissolved in 1 liter water at 100°, the solution is filtered, and the dye is reprecipitated by the addition of 50 grams of ordinary 50 per

cent zinc chloride solution and 150 grams of salt. The zinc	chloride
double salt of the dye separates completely in 24 hours in the	form of
beautiful bronzy red crystals which are filtered off, washed	with 10

per cent salt solution, and dried at 50° (no higher). The yield of pure, concentrated dye is about 44 grams.

The method described above was worked out by Bernthsen and Ulrich, who also recommended the use of aluminum thiosulfate. The practice of adding manganese dioxide or copper sulfate is generally followed. Relatively small quantities of dye are prepared in one run because rapid heating is important. The final dye is filtered off, using filters such as the one shown in Figure 27, page 151, and then placed in small bags and centrifuged.

Methylene blue is highly valued because it gives pure shades and is inexpensive. It is widely used for coloring tanned cotton. The zinc-free dye is used for discharging on silk. To produce the zinc-free dye,ordinary methylene blue is dissolved

in water, and the	zinc	is precipitated by adding	soda, leaving	the easily soluble
dye base in solution. The base is then precipitated			from the filtered solution by add-
ing salt. In large	scale	preparations, the crystallization requires		several days, and

is promoted by cooling, using lead pipes through which cold water is circulated.

Methylene green, the nitro derivative of methylene blue, is an interesting dye. The nitration is effected in the same way as that of tropaeo-

line, using the crude zinc chloride double salt without further - treatment.

The moist, crude methylene blue, as obtained in the above preparation, is mixed with 50 cc. water and 20 grams of 62 per cent nitric acid (40° Be); and 5 grams of sodium nitrite, dissolved in the minimum amount of water, is added at 25°C. The temperature is then raised to 50° with continuous stirring, and held at this point for 2 hours. The mixture is then diluted with 200 grams of saturated salt solution and filtered after 12 hours. The crude dye is redissqlved in 1 liter water at 60° (not higher), the solution is filtered, and the dye is reprecipitated by the addition of 150grams of salt and 50 grams of 50 per cent zinc chloride

314 DYES

solution. The product is filtered off after 12 hours and dried at 45° until it can just be powdered. At this point, the dye still contains about 20 per cent of water, but it cannot be dried further without causing the color strength to decrease and part of the product to become insoluble. The yield is about 38 grams of concentrated material.

Methylene blue and methylene green are diluted with dextrin since the use of salts greatly decreases their solubility. Methylene green is used chiefly for producing black shades on silk, in combination with logwood-iron mordants and also with tin phosphate. The shades produced in this way are the most brilliant and fast blacks for silks.

When diethylaniline is used in place of dimethylaniline, the pure greenish thiazine blue is formed. (Monoethyl-o-toluidine gives thionine blue.) Thiazine blue gives very pure blue shades on silk, but its importance is reduced by the availability of more fast alizarin dyes. The nonalkylated methylene blue, diaminophenazthionium chloride or Lauth violet, is used in moderate amounts for producing pure violet shades. It is still prepared by the old method involving simultaneous oxidation of p-phenylenediamine and hydrogen sulfide with iron chloride.

G. ANTHRAQUINONE DYES

(a)	Mordant Dyes
	Alizarin
	CO OH	CO OH
	S(XH	OH
CO	CO	CO
Anthraquinone-2-	1-Hydroxyanthra-	Alizarir
sulfonic acid	quinone-2-sulfonic
	acid

Alizarin (1,2-dihydroxyanthraquinone) is formed by alkali fusion of sodium anthraquinone-2-sulfonate ("silver salt"). The reaction is rather remarkable in that not only is the sulfo group replaced by hydroxyl, but a second hydroxyl is also introduced. The presence of an oxidizing agent has a favorable effect on the reaction.

The alizarin fusion was first undertaken industriallyby Caro. In the earlier days, saltpeter was used as the oxidizing agent but this was replaced some seventy years ago by chlorate, following the suggestionof Koch, and today most processess still employ.electrolytic sodium chlorate, which is cheap.

A mixture containing the equivalent of 100 grams of 100 per cent

MORDANT ANTHRAQUINONE DYES

315

"silver salt" (page 228), 260 grams of 100 per cent sodium hydroxide, 28 grams of sodium chlorate, and enough water to make a total volume of 670 cc. is heated at 185°C. with continuous stirring in an autoclave. The pressure increases to 5 or 6 atmospheres. After 48 hours, the apparatus is cooled, and a test is made to determine whether the fusion is completed. To this end, 2 cc. of the melt is removed, the alizarin is precipitated with the necessary amount of concentrated hydrochloric acid, and the filtrate is extracted twice with ether. The solution, thus freed

Fig. 33. Centrifuge with bottom discharge.

from alizarin, is diluted to 15 cc. and examined for fluorescence caused by unchanged silver salt or monohydroxyanthraquinonesulfonic acid. Only a very weak, if any, fluorescence should be visible. If necessary, the mixture is heated for an additional 24 hours at 190°. The melt is then diluted with 2 liters water, the mixture is heated to boiling, and the alizarin is precipitated with 50 per cent sulfuric acid. The dye is filtered off at 50° and washed until the washings are free from salts. The alizarin is not dried since then it does not dye well. The yield is determined by drying a test sample. Usually, the dye is made up to a 20 per cent preparation. About 70 grams of pure alizarin is obtained from 100 grams of pure silver salt.

Technical Observations: Alizarin was the first naturally occurring coloring matter which was successfully prepared synthetically on a commercial scale. This synthesis was a triumph for the then young coal-tar dye industry, and for a long time alizarin was its most important product. The world production of alizarin (as 100 per cent material) was about 2,800,000 kilograms yearly, of which the Badische AS.F. supplied 2,000,000 kilograms. In more recent times, the consumption of alizarin has been greatly reduced as a result of the competition of more easily ap-

plied red dyes of the azo series, especially some of the equally fast naphthol AS combinations.

316

DYES

Fig. 34. Kettle heated with steam or hot water (Frederking).

(a)		(b)
Fig. 35. Reaction kettle (Samesreuther). The kettle is equipped			with	external
heating or cooling coils. The	coils are mounted on copper	supports (b)		which
conduct heat to or from the	kettle. This heating system is	quite	different from

that used in the Frederking apparatus (Fig. 34). If the kettle wears out, the heating system can be used for a new kettle. These kettles are very useful, e.g., for sulfonations in which varying heating and cooling must be used (see p. 211, H acid).

ACID ANTHRAQUINONE DYES	317
Alizarin fusions are carried out in Frederking apparatus,	such as shown in

Figure 34, or in similar kettles of the Samesreuther type (Fig. 35a and b). Since the chlorate-containing melt* attacks the apparatus, the equipment is always made with an insert of alkali-resistant cast iron which can be replaced easily. There are many variations in these apparatus. The industrial preparations are run in very large batches; in some cases 2000 to 2500 kilograms of 100 per cent alizarin is made at one time in one reaction kettle. The product is made up into either a 20 per cent or a 16 per cent paste. The material is standardized by determining the dry weight and by test dyeing. In plant operations, it is possible to use much less sodium hydroxide, only 110 per cent of the theoretical amount (in our example, only about

40 grams instead of 260 grams). Alizarin, once dried,	can	easily be	regenerated
by dissolving it in borax and reprecipitating with acetic	or sulfuric acid. Dyes of
the alizarin type must be precipitated from boiling solution		because	of their low

solubility. Only in this way can a finely divided product be obtained. A solid preparation of alizarin is prepared for the Orient by mixing the dye with enough starch to make dry lumps which, when placed in boiling water, form a paste suitable for dyeing.

(b) AcidDyes

Quinizarin Green or Alizarin Cyanine Green G

OH

OH

*In many plants, saltpeter is preferred over chlorate since it attacks iron less readily

and is reduced directly to ammonia in the process. Chlorate, however, gives a more clear-cut reaction.

318 DYES

In a 1.5- to 2-liter round-bottomed flask fitted with a downward eondenser, stirrer, and thermometer, a mixture of 500 grams of p-toluidine and 60 grams (0.25 mole) of quinizarin (page 237) is heated to 80°C., and to it is added, with stirring, a mixture of 18 grams of boric acid, 30 grams of stannous chloride, and 16 grams of chalk. The mixture is held at 110° for 1 hour, then at 120° for 1 hour, and finally at 130° for 2 hours. During the heating, the water formed in the reaction distillsoff along with some p-toluidine. The reaction mixture, after cooling to 70°, is diluted with 350 cc. alcohol, cooled thoroughly, and transferred to a stoppered container, rinsing,the flask and stirrer with an additional 350 cc. alcohol. After standing overnight, the precipitated material is filtered off, stirred with 2 successive 300-cc. portions of alcohol, and finally washed on the filter with alcohol until the washings are nearly colorless.

The resulting crude base is purified by boiling it with a mixture of 70 cc. concentrated hydrochloric acid and 1600 cc. water, filtering hot, and washing the solid with hot water until it is neutral. The residue is then boiled with a mixture of 1600 cc. water and 40 cc. 40° Be sodium hydroxide solution, and again washed until neutral. The quinizarin green base, after drying, weighs 85 to 90 grams.

Sulfonation is carried out by adding 25 grams of the base to 250 grams of 10 per cent oleum with stirring. The temperature is allowed to rise to 40-45°C. and is held at this point for 2 to 3 hours in order to get rapid solution. The mixture is then allowed to stand for 24 hours and is then poured into 1 liter water, washing out the flask and stirrer with an additional 1 liter water. The diluted acid solution, which should be at about 50-60°, is treated with 250 grams of salt. On cooling, the dye separates out in an easily filterable, crystalline form. It is filtered off and pressed out, and then redissolved in about 1500 cc. hot water. The solution is neutralized with 15 grams of soda ash and filtered through a fluted filter, washing the residue with hot water. 500 grams of salt is added to the hot filtrate, and the dye, after it has precipitated completely, is filtered off, pressed out, and dried. The yield is about 50 grams.

ACID ANTHRAQUINONE DYES	319
Alizarin Saphirol B and SE
OH	OH
Oleum

OH

	SO,H	NaSH
	CO	| CO
OH	NH2	OH	N02

Alizarin saphirol B

|Na.S

CO

—SO,H

CO

OH NH2

Alizarin saphirol SE

Sulfonation and Nitration. To 188 grams of 20 per cent oleum is added, with stirring at room temperature, 50 grams of thoroughly dried, finely pulverizfed 1,5-dihydroxyanthraquinone or its mixture with the 1,8 isomer (see page 236). The temperature is raised slowly to 100°C. during the course of 1 hour and held at this point for 2 hours more, then raised to 105° for 2 hours, and finally held at 110° until a test sample gives a clear solution in cold water. The mixture is then cooled to 25-30°, and 272 grams of 100 per cent sulfuric acid is added, followed by the dropwise addition, over a period of 1 hour, of a mixture of 36 grams of nitric acid (48° Be) and 109 grams of 20 per cent oleum. (This mixture must be prepared very carefully, adding the oleum slowly with stirring to the well cooled nitric acid.) During the addition of the nitrating mixture, the reaction mixture is cooled in water so that the temperature does not exceed 30°C. When the addition is complete, the temperature is raised slowly to 35°, held at this point for 2 hours, then raised to 55° for 2 hours, and finally to 80° for 2 hours. The mixture is then cooled to 30° and poured, as rapidly as the foaming will permit, into

320

DYES

200 cc. cold water. The temperature rises to 110-115°. The resulting mixture, which is about 75 per cent sulfuric acid, is cooled and filtered, after 2 days' standing, through a sintered glass funnel (or through asbestos ) and sucked as dry as possible. The filter cake is then dissolved in about 1 liter water, and the solution is filtered to remove any undissolved residue. The filtrate should be perfectly clear and remain so on standing.

Reduction. The nitro compound is reduced by means of a concentrated solution of sodium hydrosulfide, NaSH, the preparation of which was described on page 113. A test is made in the following way to determine how much of the reducing agent is required: 25 cc. of the filtered solution of the nitro compound is pipetted into a 750-cc. Erlenmeyer flask, diluted with 350 cc. hot water, and neutralized with soda to the point where the red coloration, which is formed, just persists. A sodium hydrosulfide solution, prepared by diluting 10cc. of the concentrated solution to 100 cc., is then added, at 60-70°, from a burette until the color of the solution turns to a pure blue. Additional 1-cc. portions of the hydrosulfide solution are added until a definite blackening is obtained when the colorless spot on filter paper, formed by a salted-out test sample, is treated with ferrous sulfate. From the amount of hydrosulfide used in the test determination, the amount required for the total volume of the nitro solution is calculated.

The main body of the solution of the nitro compound is then neutralized with soda ash to the point giving a permanent red coloration, and heated to 60-65°C. The calculated amount of concentrated hydrosulfide solution is added slowly with stirring, and the mixture is held at 60-65° for 3 hours. Enough salt is then added to make the solution 10per cent with respect to salt, and the solution is cooled with stirring. The precipitated dye is filtered off, washed with 15 per cent salt solution until the washings are colorless, pressed out, and dried at 90°. The yield of alizarin saphirol B is about 105 grams.

Splitting Out of One Sulfo Group. 25 grams of alizarin saphirol B is mixed with 800 cc. hot water, 46 grams of 40° Be sodium hydroxide solution is added at 90°C., and the mixture is heated until all of the material has dissolved. A solution of 10 grams of sodium sulfide in 50 cc. water is then added slowly and the mixture is held at 95-100° with stirring, keeping the volume at about 1 liter, until a diluted test sample, when acidified with sulfuric acid, gives a filtrate which is wine red instead of blue. When this point is reached, 200 gramsof salt is added immediately, and the solution is cooled with stirring. When a tempera-

VAT ANTHRAQUINONE DYES

321

ture of 30-40° is reached, the precipitated dye is filtered off, washed with 10 per cent salt solution, pressed out, and dried. The yield of alizarin saphirol SE is 22 to 24 grams.

Technical Observations: The dyes, prepared from 1,5- and 1,8-dihydroxy- anthraquinone by tlie above procedure, are so similar that frequently a mixture of the two is used. The-SE dye can be prepared without isolating the intermediate disulfo dye (alizarin saphirol B), the reduced solution from the hydrosulfide reduction being treated directly with sodium sulfide.

(c) Vat Dyes

Indanthrene Blue RS from /^Aminoanthraquinone

	CO
	CO | NH
CO	NH | CO
	NH9
CO
	Dianthraquinonyldihydroazine
	or indanthrene blue RS

An intimate mixture of 50 grams of pure /3-aminoanthraquinone (page 229), 25 grams of potassium acetate, and 6 grams of potassium nitrate is added in small portions to a melt of 150 grams of potassium hydroxide and 20 cc. water, heated to 210°C. A crucible made of nickel or V2A steel should be used as the reaction vessel, and the stirrer should be made of the same material. Iron is not suitable for the purpose. The additions are completed in 20 minutes, and the mixture is heated for 5 minutes more at 215-220° (not higher) with thorough stirring. The melt is then poured onto 1 kilogram of ice, washing out the residue from the reaction vessel with a small amount of water. The entire melt is dissolved in the water, and 40 grams of concentrated sulfuric acid is added to salt out the dye. The solution, which is still alkaline, is then heated to 60° and treated with 30 grams of sodium hydrosulfite. The blue leuco derivative of indanthrene blue RS which is formed is insoluble in the salt solution. When the mixture is cold, the leuco compound is filtered off, using a cotton filter on a suction funnel. The product is washed with a 2 per cent sodium hydroxide solution containing 5 grams of sodium hydrosulfite per liter until the washings are clear and light blue in color. The precipitate is then stirred with 500 cc. water

322

DYES

at 60°, and a stream of air is blown through the mixture until all of the leuco derivative is oxidized as shown by the insolubility of a small test sample in a large volume of water. The dye is filtered off, washed with water, and dried. The yield is about 22 grams.

Indanthrene blue RS dyes cotton a pure, deep blue from its blue hydrosulfite vat. The dye has exceptional light fastness. Its fastness to chlorine is only moderate, but this can be improved by chlorination. This chlorination can be effected by the action of chlorine on the sulf uric acid solution in the presence of sodium nitrite (GCD brand), by the action of sulfuryl chloride on the nitrobenzene solution (BCS brand), or by the action of chlorine in chlorosulfonic acid, possibly in the presence of a carrier such as iron chloride or antimony pentachloride.

All efforts to increase the yield of indanthrene blue to more than 45 per cent of the theoretical amount have so far been unsuccessful. By-products are always formed, among them alizarin and many others. In addition, some of the dye is destroyed in the fusion.

The recently published95 preparation from l-chloro-2-aminoanthraquinone by heating with cuprous iodide (Ullman-Goldberg reaction) gives still less satisfactory results.

The chlorine fastness of indanthrene blue varies directly with the purity of the aminoanthraquinone used. Also, the finished dye can be purified by treating it in concentrated sulfuric acid solution with an oxidizing agent (manganese dioxide, etc.) which destroys the impurities. Such purified dyes are marketed as indanthrene brilliant blue. They are somewhat 'stronger, and have better chlorine fastness, than the original dye. The use of potassium acetate was first proposed by Pope98 who also pointed out the advantage of using an oxidizing agent in addition to the acetate (or formate). It is essential that the fusion is not carried out at too high a temperature or for too long a time. Hence, in the plant only small batches are run, e.g., about 20 kilograms. The reaction vessels are either pure nickel or stainless steel.

Indanthrene Yellow GK97
NH,	NH
' i\ Benzoyl chloride
	CO
	;o—NH
1,5-Diamino-	Indanthrene yellow GK
anthraquinone
95 du Pont, Ger. Pat. 656,944 (1938)	[Frdl, 24, 868; C.A., 32, 4176 (1938)].

9« Pope and Scottish Dyes, Ltd., Ger. Pat. 382,178 (1923) [Frdl, 14, 871 (19211925)].

»7Hefti, Helv. Chim. Acta, 14, 1404 (1931). Schmidt (Bayer), Ger. Pat. 225,232 <1910) [Frdl, 9, 1197 (1908-1910); C.A., 5, 592 (1911)].

INDIGOID DYES

323

To 1,5-diaminoanthraquinone (page 237) in 20 parts of 1,2-dichloro- benzene (or nitrobenzene) is added slowly at 140° the calculated amount of benzoyl chloride. Hydrogen chloride is generated (hood!). When the gas evolution has ceased (about 1hour), the mixture is cooled and the dye is filtered off.The yield is quantitative.

From its weakly alkaline hydrosulfite vat at 45°C., 1,5-dibenzoylaminoanthra-

quinone gives yellow shades with excellent		characteristics. The	designation K
signifies that the	dye is one which must not	be used at too high	a temperature
or it is destroyed	(Kaltkiiper).

In an analogous manner, indanthrene red 5 GK is prepared by benzoylation of 1,4-diaminoanthraquinone (page 232), and algol yellow WG by benzoylation of 1-aminoanthraquinone (page 231).

H. INDIGOID DYES

Indigo

1.Heumann Synthesis98

(a)Phenylglycine-o-carboxylic Acid"

COOH

NaCl

NH—CH,~COONa

A paste of 137 grams (1.0 mole) of anthranilic acid (page 174) in a small amount of water is exactly neutralized with approximately 120 grams of 40° Be sodium hydroxide solution. A second solution is prepared by dissolving 94.5 grams (1.0 mole) of chloroacetic acid in 200 cc. water and exadtly neutralizing it by the addition, with stirring, of about 55 grams of soda ash. The two solutions are mixed and held at 40°C. for 4 days. The monosodium salt of phenylglycine-o-carboxylic acid which crystallizes out is filtered off with suction, washed with a small amount of cold water, and dried to constant weight in a steam heated oven. The yield is about 75 per cent of the theoretical amount.

^Badische A. und S. F., Ger. Pat. 56,273 (1891) [Frdl, 3, 281 (1890-1894)]. uaBadische A. und S. F., Ger. Pat. 127,178 (1901) [Frdl, 6, 538 (1900-1902)].

324

DYES

(b) Indoxylcarboxylic Acid

COOH

CO

CaO	—COONa + Ca(OH)2

NH—CH2—COONa

NH

A mixture of 25 grams of sodium hydroxide (completely dehydrated by fusing in an iron crucible), 25 grams of potassium hydroxide (similarly dehydrated), 7.5 grams of caustic lime (dehydrated by igniting without sintering in a porcelain crucible), and 25 grams of thoroughly iried monosodium phenylglycine-o-carboxylate is ground in a ball mill with exclusion of moisture. The resulting intimate mixture is heated in a vacuum baking apparatus (Fig. 30, page 181) in a graphite bath for 2 hours at 150°C., then for 6 hours at 230-235°. A homogeneous, yellow brown, hard mass is formed.

(c)	Indigo
CO	CO\	/CO	2 NaHCOa
XCH—COONa + Oa =	NC=C'
NH	/	\
	NH	NH

The fusion mixture from (b) is dissolved in 2 liters water at 80°C., and a vigorous stream of air is passed through the solution until no more indigo is formed in a filtered test sample when shaken with air. The precipitated dye is filtered off, washed with water, boiled with dilute hydrochloric acid to remove all of the lime, filtered again, washed thoroughly with water, and finally dried in a steam heated oven. The yield is 12 to 12.5 grams, or 80 to 82 per cent of the theoretical amount based on the phenylglycine-o-carboxylic acid.

The foregoing procedure, discovered by Heumann and worked out by the Badische A.S.F., was the first process used on a large scale for the preparation of indigo. It was kept in operation for a long time, along with the related process (Deutsche Goldund Silberscheideanstalt) employing potassium and sodium hydroxides in combination with sodium amide, and only recently has it been replaced by other processes.

2. Traugott Sandmeyer Synthesis1 Q(>

Although the Sandmeyer indigo synthesis is no longer used, it is such an interesting example of the combined efforts of science and in-

INDIGOID DYES

325

dustry that it deserves a place in this book. The process involves		the
following steps:
(a) Aniline is converted to thiocarbanilide (A. W. Hoffmann)		by
heating with carbon bisulfide:
NH2 -f CSZ	-NH

1

CS—NH

Thiocarbanilide

m.p. 151°

(b) Sulfur is split from the thiocarbanilide by treatment with basic lead carbonate, and simultaneously hydrocyanic acid is added, resulting in the formation of Laubenheimer's hydrocyanocarbodiphenylimide:

HCN—H2S Y-NH

C=N-

/

N

Hydrocyanocarbodiphenylimide, m.p. 137°

(c) The hydrocyanocarbodiphenylimide is converted to thiooxamidodiphenylamidine ("thioamide") by treatment with yellow ammonium sulfide:

—NH

C-N-

NH2

"Thioamide," m.p., 161-2°

( d ) The "thioamide" is converted smoothly to a-isatinanilide by the action of concentrated sulfuric acid:

4-H2SO4(conc.)

C-N—<

S02 + S + (NH4)2S04j

CO

100 Sandmeyer, Z. Farben- u. Textilchem., No. 7, p. 129 (1903). Also, Helv. Chim.	o-Isatinanilide,
Acta, 2, 234 (1919).	m.p. 126°

326

DYES

(e) The a-isatinanilide can be converted to indigo in various ways. It can be reduced in alcoholic solution with dilute ammonium sulfide solution, or converted to a-thioisatin which immediately goes to indigo when treated with alkali. The latter method is shown here because it was used industrially:

NH

4- Aniline

CO

Indigo

(a) Thiocarbanilide

A mixture of 186 grams of pure aniline and 100 grams of pure carbon bisulfide is boiled under reflux until evolution of hydrogen sulfide ceases, which requires about 2 days. The temperature of the oil bath is then raised to 160°C. and the excess carbon bisulfide is distilled off. The residue of melted thiocarbanilide is poured out into a flat dish and pulverized after cooling. It is desirable to recrystallize the crude product from alcohol which gives lustrous crystals melting at 151°. The yield is about 200 grams of purified material. (Recrystallization of the thiocarbanilide is usually omitted in industrial preparations, although such purification would prevent the formation of small amounts of oily by-products in subsequent steps.)

(b) Hydrocyanocarbodiphenylimide

A solution of 350 grams of lead nitrate in 1 liter hot water is treated carefully at 95°C. with about 120grams of soda ash and the precipitated basic lead carbonate is washed thoroughly with water. The moist material is transferred to a 2-liter flask fitted with stirrer and reflux condenser (Fig. 6) and mixed to a homogeneous paste with 600 grams of 90 per cent alcohol. One mole (228 grams) of very finely pulverized thiocarbanilide is added rapidly, followed by the addition, at 25°, of 1.3moles of sodium cyanide (about 60 grams of technical material).*

INDIGOID DYES

327

With vigorous stirring, the temperature is raised to 77°C. during the course of 1 hour, and then a small test portion is removed and filtered. The colorless filtrate should not be blackened by the addition of a pinch of basic lead carbonate. If it is, heating is continued for an additional hour, and the test is repeated. If the test is still positive, additional lead carbonate and sodium cyanide are added, but this should not be necessary if the correct amounts of reagents were used originally.

When the reaction is completed, the mixture is heated to boiling and filtered hot, extracting the residue with two 500-cc. portions of alcohol. The hydrocyanocarbodiphenylimide is allowed to crystallize out. The first fraction is entirely pure and weighs about 160 grams. The mother liquor, after concentration, yields an additional fraction of about 40 grams of nearly pure product. The yield is about 98 per cent of the theoretical amount. The hydrocyanocarbodiphenylimide crystallizes in yellowish prisms melting at 137°. The mother liquors contain hydrocyanic acid and must be handled carefully.

(c) "Thioamide"

The addition of hydrogen sulfide to the hydrocyanocarbodiphenylimide takes place very easily if the latter is finely powdered. The material is therefore ground in a roller mill or sifted. 200 grams of the finely divided material is emulsified by vigorous stirring at 35°C. in 500 grams of yellow ammonium sulfide solution. (The reaction can be greatly accelerated by the addition of an equal volume of alcohol.) The ammonium sulfide solution is prepared by passing 35 grams of hydrogen sulfide into a mixture of 460 grams of 20 per cent aqueous ammonia and 25 grams of powdered sulfur. If the hydrocyanocarbodiphenylimide is powdered finely enough, the addition of hydrogen sulfide takes place quantitatively within 12 hours, as shown by the fact that a washed sample is soluble in dilute hydrochloric acid. The product is filtered off and washed thoroughly with water. It is sufficiently pure for use in the next step. The yield is about 220 grams. It crystallizes from alcohol in yellow prisms melting at 162°.

(d) a-Isatinanilide

The ring closure to an isatin derivative takes place only under certain, accurately controlled conditions. It is important that the reaction be carried out in hot sulfuric acid.

* The HCN content of the sodium cyanide must be determined!

328 DYES

In the course of 15 minutes, 200 grams of pure, dry, finely divided "thioamide" is added to 800 grams of 94 per cent sulfuric acid (66° Be) at a temperature of exactly 94°C. Considerable heat is generated and the mixture must be cooled. When the mixing is complete, the mixture is heated at 106-108° for one hour, after which no more sulfur dioxide is evolved. The solution is then cooled to 20°, and the product is converted to the hydrochloride of a-isatinanilide by pouring it in a thin stream into a well stirred mixture of 1 liter water, 2 kilograms of ice, and 500 grams of salt. The hydrochloride of a-isatinanilide separates as a light reddish brown precipitate mixed with finely divided sulfur.

If the anilide is to be purified, it is filtered off and washed thoroughly with 20 per cent salt solution. The salt, freed from acid, is stirred with water containing enough soda to give a weakly alkaline reaction, and the solid mixture of the anilide and sulfur is again filtered off, washed thoroughly, and dried. It is then extracted with cold carbon bisulfide (removing the sulfur), and finally crystallized from alcohol. The purified material is in the form of dark colored needles melting at 126°C. The yield is about 150 grams of pure material from 200 grams of "thioamide." When the anilide is heated with a small excess of dilute hydrochloric acid, the aniline group is split off as aniline, and pure isatin, melting at 200-201°, is precipitated. It may be recrystallized from hot water in which it is very soluble.

Isatin is used as such in the preparation of many valuable vat dyes. Still more important are the vat dyes prepared directly from a-isatinanilide by condensation with /3-hydroxythionaphthenes. As first observed by G. Engi, isatin and a-isatin- anilide lead to different dyes on condensation. In the case of isatin itself, the p group is reactive in the condensation reaction, while with a-isatinanilide, aniline is split out and a condensed dyes are formed. These a condensation products are much .more valuable as dyes than the isomeric/8 compounds.

From isatin

From a-isatinanilide

NH

		C0()8)
Thioindigo scarlet R	(Kalle)	Tribromo derivative = Ciba Violet B
dibromo derivative	= Ciba	Dibromo derivative = Ciba Violet 3B
RedG		Monobromo derivative = Ciba Grey G

INDIGOID DYES

329

(e) a-Thioisatin and Indigo

To prepare indigo from the sulfuric acid solution of isatinanilide, it is not necessary to isolate the pure anilide or its hydrochloride. A solution of sodium hydrosulfide is prepared by introducing hydrogen sulfide into a solution of 45 grams of sodium hydroxide in 150 cc. water. This solution is mixed with the sulfuric acid solution of a-isatinanilide obtained from 200 grams of "thioamide" by pouring the two solutions into 6 liters ice water. A definite, but slight, excess of hydrogen sulfide should be present at all times. The reduction requires about 30 minutes during which the thioisatin separates as a voluminous,brown precipitate. Aniline sulfate remains in the solution. The thiosatin is filtered off when a filtered test sample of the reaction mixture gives no further precipitate with more sodium sulfide, usually after about 1 hour. The precipitate is washed until the washings have a specific gravity of only 1.007 (1° Be) and is then stirred into 3 liters water. A concentrated solution of soda is added until a permanent, strongly alkaline reaction is obtained. This requires about 30 grams of soda. The formation of indigo takes place very rapidly, but it is desirable to heat the mixture at 60°C. for 1 hour and then allow it to stand overnight. The indigo and sulfur mixture is filtered off, washed thoroughly, and dried at 80°. The dry material is extracted with twice its weight of carbon bisulfide, leaving about 80 grams of pure indigo.

Technical Observations: The reactions involved in the Sandmeyer synthesisof indigo are surprisingly smooth. The over-all yield of dye, calculated on the basis of aniline, is about 80 per cent. The process was used for a short time by Geigy, the cost of the dye being about 10.8 francs per kilogram of 100 per cent product. This product reduced to a vat more satisfactorily than other commercial indigos and was favored by dyers. The whole process was carried out without using alcohol, since all of the materials involved reacted well in aqueous solution when sufficiently finely divided. The chief difficulty was not with the hydrocyanic acid, but with the hydrogen sulfide. This compound is a dangerous industrial poison because its odor is not noticed after short exposure. The lead sulfide was treated with concentrated hydrochloric acid to form lead chloride and hydrogen sulfide which were returned to the process. The process of the Deutsche Goldund Silberscheideanstalt replaced the Sandmeyer process soon after large scale manufacture of indigo was started. The yields in the new process were as high as 85 per cent, so the older process could not compete with it.

330

DYES

Thioindigo

NH,			COOH HOOC
	f^N	NaaSa 4	/ Dithiosalicylic acid
HN02	\	NaOH	Fe
	O
COOH			SH
Anthranilic
acid			COOH
			Thiosalicylic acid
			C1CH2-COOH
			S-CIVCOOH
CH		NaOH
COH			COOH
3-Hydroxythio-			o-Carboxyphenyl-
naphthene			thioglycolic acid
			(O acid)

Thioindigo

(a) o-Carboxyphenylthioglycolic Acid (O Acid)

Anthranilic acid is diazotized and the diazonium salt is converted to a mixture of thiosalicylie acid and dithiosalicylic acid by the action of sodium polysulfide. For these reactions, the following three solutions are prepared:

I.68.5 grams anthranilic acid (0.5 mole)

500 cc. water

78.5 grams hydrochloric acid (21° Be)

II.34.5 grams sodium nitrite (100 per cent)

75 cc. water

III.85 grams sodium sulfide, cryst (Na2S • 9H2O)

11.1 grams flowers of sulfur

100 cc. water

Solution I is prepared by adding the anthranilic acid to the aqueous solution of the acid. Solution III is made by boiling the sulfur in the

INDIGOID DYES

331

sulfide solution until clear, and then adding JL2 grams of 40° Be sodium hydroxide solution to it. Solution I is cooled to 0°C. with vigorous stirring in a 1-liter beaker, and Solution II is added slowly to it. The temperature rises to about 5°, and starch-iodide paper gives a weak blue reaction. Solution III is now mixed with 500 grams of ice in a 4-liter beaker and, with vigorous stirring, the diazonium solution is added portionwise in the course of 10minutes. The mixture foams heavily and a yellow precipitate is formed. The mixture is stirred for 2 hours and then made acid to Congo red by adding 90 grams of 21° Be hydrochloric acid. The precipitated thioand dithiosalicylic acids are filtered off and washed until free from acid. Before the precipitate is worked up, the dithiosalicylic acid must be reduced to thiosalicylie acid. The mixture of acids is placed in a 2-liter beaker with 500 cc. water, 25.8 grams of soda ash is added gradually with stirring, and the solution, which must be acid to litmus, is heated to boiling. Reduction is effected with 100 grams of Bechamp iron (page 77) at 95° for 2 hours, replacing, from time to time, the water lost by evaporation. To determine when reduction is complete, a sample is removed, made strongly alkaline, boiled for a short time, and filtered. The filtrate is then treated with hydrochloric acid, and the precipitated acid is filtered off, washed, and partially dried on a clay plate. It must be easily soluble in cold alcohol. (Dithiosalicylic acid is very difficultly soluble in alcohol.)

When the test is satisfactory, the reaction mixture is made alkaline by the addition of 60 grams of 40° B£ sodium hydroxide solution, and at 95°C. a solution of 52 grams of chloroacetic acid and 29 grams of soda ash in 150cc. water is added. The mixture, which must be alkaline at all times, is held at 90° for 30 minutes, and is then cooled and allowed to settle. The residue of iron is filtered off and washed with dilute sodium hydroxide solution. The O acid is precipitated from the filtrate by adding 150 grams of 21° Be hydrochloric acid. This precipitation is most satisfactory when the solution is cold, and it is desirable to carry it out as slowly as possible and with good mechanical stirring. The product is filtered off, washed until free from acid, and dried at 80°. The yield is about 85 grams, or 80 per cent of the theoretical amount.

(b) 3-Hydroxythionaphthene

A mixture of 50 grams of well dried O acid and 100 grams of technical sodium hydroxide is ground in a ball mill for 24 hours. The mixture is then placed in a vacuum baking apparatus (Fig. 30, page 181) which is evacuated and heated, as uniformly as possible by means of an oil