Fundamental processes of dye chemistry. 1949

172	INTERMEDIATES

catalyst is not heated above the temperature at which it is to be used later, a precaution to be observed in many catalytic reactions.

Rule: A catalyst should not be heated above the reactiontemperature.

Performing the Reaction. Apparatus

The diagram in Figure 28 best explains the procedure. Naphthalene is placed in flask B, which is heated to 110°C., and a stream ofmoist air is passed through the gas introduction tube (A). The presence of water vapor is important for the oxidation. The necessary air is most

Th

Fig. 28. Apparatus for preparation of phthalic anhydride by catalyticoxidation: A, air inlet tube; B, melted naphthalene; C, graphite bath; D, electrical heating element; E, ground; F, spark induction coil; G, glass wool; H, catalyst; J, bulb receier; K, aluminum foil, grounded; L, gas exit tube; M, thermocouple; N, rigid wire; Th, thermometer. One side of the spark induction coil is connected to N, the other is grounded.

simply supplied by a water pump which is readily available in the laboratory, and the quantity of air passed through the apparatus is measured with regular laboratory equipment which need not be described here. The air stream should be regulated so that 200 liters of air and 5 grams of naphthalene are passed through the apparatus in 1 hour. It is important that the ratio of air to naphthalene be controlled so that an explosive mixture is not formed, since this would cause a loss of phthalic anhydride. The mixed gases (air and naphthalene vapor) are passed into the catalyst tube, which should be of about 5-cm. internal diameter and should contain a charge of catalyst about 10 cm. in length. The pumice particles are held in place by glass wool. Heating of the catalyst is done electrically as is the usual practice in technical laboratories.

BENZENE DERIVATIVES FROM NAPHTHALENE

173

The gases coming from the catalyst are passed first into a round receiver (J), and then go to the precipitator. The latter consists of a glass tube of about 8-cm. internal diameter, the inside wall of which is lined with a smooth aluminum foil which can be removed easily and which is well grounded through a metal wire to a water pipe. A wire or silvered glass rod projects into the tube, and to this a potential of 10,000 volts is applied. This potential, which can be alternating for the present purpose, is generated in the usual manner, most simply by an induction coil or a transformer with interrupter. It is important that the air spark length of the induction coil be at least 25 mm. The internal width of the tube carrying the precipitator must therefore be sufficiently great so that sparks do not jump from the wire to the metal foil. Such sparking may cause explosions which, although not involving danger, may easily burn part of the reaction product.

When the apparatus, to which careful attention must be given, is assembled correctly, the oxidation can be started. The catalyst tube is heated to 450°C. (thermocouple), and the stream of mixed air and naphthalene vapor is started. The oxidation takes place as the vapors pass through the catalyst. Part of the phthalic anhydride collects in the bulb receiver, but most of it is collected in the Cottrel precipitator where it frequently forms beautiful needles, up to 5 cm. in length, in a small space at the extreme first part of the precipitator. The operation can be continued as long as desired, since the catalyst retains its activity for weeks provided that pure naphthalene is used. The yield of phthalic anhydride is about 90 to 95 per cent of the theoretical amount, the product being chemically pure if the oxidation temperature is maintained at the correct point and the throughput is not too high. If too much naphthalene is put through per unit of time, the product becomes yellowish due to admixed 1,4-naphthoquinone. The product is removed from time to time by drawing out the aluminum foil. 20 grams of pure phthalic anhydride can easily be prepared in 5 hours in the laboratory.

Phthalimide from PhthalicAnhydride

NH

In a three-necked flask fitted with thermometer, gas introduction tube, and wide outlet tube bent downward, 148 grams (1.0 mole) of pure phthalic anhydride is melted and heated to about 170°. At this

174	INTERMEDIATES

temperature a rapid stream of ammonia is passed into the molten anhydride. The ammonia is completely absorbed and steam escapes, carrying with it some phthalic anhydride. Sublimed material which condenses in the neck of the flask is melted down from time to time. During the introduction of the ammonia, the temperature is raised slowly until it reaches 240°. At this point, the phthalimide does not solidify since its melting point is 230°. When ammonia begins to escape, the reaction is nearly complete. The stream of ammonia is continued for 10 minutes more, and the melt is immediately poured into a porcelain dish where it is allowed to solidify. About 130 to 135 grams of phthalimide is obtained, melting at 223° (uncorrected) and containing as an impurity only traces of unchanged phthalic anhydride.

Anthranilic Acid from Phthalimide

NH.

H + NaOCl + 3NaOH = I

+ Nad + NaaCO3 + HtO

\A

COONa

A solution is made by stirring 73.5 grams (0.5 mole) of finely powdered phthalimide with 250 cc. water and 125 grams of ice and then adding 55 cc. sodium hydroxide solution (40° Be). As soon as solution is complete, 250 grams of ice is added and then, in one portion, a chlorate-free sodium hypochlorite solution, which corresponds to 37 grams of active chlorine and contains two moles of NaOH for each mole of NaOCl.* An additional 20 cc. 40° Be sodium hydroxide solution is added just before use. A drop of the reaction mixture added to a few drops of aniline water should give only a very weak violet coloration. After the solution is thoroughly mixed, it is allowed to stand in an ice bath for 3 to 4 hours, and then a few drops of bisulfite solution are added to destroy the small excess of hypochlorite. The solution is then heated to 80° and neutralized at this temperature with concentrated hydrochloric acid (foaming) just to the point where litmus paper is not turned blue (about 130 cc. acid). The hot solution is filtered to remove any inorganic impurities, and the filtrate is acidified by adding 40 cc. concentrated hydrochloric acid and 12 cc. glacial acetic acid. (An

*The hypochlorite solution is easily prepared by adding 71 grams of chlorine to a mixture of 400 cc. sodium hydroxide solution (40° Be) and 400 grams of ice, cooled in an ice bath. The concentration of active chlorine is determined iodo-

metrically. Commercial hypochlorite solutions, which have been held for a long time, contain appreciable amounts of chlorate and are not suitable for the present purpose.

a-NITRONAPHTHALENE AND a-NAPHTHYLAMINE

175

excess of hydrochloric acid must be avoided, since it would dissolve the anthranilic acid.) Anthranilic acid begins to separate from the hot solution as a crystalline, light brown precipitate becoming more copious on cooling. After standing overnight, the product is filtered off, washed with cold water, and dried in a steam heated drying oven. The yield of anthranilic acid melting at 144-145° is 57 to 58 grams, or about 84 per cent of the theoretical amount.

The anthranilic acid remaining in solution in the filtrate can be recovered by adding copper acetate to form the insoluble copper salt. This salt is filtered off, suspended in water, and decomposed by passing in hydrogen sulfide. The copper sulfide is filtered off and the filtrate is concentrated to a small volume. When the resulting solution is cooled, a few grams of anthranilic acid crystallize out.

B. COMPOUNDS OF THE NAPHTHALENE

SERIES

In the preparation of naphthalene derivatives, good yields are obtained only if the naphthalene used as the starting material is of the highest purity. If a good grade of naphthalene is not at hand, it is recommended that the material be purified, first by distillation, and then by treating it with 5 per cent of its weight of concentrated sulfuric acid. Generally, however, pure naphthalene from tar distillation is available today.

14. a-Nitronaphthalene and a-Naphthylamine55

NH«

In general, naphthalene reacts much more easily than benzene, and nitration of naphthalene takes place so energetically that polynitro compounds are formed easily. On the other hand, since the reaction is carried out at a temperature below the melting point of naphthalene, the larger particles may not be attacked by the nitric acid under the

55See also O. N. Witt, Chem. Ind. Ger. Nachr.-Ausgabe, 10, 215 (1887); L. Paul Z. angew. Chem., 10, 145 (1897).

176	INTERMEDIATES

conditions employed. Hence, it is necessary to pulverize the naphthalene to such a degree that it will pass through a sieve having 400meshes per square centimeter.

Naphthalene crystallized as plates is very difficult to pulverize. If such material is to be used, it should first be melted and allowed to solidify in a compact crystalline mass which is much more easily pulverized.

To a mixture of 103 grams of 62 per cent nitric acid (40° Be) and 300 grams of 80 per cent sulfuric acid, 128 grams of naphthalene is added. The mixture is stirred continuously for 6 hours at 50°C., and then the temperature is raised to 60° during the course of 1 hour. On cooling the mixture, the nitronaphthalene floats as a porous cake on the surface of the acid. It contains about 90 to 92 per cent of the a-nitro compound, 4 to 5 per cent of the ^-nitronaphthalene, 2 to 3 per cent of dinitronaphthalene, and about 0.5 per cent of 2,4-dinitro-l-naphthol (Martius yellow).

The crude product is melted several times with boiling water to remove the acid and simultaneously distill out any residual naphthalene. The melted product is then poured into cold, well stirred water, causing it to solidify in the form of small balls.

If purified nitronaphthalene is desired, the	crude	material is dried by melting
at 120° in an air oven. It is then treated with	10 per	cent of its weight of ligroin

(b.p. about 150°). (Commercial xylene or cumene can also be used.) The solution is filtered hot through a smooth filter and allowed to stand for some time. The resulting cake of crystals is pressed out strongly in a cotton cloth. The purification operation is repeated until the nitronaphthalene has a melting point of 61°. It is then in the form of yellow, glistening crystals. Part of the product is lost in the mother liquors and can be recovered by distilling off the solvent.

The crude nitronaphthalene is reduced by the Bechamp method using iron and a small amount of hydrochloric acid. In an iron reduction vessel equipped with an anchor-type stirrer (Fig. 11) is placed 200 grams of iron turnings, 100 cc. water, and 10 cc. concentrated hydrochloric acid (30 per cent). The mixture is heated in a boiling water bath, and the nitronaphthalene is added in small portions, preferably in the manner described for the reduction of p-chloro-o-nitrophenol (page 110). With continuous stirring, 173 grams (1.0 mole) of nitronaphthalene (air-dried material) is reduced in a period of 4 hours. It is not advisable to operate more rapidly or undesired azo compounds may be formed. The mixture is now made distinctly alkaline by the addition of soda and removed from the reduction vessel. The a-naph- thylamine can be separated most satisfactorily in the laboratory by distillation with superheated steam. For this purpose, the whole reaction mixture is transferred to a kettle such as that shown in Figure 29. The

a-NITRONAPHTHALENE AND a-NAPHTHYLAMINE

177

water is completely driven off, while the mixture is being stirred, by heating the oil bath to about 200°, and then steam, superheated to 250°, is passed in (Fig. 23a). (The figure shows a schematic apparatus without stirrer. Stirring is recommended, however, in order to make the separation of iron oxide and naphthylamine easier.) A rapid distillation will easily carry over one-half to one part of naphthylamine with

Fig. 29. Cast iron reaction kettle with stirrer for use at pressures up to 2 atm.; weight, 12 kg. The oil bath is made of copper.

one part of water. A small amount of very finely divided iron powder, graphite from the cast iron, and iron oxide always comes over with the base. The distillation is complete when no more product, or only colored material, comes over with a steam temperature of 260°. The whole distillation requires 1 to 1.5 hours, depending on the type of heating employed. The kettle then contains a very fine, black mass which is pyrophoric and which therefore cannot simply be thrown out. The naphthylamine is separated from the mother liquor after cooling,

178	INTERMEDIATES

melted, and dried at 110° in an air oven. Vacuum distillation of the dried product gives the base as a colorless, crystalline material. The yield from 1 mole of naphthalene is about 110 grams of pure a-naph- thylamine melting at 50°.

The reaction mixture can also be worked up in the following way. When the reduction is complete, the mixture is neutralized with soda and all of the water is evaporated under vacuum. The residue is then extracted three times with benzene, and the extract is distilled at atmospheric pressure to remove the benzene. The a-naphthylamine is then distilled under reduced pressure.

Technical Observations, (a) Nitronaphthalene. The waste acid from the nitration is always partly re-used, being brought back to 80 per cent sulfuric acid by the addition of stronger acid. The unused part of the acid is used in acidifying alkali fusion mixtures, and similar purposes. The nitration is almost quantitative if the starting material has been ground up correctly (disintegrating at 60°). Nitronaphthalene is used in the preparation of 1,5- and 1,8-dinitronaphthalenes, as well as of the diazo compound of l-amino-2-naphthol-4-sulfonic acid. In the latter preparation, nitronaphthalene, heated with bisulfite, gives l-naphthylamine-2,4- disulfonic acid along with some naphthionic acid. The disulfonic acid is diazotized and converted to the diazo compound of l-amino-2-naphthol-4-sulfonic acid by treatment with sodium bicarbonate and sodium hypochlorite. This remarkable series of reactions is shown below:56

NH2

HC1 + NaNO2

S03Na

-f HOC1 NaHCO,

O

This process, although good, has been replaced by the less expensive Sandmeyer process (page 202).

(b) a-Naphthylamine. The reduction is carried out in apparatus similar to that already described. However, paddle or propeller stirrers cannot be used because of the pasty consistency of the reaction mixture, and only anchor-type stirrers are usable, such as the one shown in Figure 31. The steam distillation is done in apparatus such as the one shown in Figure 24, in which the steam is heated in a

superheater. Various satisfactory models of this equipment are supplied by				different
manufacturers.
In large scale operations, a large part of the		naphthylamine can be drawn
off directly	as a liquid. The residue of iron, iron	oxide, and a-naphthylamine is
mixed with	sawdust and extracted several times with		benzene. The mixture of
56Badische	A. und S. F. Ger. Pat. 160,536 (1905),		157,325 (1904),	156,440

(1904) [FrdL, 8, 656-7 (1905-1907)].

a-NITKONAPHTHALENE AND a-NAPHTHYLAMINE	179
iron, iron oxide, and sawdust is freed of benzene by steam and then sent to	the

foundry where it is used as a binding material for briquetting of iron turnings for the cupola furnace.

a-Naphthylamine is used as an end component or especially as a middle component, in the preparation of many important azo dyes; it is also used in the preparation of dyes of other classes. a-Naphthylamine is the starting material for the synthesis of a whole series of dye intermediates, some of which are described below. Like aniline, a-naphthylamine has found a new and interesting application outside of the dye field. It is used in North America and Australia in refining some of the poorer ores by the flotation process. In this process, the finely stamped ore is mixed vigorously (emulsified) with about one-half of one per cent of its weight of naphthylamine and crude xylene and a large quantity of water. The heavy ore, despite its high specific gravity, collects with the foam on the surface and can easily be scooped off.

Phenyl-a-naphthylamine

NH2	NH	NH
		+ NH,
		SOaH>

In a round-bottomed flask fitted with a thermometer and a vertical tube, a mixture of 143 grams (1.0 mole) of a-naphthylamine, 175 grams of aniline, and 3 grams of sulfanilic acid is boiled vigorously for 42 hours. Ammonia is liberated and the boiling point of the mixture increases to 215°C. from an initial value of about 195°. When the reaction is completed, the mixture is fractionated carefully in vacuum. Three fractions are taken: about 80 grams of aniline, about 10 grams of an intermediate fraction containing some aniline, a little a-naphthylamine, and mainly phenyl-a-naphthylamine, and finally, 190 to 200 grams of phenyl- a-naphthylamine which solidifies at about 53°. The residue is worthless and is discarded. This method, which is generally used today, gives a purer product than is obtained when the condensation is carried out with hydrochloric acid. Little or no diphenylamine is formed, and the reaction can be carried out on a large scale in an iron vessel. At 12 mm. pressure, the boiling point of aniline is about 73°, of a-naphthylamine about 160°, and of phenyl-a-naphthylamine about 224°.

Phenyl-a-naphthylamine, as well as the alkyl derivatives of a-naphthylamine, is used in preparing basic diphenylnaphthylmethane dyes (Victoria blue).

180	INTERMEDIATES
	a-Naphthol from a-Naphthylamine
	NH« S04H	OH
		HftO
		a-Naphthol

A mixture of 143 grams of a-naphthylamine, 110 grams of 66° Be sulfuric acid, and 1 liter of water is heated at 200°C. under 14 atmospheres pressure. The naphthylamine should first be melted in the water and the acid added in a thin stream with good stirring. The autoclave should be either leaded or enameled, although the cover can be of iron, since the sulfuric acid is not volatile. It is necessary that the autoclave be heated in an oil bath, because any overheating might cause the lead to melt.

After 8 hours, the mixture is allowed to cool and the a-naphthol is separated from the mother liquor. Ammoniumsulfate is recovered from the latter. The product is melted with a little water and, after cooling, separated from the liquid. It is almost chemically pure. Completely pure material is obtained by vacuum distillation. The yield of a-naph- thol melting at 94° is 94 to 95 per cent of the theoretical amount.

Technical Observations. The process described above is the best and least expensive. There is, however, another method which is analogous to the preparation of /3-naphthol. Sodium naphthalene-a-sulfonate is fused with caustic soda at 290° (not to exceed 300°). The sulfonation is carried out at 80-90°, and the product is salted out in as concentrated a solution as possible. The sulfonate can also be isolated by removing the excess acid with lime or chalk, treating with soda, and evaporating to obtain the product. The sulfonate, thus obtained, can be used in the fusion without further purification, but the resulting a-naphthol is impure.

Naphthionic Acid froma-Naphthylamine

Naphthionic acid is prepared from naphthylamine acid sulfate by the baking process, i.e., long, dry heating, preferably under reduced pressure.

NH

NH2-H2S04

NH,

—H.O

S03H

(a) Preparation of the Acid Sulfate

In a three-neck flask fitted with thermometer, stirrer, and vertical condenser, 73.5 grams of 70 per cent sulfuric acid is heated to 120-125°,

a-NITRONAPHTHALENE AND a-NAPHTHYLAMINE

181

and then a warm (50°) solution of 75 grams of a-naphthylamine in about 15 grams of benzene is added with thorough stirring, during a 30-minute period. The formation of clumps of undissolved base is completely avoided in this way, and the benzene distills out slowly. The light reddish solution soon solidifies, and the resulting solid is dried at 120° in vacuo for 18 hours. A very pure acid sulfate, which is of the greatest importance in the baking process, is obtained by the use of 70 per cent sulfuric acid and pure a-naphthylamine.

Oil bath thermometer

Vacuum

Thermometer for temperature of material

packing

l/^Material being baked

Fig. 30. Vacuum baking apparatus.

(b) Baking Process

The conversion of a-naphthylamine sulfate into naphthionic acid is carried out in a vacuum baking apparatus constructed as shown in Figure 30. 75 grams of the finely powdered sulfate is heated for 8 hours at 180° under a pressure of 10-15 mm. After the heating a light gray mass remains. This is dissolved in about 500 cc. water to which 20 grams of anhydrous soda has been added. The solution is heated to boiling and filtered, and then extfacted with benzene to remove unchanged naphthylamine. The solution is again heated to boiling and hydrochloric acid is added until a slight turbidity is formed. Decolorizing carbon is added, the solution is filtered hot, and the filtrate is cooled and acidified with hydrochloric acid. The colorless naphthionic acid which separates is filtered off and dried at 100°. The yield is 60 to 65 grams, or 85 to 95 per cent of the theoretical amount.

The general principles of the baking process are discussed on page 127 ff.

182	INTERMEDIATES
l-Naphthol-4-sulfonic Acid (Nevile-Winther Acid)			from
	Naphthionic Acid (Bucherer Reaction)
NH	OSO2Na	OH SO3Na	OH
	or	II
	S03Na	SO3Na	SO3Na

	Naphthionic	Nevile-Winther acid
		(l-naphthol-4-sulfonic
	acid
		acid)

A mixture of 100 grams of 100 per cent naphthionate in 200 cc. water and 600 grams of sodium bisulfite solution (25 per cent SO2) is boiled under reflux for 1 day. Sufficient 30 per cent sodium hydroxide solution is then added to cause the solution to give a red test with thiazole paper, and the solution is boiled as long as ammonia is liberated, then acidified with hydrochloric acid. Crystalline Nevile-Winther acid is obtained on cooling. It is freed from residual naphthionic acid by dissolving in water and filtering. The yield is about 80 per cent of the theoretical amount.

Naphthols as well as naphthylamines are converted to labile intermediate compounds by the action of bisulfite. These intermediates were considered to be sulfurous acid esters of naphthols (Formula I) by Bucherer, the discoverer of the reaction, but Woroshtzow formulated them as addition products of bisulfite with the keto forms of the naphthols (Formula II). These intermediates yield the corresponding naphthylamines with ammonia, and are hydrolyzed to the naphthols by caustic alkali. Thus, it is possible to convert naphthols into naphthylamines (pages 200 and 203), as well as naphthylamines into naphthols.57

Furthermore, ammonia can be replaced by primary or secondary	aliphatic or
aromatic amines (the latter, however, only in the p series), thus affording a means
of preparing alkylated or arylated naphthylamines starting with either	the primary

naphthylamine or the corresponding naphthol.58

The Bucherer reaction is also applicable with certain compounds in the benzene and anthracene59 series, but it is of practical significance only with naph-

thalene compounds. The	reaction can be used with both a- and j3-naphthols or
57 Bayer & Co., Ger. Pat.	109,102 (1899) [Frdl., 5, 164 (1897-1900)]. Badische
A. und S. F., Ger. Pat.	115,335 (1900), 117,471 (1900), and 126,136 (1901),

[Frdl., 6, 187-190 (1900-1902)].

58 Badische A. und S. F., Ger. Pat. 121,683 (1901) and 122,570 (1901) [Frdl, 6, 192-194 (1900-1902)].

59I.G. (Limpach and Hager), Ger. Pat. 550,707 (1930) [Frdl., 19, 1899 (1934);

C.A., 26, 4962 (1932)]. Helv. Chim. Acta, 29, 1756 (1946).

SULFONIC ACIDS OF NAPHTHALENE

183

-naphthylamines. It does not work, however, if a sulfo group is present in the position ortho or meta to an a—OH or —NH2, or meta to a /3—OH or —NHa- Thus, tor example, in J acid (2-amino-5-naphthol-7-sulfonic acid) or Gamma acid (2- amino-8-naphthol-6-sulfonic acid), the reaction involves only the amino group and not the hydroxyl group which is "protected" by the meta sulfo group (cf. phenylgamma acid, page 209). Similarly, the reaction with 2,8-dihydroxynaphthalene-6- sulfonic acid involves only the hydroxyl group in the 2 position (page 208).

l-Naphthol-4-sulfonic acid can be prepared also by diazotizing naphthionic acid. It is used chiefly as an azo dye component.

15. The Sulfonic Acids of Naphthalene

The riaphthalenesulfonic acids find very little direct use in the preparation of dyes, but they are of great technical importance as steps in the preparation of important intermediates such as /3-naphthol and /?-naphthylamine, dihydroxynaphthalenes and aminonaphthols, and numerous naphthylamine-, naphthol-, and aminonaphthol-sulfonic acids.

As was pointed out in the section on orientation rules, the position in the naphthalene nucleus taken by an entering sulfo group depends on the sulfonation temperature. The a position is favored at low temperatures, the ft position at higher temperatures. Accordingly, sulfonation to produce the a-sulfonic acid is carried out at as low a temperature as possible, at least below the melting point of naphthalene, and the naphthalene must be used in a finely pulverized form (see also the preparation of 1,5- and 1,8-naphthylaminesulfonic acids, page 214). The preparation of the /2-sulfonic acid, on the other hand, is carried 6ut at the highest temperature possible without causing decomposition, about 16Q~170°C. Even under these conditions, some of the a-isomer is always formed, the amount being at least 15 per cent according to the results of various experimenters. The complications involved in this sulfonation have been cleared up largely by the work of O. N. Witt.60

The procedure to be followed usually varies, depending on whether the naphthalene-/3-sulfonic acid is to be isolated as such or nitrated or sulfonated without actual isolation. In the latter case, since an excess of sulfuric acid must be used anyway, the procedure of Witt60 is advantageous and is used widely in industry. In this process, an excess of sulfuric acid is used in the sulfonation for the purpose of converting the a-sulfonic acid into the disulfonic acid as completely as possible, since the a-compound is further sulfonated much more easily than the 0-sulfonie acid. On the other hand, when the /3-sulfonic acid must be isolated as such, cost considerations demand that the minimum amount of sulfuric acid be used (see 0-naphthol, page 187).

60 Witt, Ber., 48, 743 (1915).

184	INTERMEDIATES

1,6- and 1,7-Naphthylaminesulfonic Acids (Cleve Acids)

jS03H

S03dH

The 1,6- and 1,7-naphthylaminesulfonic acids have long been very important as components for azo dyes, particularly as the middle component in polyazo dyes. They are used in the preparation of the impor-

tant black cotton dyes of the type of Columbia black FF,	and also of
a whole series of deyeloped dyes, such as the important	naphthogene

blue, Zambesi black V, and others. Sulfonic acids of this type are also frequently used in preparing dyes of the Sirius blue series (Bayer).

The sulfonation is best accomplished according to Witt's procedure, as described in the following paragraphs.

In a sulfonation and nitration vessel as described on page 101*(Fig. 19), 128 grams of naphthalene of the highest quality is heated over a free flame to 165°C. with continuous stirring. This temperature is maintained while 206 grams of 94 per cent sulfuric acid (66° Be) is run in slowly. The addition should be made over a period of at least 30 minutes, or too much of the a acid is formed and the yield is lowered. The mixture is then heated for 30 minutes at 165° in order to convert as much as possible of the a acid into the disulfonic acid so that the final product contains the 1,6 and 1,7 acids, practically free from isomeric a-sulfonic acids. The mixture is allowed to cool to 60° while being stirred, but external cooling is not applied since this causes the formation of a precipitate adhering to the walls of the vessel. If such a precipitate does form, it must be scraped from the walls, and if necessary, from the stirrer, and broken up. When the temperature has dropped to 60°, the reaction mixture is diluted with 300 grams of 90 per cent sulfuric acid. (In industrial processes, the monosulfonic acid at this stage is forced by air pressure over into the nitrating vessel. The diluting acid is added first, and it is important that the mixture is not cooled too much since separation of the /2-sulf onic acid may occur under some conditions causing the mixture to solidify and clog the tube.) The mixture is now cooled further with continuous stirring, and when the temperature has dropped to 25°, very slow, dropwise addition of 103 grams (1.0 mole)

SULFONIC ACIDS OF NAPHTHALENE

185

of 62 per cent nitric acid (40° Be) is started. Since the nitro compound formed is much more soluble in sulfuric acid than is the naphthalenesulfonic acid, there is no longer any danger of the reaction mixture solidifying after the nitration has been started. Therefore, the reaction mixture is cooled in ice to 10° after a few grams of nitric acid has been added, and the nitration is completed at this temperature. When about half of the nitric acid has been added, the mixture is examined to see that no deposit has formed on the walls of the vessel and no large lumps are present. If present, these aggregates must be broken up since solid crusts or thick lumps of crystallized naphthalenesulfonic acid will resist nitration even on very long standing. Similar precautions must be taken in processes which use less sulfuric acid for reasons of economy and in which, therefore, the danger of solidification is still greater. When all of the nitric acid has been added (about 2.5 hours), the mixture is allowed to stand for at least 12 hours, after which a nitrometer test should show the presence of not more than about 2 per cent of the nitric acid used. The viscous, but clear, brownish solution is poured into 2 liters water. Practically no nitrous acid should be generated.

The resulting acid solution is heated to 75° and a 20 per cent solution of ferrous sulfate is added until no more nitric oxide is formed. A stream of air is then blown through the liquid until a drop of the solution,greatly diluted with water, fails to give an immediate blue coloration on starchiodide paper. It is essential that these conditions be met, otherwise side reactions occur in the subsequent reduction.

The ferric salts formed from the ferrous sulfate also give a reaction with starchiodide paper, but the coloration appears only after one or two seconds, and then it begins to form around the edge of the test drop. An experienced person will not mistake the two reactions; it is safer, however, for the beginner to use, instead of starch-iodide paper, the so-called "sulfone reagent" (4,4'-diaminodiphenylmethane- 2,2'-sulfone) which does not react with ferric salts and which is not destroyed by

the strong acid. For further details, see page 243, in the section on the diazotization of aniline.

The reaction mixture is now treated with 50 grams of magnesium carbonate, and then with enough finely powdered chalk (about 320 grams) to neutralize all of the free sulfuric acid. (Slaked lime can also be used, but care must be taken not to make the mixture strongly alkaline because free alkali or alkaline earth hydroxide decomposes the nitrosulfonic acids.) The thick paste of calcium sulfate is filtered on a large suction funnel, and the residue is pressed out thoroughly and washed repeatedly with hot water until the washings are only slightly yellow in color. Complete washing out is not to be recommended because the volume of wash water required is too large; the total volume of filtrate and washings should be about 2.8 liters.

186	INTERMEDIATES
Reduction.	The solution of the nitrosulfonic acids is made slightly

but distinctly acid to Congo red by the addition of hydrochloric acid. In a 2.5-liter reduction beaker, preferably of copper, are placed 250 grams of finely powdered gray cast iron (see pages 75 and 77) and 250 cc. water. The mixture is heated to boiling and the iron is etched by the addition of 10 cc. glacial acetic acid. After boiling for a short time, 20 grams of crystalline sodium acetate is added and then the weakly acid solution of the nitrosulfonic acids is added, over a period of 1 hour, while the mixture is boiled and stirred vigorously (the iron must be churned up!). A considerable part of the solution evaporates but the total volume should not decrease to less than 1 liter. When the addition is completed, boiling is continued for 20 minutes, and then a test is made to see whether a drop of the solution is nearly colorless on filter paper. It will not be completely colorless, but it should, in no case, be strongly brown or yellow. The boiling solution is neutralized by the addition of calcined magnesia (about 20 grams) until a test drop on filter paper shows the presence of an easily filterable iron oxide precipitate and the solution is weakly* but distinctly, alkaline to litmus. The solution should also be tested on filter paper with sodium or ammonium sulfide to determine whether all of the iron has been precipitated. If this is not the case, the residual iron is precipitated with a small amountof ammonium sulfide. It is absolutely necessary to remove the last traces of iron to avoid rapid oxidation of the Cleve acids in subsequent steps.

The solution of the magnesium salts of Cleve acids is filtered with suction and the solution evaporated, if necessary, to 1 liter, using a porcelain dish over a free flame and a wooden propeller over the dish.

The concentrated solution is made strongly acid to Congo red by the addition of about 100 cc. concentrated hydrochloric acid and left to crystallize for a day while being stirred continuously. Both of the Cleve acids (1,6- and 1,7-naphthylaminesulfonic acids) separate slowly, although they are rather insoluble in water once they have precipitated. After 24 hours, the precipitate is filtered off and washed thoroughly with a large volume of water.

To separate the crude acids, the precipitate is dissolved in 800 cc. hot water and enough ammonia (or soda solution) is added to make the solution distinctly alkaline. Sodium chloride is now added in sufficient amount to make the solution 10 per cent with respect to salt. The difficultly soluble salt of the 1,7 acid separates in the course of a day in the form of greasy, lustrous, yellowish plates which are filtered off and washed with 10 per cent salt solution and then with a small

SULFONIC ACIDS OF NAPHTHALENE

187

amount of cold water. Hydrochloric acid is added to the filtrate to make it distinctly acid to Congo red, and the solution is allowed to stand with occasional stirring for 2 days. The resulting precipitate of the

1.6Cleve acid is filtered off, washed with water, and dried at 100°. The yield of 1,6 Cleve acid is about 80 grams (mol.wt. 223), that of

1.7Cleve acid about 75 grams of the sodium salt (mol.wt. 245).

The mother liquor contains appreciable quantities of the impure products which are discarded or, in large scale operations, recovered by evaporation.

Technical	Observations. The pure 1,6- and 1,7-naphthylaminesulfonic acids
give practically	identical dyes. Frequently it is advantageous to use a		mixture of
the two acids because the resulting dyes are stronger,		particularly in	the case of
black polyazo dyes (e.g., Columbia black or Zambesi black V).
The 1,7 Cleve acid is usually obtained in higher		purity than is the 1,6 acid,
so the 1,7 acid	is generally used in preparing complex	dyes. The presence of 1,8-

naphthylaminesulfonic acid in the 1,7 Cleve acid is easily recognized by diazotization, and heating the diazonium compound. The 1,8 compound yields naphthsultone, which forms an insoluble precipitate which can be filtered off and weighed (see page 217).

Naphthalene-/?-&ulfonic Acid and /?-Naphthol

S03H

In the preparation of naphthalene-/?-sulfonic acid, the sulfuric acid must be used up very completely since /?-naphthol is so cheap that only the cheapest process for its preparation will survive.

In the apparatus described on page 101, 256 grams (2 moles) of naphthalene is heated to 165°C. over a free flame with continuous stirring, and over a period of 30 minutes 280 grams of 94 per cent sulfuric acid (66° Be) is added, while the temperature is held between 163 and 168° by careful regulation of the flame. The dropping funnel is now removed and in its place is installed a bent glass tube fitted tightly into the cover (with a cork or asbestos paper). During the course of the sulfonation, water and naphthalene distill out through this tube. The mixture of naphthalene and sulfuric acid is heated with continued stirring for an hour at 165°, then an hour at 167°, another hour at 170°, and finally an hour at 173°. During this time, about 30 grams of water and 25 grams of naphthalene are collected in the receiver. An appreciable amount of naphthalene deposits on the cover of the reaction flask, but this is neglected. The flame is now removed and the apparatus dismantled. The resulting mixture contains, in addition to naphtha-

188	INTERMEDIATES
lenesulfonic acid, a certain amount of sulfone, free		sulfuric acid, and
some disulfonic acid,	along with tars. It should be	almost colorless.

The mixture is poured, still hot, into 1.8 liters water.

The resulting solution of the free sulfonic acid is partly neutralized by the careful addition, with good stirring, of 60 grams of soda ash. Then, 360 grams of salt is added slowly. The solution soon solidifies in large lumps which make further stirring difficult. Nevertheless, stirring must be continued until the mixture appears completely homogeneous, for only in this way can an easily filterable precipitate be obtained and complete solution of the salt be achieved. The duration of stirring depends on its speed, but should be at least 6 hours to ensure complete separation. The precipitate is then filtered off on a large suction funnel with a cotton filter, sucked as dry as possible, then transferred to a moistened, strong, cotton cloth and pressed out in a screw press, carefully at first and then as strongly as possible. The pressing should be carried on for at least 2 hours, otherwise too much mother liquor remains in the precipitate. The resulting hard mass is broken up and dried completely at 100-120°.

The yield of "/? salt" is about 165 per cent calculated on the basis of naphthalene, or about 400 to 420 grams. From the mother liquor, containing some of the a acid in addition to tar and a trace of ft acid, Glauber salt may be recovered.

Alkali fusion of sodium naphthalenesulfonate is one of the most important organic-technical operations. With the low price of naphthol, it is not surprising that only a few manufacturers undertake the prepara-

tion. Very cheap materials, such as coal,	soda, and sulfuric acid, are
essential.
2NaOH =	Na2SO3 + H2O
side reaction:	•f Na2SO4

The sodium sulfonate must be pulverized very finely in order to get good fusion with the alkali. In the laboratory, this is accomplished most easily by grinding the salt in a coffee grinder.

The fusion apparatus described on page 87 (Fig. 14) is placed directly over a small Fletcher burner and charged with 200 grams of solid, chlorate-free sodium hydroxide in large sticks, and 60 cc. water. (Fusions using chlorate-containing alkali give lower yields and are,

SULFONIC ACIDS OF NAPHTHALENE

189

moreover, very dangerous. Explosive!) The alkali is melted with a large flame. The melt is water clear, and foams as the temperature is raised gradually to 270°C., at which point the foaming ceases. The powdered sodium sulfonate is now added, with continued stirring, in teaspoon portions, and the temperature is raised slowly to 290°. The dry sodium salt is seen to disappear gradually as the dark colored, lustrous

Influence of NaCl and Na2SO4 on the Solubility of Sodium Naphthalene-/2-Sulfonate (American Data)

£ Salt and NaCl ( grams in 100 grams of solution )

25°C,		30°C,		40°C		50'>c.		65°C.
ft Salt	NaCl	ft Salt	NaCl	ft Salt	NaCl	ft Salt	NaCl	ft Salt	NaCl
5.58	0	6.24	0	7.98	0,	9.75	0	14.6	0
3.46	2.38	1.21	4.84	1.46	5.62	4.15	2.9	8.47	2.93
0.31	9.19	0.16	13.08	0.65	8.47	2.17	5.42	6.12	3.81
0.15	13.16	0	26.5	0	26.70	1.05	8.39	1.96	7.19
0	16.81					0	26.8	1.26	10.83
0	26.43							0	27.2
	p Salt and Na2SO4 ( grams in 100 grams of solution)
25'°C.		30°C.			40°C.		50°C.		65°c.
ft Salt	Na2S04	ft Salt	Na2SO4	ft Salt	Na2SO4	ft Salt	Na2SO4	ft Salt	Na2S04
3.42	1.97	1.97	4.81	4.3	2.85	5.72	2.87	11.75	1.68
2.41	3.06	0.26	13.23	2.18	5.83	3.49	5.35	7.37	5.28
1.78	4.34	0	29.1	1.2	8.48	1.93	8.24	6.7	5.45
0.93	7.4			0.77	10.92	1.42	10.01	1.90	12.0
0.62	9.25			0	32.5	0	31.9	3.14	10.86
0.52	10.52							0.25	26.96
0.10	13.15							0	31.0
0	21.9

sodium naphtholate is formed. The watery consistency of the mixture permits the addition of much more sodium sulfonate than mostprocedures specify. In laboratory preparations, 1.5 parts of the sulfonate (300 grams in this case) to 1 part of alkali can easily be added. (In plant processes using correctly designed equipment and proper firing, 2.8 parts of the salt can be used for each part of alkali, and charring or excessive thickening of the mixture is not encountered.) When a temperature of 290° has been reached, about one-half of the sulfonate should have been added. The temperature is then raised carefully to 300° and, when three-fourths of the salt (225 grams) has been added,

190 INTERMEDIATES

to 305°. Finally, the temperature is raised to 318°when all of the salt has been added. Higher temperatures should not be used. The melt becomes gritty due to the sodium sulfite which has separated, and the naphtholate gradually replaces the sulfonate, which disappears slowly. The melt is held at 318°for 15 minutes, carefully avoiding overheating. The whole operation requires about 1 hour. Too rapid addition of the sulfonate results in charring and lowering of the yield.

The reaction mixture is poured out into a low dish, and after cooling, it is broken up and returned to the fusion kettle with 500 cc. water. Most of the mixture dissolves easily on careful warming, leaving a crust of undissolved sodium sulfite. The solution is poured off and more water is added until all the material is dissolved. More than 2 liters water should not be required. The combined solutions are placed in a porcelain dish and heated to boiling over a Fletcher burner. Sulfuric acid (50 per cent) is added until the reaction to thiazole paper almost disappears. The solution is then cooled slightly and filtered through a large suction funnel into a warm flask. The volume of neutralized solution ready for filtration is about 3 liters. It is colorless or light yellow.

The clarified solution is heated to boiling and enough 50 per cent sulfuric acid is added to make the solution strongly acid to litmus. (If there is not time to filter off the /3-naphthol within an hour's time, hydrochloric acid should be used. Otherwise, sodium sulfate is also precipitated.) There is no odor of sulfurous acid. The /2-naphthol is insoluble in neutral sulfite solution in the presence of some bisulfite. It separates as an oil at first, but solidifies immediately. After an hour, the precipitate can be filtered off without losing more than traces. The product is collected on a suction funnel with a cotton filter, washed thoroughly with cold water, and dried at low temperature, either in a vacuum drying oven or in an ordinary drying oven. If the drying temperature is too high, the product melts and sublimes.

The yield of dry, crude naphthol from 300 grams of ft salt is about 150 grams (93 per cent pure), of distilled product about 135 grams, m.p. 122°.

The crude product suffices for some uses, but it must be thoroughly purified to meet the specifications for the commercial product. For this purification, only vacuum distillation is used today.

Technical Observations. The sulfonation of naphthalene is carried out in huge cast iron kettles holding 1000 to 3000 liters. Heating is done either directly with generator gas or by means of a steam jacket (double wall) (Figure 31) which must withstand at least 6 atmospheres in order that the required temperature of 174° can be attained.

Instead of converting the a-sulfonic acid into the p isomer by long heating,

SULFONIC ACIDS OF NAPHTHALENE

191

another scheme is used in the industry. After several hours of hearing, steam is blown into the sulfonation mixture, whereby the a acid is split into naphthalene and sulfuric acid, leaving the /3 acid unchanged. In this way, a mixture of the /3 acid and sulfuric acid is obtained, substantially free from the isomeric sulfonic acid. The technical process is different from the laboratory procedure in other respects also. Instead of using salt, which is much too costly, to precipitate the

Fig. 31. Sulfonation and nitration kettle with steam jacket (double wall).

naphthalene-/3-sulfonic acid, the sulfonation mixture is treated with the waste liquor from the naphthol, which contains sulfite. Sulfur dioxide is given off and is passed into the diluted /3-naphthol melt. The /3-naphthol is thereby precipitated, and as pointed out,the resulting mother liquor serves in precipitating the /3- naphthalenesulfonate. Only in this way can the /3-naphthol be manufactured cheaply. Also, careful extraction of the crude /3-naphthol with dilute sodium hydroxide recovers the a-naphthol present, since the latter is more easily soluble in alkali. The determination of mixtures of a- and /3-naphthol is described in the Analytical Section In some 0-naphthol plants, the washed crude naphthol is fused in closed iron Kettles (the aqueous mother liquor is drained off) and dried in vacuum. It is then

n«° I ^iUg a Sma11 fter Press' In this w*y, most of thesalts present in the naphthol are retained in the filter press, and the subsequent vacuum dis-

	*?	^^ T* little	Pitch fo™ation. Technical /3-naphthol is
smuoh	PU?'	^S J6en marketed in recent Xears in the fl^e form which
is much easier to work with and is free			from dust.

Sulfonation of /8-Naphthol

Sulfonation of ^-naphthol under various conditions of temperature and sulfuric acid concentration gives rise to a whole series of monoand