The Chemistry of Dyestuff. Dyestuffs. XXV. Vat Dyestuffs.

A Manual for Students of Chemistry and Dyeing
M. Fort, M.Sc. (Leeds) Late Lecturer in Dyeing in the Bradford Technical College and L. L. Lloyd, Ph.D. (Bern) Lecturer in Organic and Technical Chemistry in the Bradford Technical College
Cambridge: at the University Press 1919
(First edition 1917, reprinted 1919)
Under this group are included all those dyes which are insoluble in water, but can be applied to the. fibre through the intermediate production of a reduction or leuco compound which is soluble in dilute caustic alkali. The leuco compound is subsequently re-oxidised on the fibre to the original dyestuff. For this purpose a dyestuff must contain the ketonic or thioketonic group, which on reduction gives respectively a phenolic or tliiophenolic group capable of solution in alkali. The alkali compound of the phenol or thiophenol is taken up by the fibre and is then re-oxidised by air to the original ketone or thioketone. In many cases it is advisable also to treat the dyed material with some oxidising agent, such as bichrome or perborate, etc.

Indigo, which is the oldest and best-known member of this series, is very fast to light and to most other agents. It is faster however when dyed on wool than when dyed on cotton, wool probably having a more pronounced chemical affinity for indigo than cotton. The fastness to light of indigo is surpassed by many other vat dyes, but such fastness is by no means invariable in the class of vat dyestuffs.

The vat dyestufFs may be subdivided into three classes:
I. The Indigoid Vat Dyestuffs.
II. The Anthraquinone Vat dyestuffs.
III. The Sulphurised Vat Dyestuffs.

No really sharp class distinction, however, can be drawn in the case of certain dyes of these series.

The Indigoid Class may be applied to wool as their leuco compounds are soluble in very dilute alkali. The amount of alkali permissible without risk of injury to the wool must be exceeded in many cases however, and various "restrainers" have been added in dyeing, e.g., glue, glucose, formaldehyde, and soluble oils.

The Anthraquinone Class are mainly in use for cotton dyeing, and are not generally applied to wool because they usually require too much caustic alkali to dissolve their leuco compounds. They may, however, in certain cases be applied to wool, although great difficulties have to be overcome.

The Sulphurised Class are also more important for cotton dyeing.

The Indigoid Class. - The class of indigo derivatives is characterised by the presence of one or two fivemembered rings, in which a ketonic group is present along with either carbon, nitrogen, or sulphur, one of the carbon atoms of this ring being combined by a double bond to a similar carbon or to a ring component carbon atom, as for example to anthracene or naphthalene, provided there is a ketonic group ortho to the double-bonded carbon. Several types are possible therefore as represented by the following chromogenes:

When synthetic indigo was first placed on the market many virtues were claimed by dyers for natural indigo as compared with the chemically pure synthetic product. The valuable red bloom on indigo navies is more easily obtained with the older dyes, and this was attributed to the natural indigo containing indirubin (indigo red). The natural indigo also contains small amounts of indigo brown, indiglutin, etc. It has been shown, however, that indirubin, on reduction in the indigo vat, gives indoxyl which oxidises later in presence of air, giving indigotin or indigo blue. The indiglutin, a degradation product of indigo, is perhaps the most important impurity apart from clay andgums which may exert a colloidal influence. Owing to the varying quantities of substances other than indigotin present in natural indigo, the shade produced from it varies with different samples, and also varies during dyeing from the same vat. The shade produced by synthetic indigo is always the same, if the same percentage of dye is fixed on the fibre, and the same method of dyeing is employed. The shade of indigo varies, however, with the manner of application and the after-treatment. For example, steaming the dyed goods for a short time makes the blue a little more violet, and also increases its fastness against light.

Indigo (indigotin) is present in both plant and animal life in the form of its glucoside indican. This glucoside is formed in small quantity by a process of deassimilation in the stalk and leaves of indigofera tinctoria and indigofera sumatrana, which are cultivated chiefly in India and Java. The leaf of the indigo plant contains, on an average, about 0,5 per cent, of colouring matter. Two crops are gathered from the same plants each year, the second crop some two or three months after the first. The cut plant is put into vats and extracted with water, the temperature of which is about 30° to 35°C.; the length of time required for extraction varies from nine to fourteen hours. Towards the end of the extraction marsh gas and hydrogen are evolved, and about this period the fermentation diminishes and the liquid subsides. The liquid is then run off into the beating vats, in which oxygen from the air acts as the oxidising agent and precipitates the indigo from the vat. It is then filtered, boiled with water, pressed, dried in the shade, and cut into blocks.

The amount of indigotin present in natural .indigo varies very widely. Java indigo usually contains most, as a rule 70 per cent, and upwards, while good Bengal indigo contains 60 to 65 per cent, and Madras indigo 30 to 40 per cent. As much as 10 per cent, of indirubin is found in some commercial samples.

The shortage of artificial indigo, due to the war, has done much to revive indigo cultivation again, which dwindled rapidly after the synthetic product appeared on the market.

The Badische Anilin und Sodafabrik manufactured indigo in 1897 by the Heumami method which consists in the fusion of phenylglycine with caustic soda, followed by oxidation of the alkali melt, after solution in water, with air.

Phenylglycine by loss of water gives indoxyl thus: [-]
indoxyl on oxidation, e.g., with air, gives indigo: [-]

The yield by this method was extremely poor, but by replacing the phenylglycine by its ortho carboxylic acid a fairly good yield is obtained. The phenylglycine-ocarboxylic acid is obtained either from naphthalene or from o-chlorobenzoic acid Naphthalene is oxidised by sulphuric acid in presence of mercury to furnish phthalic anhydride, and the latter is converted by ammonia into phthalimide [-], which by treatment with sodium hypochlorite gives anthranilic acid [-], this being converted into phenylglycine-o-carboxylic acid by condensation with chloroacetic acid. The ochlorobenzoic acid produces the same compound by heating with glycine in presence of copper salts.

A better yield of indigo is obtained by condensation of phenylglycine by heating with sodamide and caustic soda, or by heating methylanthranilic acid with sodamide and caustic soda. The use of sodamide for indigo manufacture was discovered by the Deutsche Gold u. Silberscheide Anstalt and purchased by Meister, Lucius and Bruning. The sodamide may be produced in the reaction mixture of sodium and the sodium salt of phenylglycine by action of ammonia.

Sodium anilide (C6H5NHNa) also finds application in the indigo synthesis, this compound increasing the yield of indigo similarly to sodamide (S. C. I.).

Whatever method is employed it is essential to carry out the condensation in absence of air to prevent
the formation of isatin [-], an oxidation product of indigo, which causes the formation of indirubin.

Indigotin is also placed on the market in a reduced form, Indigo White, which may be prepared from indigo by reduction with iron powder and caustic soda, or by glucose and caustic soda, such as Indigo White BASF (B.) or Indigo MLB/W (M.).

When indigo is reduced in the vat two atoms of hydrogen are taken up by the molecule of dyestuff, the following compound [-] probably being produced. This transforms into Indigo White [-] the leuco compound dissolving in the alkali present to give a yellowish green vat.

In order that synthetic indigo may be readily reduced it is placed on the market in as fine a state as possible, and in many cases compounds are added to preserve the indigo in colloidal form during oxidation, e.g., benzylaniline sulphonic acid (B., 1912).

Indigo Salt T (K.) is obtained by the action of dilute caustic soda upon a solution of ortho-nitrobenzaldehyde in acetone.

Indigo, when pure, is a crystalline substance with a coppery hue subliming to give violet vapours on heating. This property of sublimation is common to indigoid vat dyestuffs, and is useful as a means of distinguishing from anthraquinone and sulphurised vat dyestuffs which with the exception of Anthraflavone (B.) do not sublime. Another means of distinguishing lies in the different coloured leuco compounds obtained in the vat on reduction. While insoluble in water and common solvents, the indigoids are, generally speaking, more easily dissolved than anthraquinone and sulphurised vat dyestuffs. Thus indigo itself is soluble in boiling glacial acetic acid, pyridine, phenol, benzaldehyde, nitrobenzene, etc., but only sparingly soluble in any of these in the cold.

On dissolving indigo in concentrated sulphuric acid at 70°C. it becomes converted into a mono-sul phonic acid ( SO3H para to NH ). This, as sodium salt, constitutes the commercial red or purple Indigo Extract. The disulphonic acid obtained by longer sulphonation is more used - Indigo Carmine. It finds some use as an acid dye of no particular fastness.

Indigo is readily converted into isatin by treatment with oxidising agents, e.g., nitric acid, chromic acid, etc. This is made use of technically in discharging indigo dyed cloth, and in the estimation of indigo (as sulphonate) by titration with permanganate.

[The common facts relating to the chemistry of indigo are to be found in all organic chemistry textbooks, and for that reason a somewhat curtailed treatment has been accorded here.]

Derivatives of Indigo. - The halogen derivatives are perhaps the most important. These may be produced from halogen compounds or the indigo may be halogenated, the halogenation being carried out in a solution or in presence of nitrobenzene, dichlorobenzene, concentrated sulphuric acid or chlorosulphonic acid. By progressive bromination of indigo it is possible to introduce from one to six atoms of bromine into the indigo molecule.

The indigo derivatives are named according to the positions of the groups, the positions being numbered as follows: [-]

Bromine first enters into position 5 forming a monoor di-bromo-derivative continuing through the following series: 55'7 to 55'77', finally to 5'57'74' and 5'57'74'4. The 66' compound cannot be obtained by bromination of indigo, but is produced from 4-bromo-2-amino-benzoic acid. The 66' dibromo-indigo was found by Friedlaender to be one of the dyestuffs present in the shell-fish murex brandaris, from which Tyrian Purple was obtained by the ancients.

The chloro indigos are obtained from the halogen derivatives of phenylglycine.

The halogenated indigos are brighter in shade and faster to bleaching agents than indigo itself, but by increasing the substitution the relative tinctorial power is decreased.

Homologues of indigo do not appear to have any special value; a 77' dimethyl indigo, Indigo MLB/T (M.) or Indigo pure BASF/G (B.), is, however, manufactured. It dyes greener shades, which are faster to chlorine than indigo.

Amino derivatives of indigo give brown vat dyes; a brominated diamino-indigo, Ciba Brown R (S.C.I.), is on the market. It dyes cotton and wool reddish brown, which is fast to light and washing but not to bleaching.

Naphthalene indigos have been obtained from aand β-naphthylamine; they are green dyestuffs, but are comparatively fugitive and therefore are of little value. The beta-compound, however, gives valuable vat dyes on brominatiou. Ciba Green G (S.C.I.) and Helindone Green G (M.) belong to this class.

The naphthalene indigos show less disposition to sublime on heating than the simple indigos. Ciba Green G when heated gives some reddish violet vapours, but most of the dyestuffs carbonise without subliming.

Indigo Yellow 3G (S.C.I.) is prepared by heating indigo with benzoyl chloride in nitrobenzene solution in presence of copper powder. It gives a bluish-red vat on alkaline reduction. It is of particular interest, since it may be dyed along with indigo from the same vat giving uniform green shades. Generally a mixture of indigoid dyes gives uneven shades due to unequal affinity of these dyes for the fibre and the special amounts of alkali and particular temperatures required. It has most probably the following constitution:

Most natural indigos contain Indirubin. This compound is obtained by condensation of isatin with indoxyl, [-] and is also obtained during the production of synthetic indigo when air is admitted to the caustic melt. Indirubin, when vatted, is to a large extent converted into indigo-blue, this is accompanied with loss of dyestuff. The amount of indirubin dyed and fixed on wool is therefore small and also it is of little value, as it gives shades loose to washing. Certain derivatives of indirubin are sufficiently fast, however, to be employed commercially as vat dyestuffs, notably halogen derivatives. By bromination the fastness to washing is improved. Ciba Heliotrope B (S.C.I.) is a tetrabrominated indirubin giving a yellowish olive vat.

In 1905 Friedlaender discovered Thioindigo Red B (K.), this being the first of a large series of vat dyes. It is similar in constitution to indigo, the NH group being replaced by sulphur. Its derivatives are named similarly to those of indigo.

It is termed bisthionaphtheneindigo and is obtained from thiosalicylic acid, which, on heating with sodium chloroacetate, gives phenylthioglycol-ortho-carboxylic acid. This, on heating, gives thioindoxyl, which may be oxidised to Thioindigo Red (K.) (cf. indigo synthesis).

Thioindoxyl on oxidation, preferably with potassium ferricyanide, gives Thioindigo Red.

Many derivatives of Thioindigo Red have been prepared, introducing a variety of shades.

Helindone Grey BR (M.) is a dichloro-diamidothioindigo dyeing from a yellow vat.

Helindone Violet (M.) is dichlorodiinethyl-dimethoxy-thioindigo.

Helindone Scarlet S (M.) is diethylthio-oxy-thioindigo.

Ciba Bordeaux B (S.C.I.) is 5.S'-dibrom-bisthionaphtheneindigo. It forms a yellowish orange vat on reduction.

By the condensation of the thionaphthene ring with the indoxyl or indol ring a variety of vat dyes are obtained, these being the thionaphthene-indol indigos; the combination of the two ring systems may be at the 2 or 3 positions both in the indol ring and the thionaphthene ring. Oxythionaphthene or its derivatives is condensed with isatin or its derivatives, or with alpha-isatin derivatives.

One of the most important is Ciba Red G (S.C.I.) or Thioindigo Scarlet G (K.), which is obtained by the condensation of isatin with alpha-oxy-thionaphthene followed by bromination.

It may be termed 2-thionaphthene-5.7-dibrom-3-indol indigo. It dyes from a pale yellow vat and is a valuable dyestuff, being fast to light and bleaching.

Other dyestuffs of this series are:

Ciba Violet B (S.C.I.) or tribrom-2-thioriaphthene-2-indol indigo.

Ciba Grey G (S.C.I.) or brom-2-thionaphthene-2-indol indigo.

Acenaphthoquinone also condenses with indoxyl or oxythionaphthene or their derivatives to produce vat dyes. One of good fastness to almost all agencies is Ciba Scarlet G (S.C.I.) or 2-thionaphthene-acenaphthene indigo.

Vat dyes may also be obtained by the condensation of isatin or its brom derivatives with naphthol or anthranol or certain derivatives of these compounds.

Alizarin Indigo 3R (By.) or 2-naphthalene-2-indol indigo gives a yellow vat on reduction. It is obtained when dibromisatin is condensed in benzene solution with alphα-naphthol.

Helindone Blue 3GN (M.) is obtained by the condensation of oxyanthranol with isatin anilide.

Alizarin Indigo G (By.) is a brominated 2-anthracene-2-indol indigo.

Anthraquinone Vat Dyes.

The dyes of this series may be roughly divided into three types: a ring system in which nitrogen is absent, a ring system in which nitrogen is present (mainly dyes obtainable from amino anthraquinones), and sulphurisation products of anthraquinone and its derivatives. Many of these dyes are particularly interesting on account of their high molecular weight and their complex structural formula. Many coloured bodies derived from anthraquinone and such of its derivatives as furnish a leuco body in the hydrosulphite vat, do not possess the necessary affinity for the fibre.

The simpler vat dyestuffs of this series may be compared with the parent body anthraquinone; this body is insoluble in alkalis, but may be reduced by alkaline reducing agents giving oxanthranol.

The oxanthranol is soluble in alkalis, forming a blood red solution; this compound on exposure to air gives the original quinone again. Compounds similar to this, if of sufficient tinctorial power, may therefore be used as vat dyes.

Anthraflavone G (B.) is a stilbene derivative obtained by heating β-methylanthraquinone with alcoholic potash from 150 to 170°C. It dyes cotton a fast greenish yellow from a dark reddish-brown vat.

Indanthrene Gold Orange G (B.) is obtained from 2.2'-dimethyl-l.1'-dianthraquinonyl by heating with or without dehydrating agents. It gives a magenta coloured vat.

In this and certain other formulae for vat dyes the quinone form =O is written instead of the ketonic -CO- in order to simplify complex formula.

Indanthrene Gold Orange R (B.) is a chloro derivative of the above, and Indanthrene Scarlet G (B.) a bromo derivative of it; it gives a reddish-violet vat.

Indanthrene Dark Blue BO (B.) is another example of the peculiar complex condensation that may be carried out with anthracene derivatives.

Benzanthrone, obtained by heating anthranol, glycerine and sulphuric acid, is heated with caustic potash, whereby two molecules condense together.

Indanthrene Green B (B.) is a nitration product of the previous dyestuff; it is particularly interesting, since by the action of oxidising agents upon cotton dyed with this colour a fast black is obtained, namely, Indanthrene Black B (B.).

Indanthrene Violet R extra (B.) is isomeric with Indanthrene Dark Blue BO (B.); it is obtained by a caustic potash melt of halogen derivatives of benzanthrone.
It was found at the Bayer Farben Fabrik that simpler compounds, benzoylated oxyor amidoanthraquinones, had the properties of vat dyes.

Algol Yellow WG (By.) is benzoyl-1-amidoanthrrquinone.

Algol Scarlet G (By.) is benzoyl-l-amido-4-methoxy anthraquinone.

Algol Red 5G (By.) is dibenzoyl-1.4-diamidoanthraquinone.

Algol Red R extra (By.) is dibenzoyl-1.5-diamido-8-oxy anthraquinone, and gives a red vat.

The first two members of the anthraquinone vat series to be discovered contain nitrogen, and were obtained from β-amidoanthraquinone by fusion with caustic alkali. At temperatures from 200° to 250°C. a blue dye is obtained, and at 330°-350°C. a yellow one. These two dyes were discovered in 1901 by R. Bohn at the Badische Anilin imd Sodafabrik.

Indanthrene Blue R (B.) is one of the oldest of the anthraquinone vat dyes, and is still the most important and the most largely used of these colours. It is obtained by heating β-amidoanthraquinone with caustic potash and potassium nitrate. The melt is dissolved in water and the dyestuff which separates is filtered off. It is purified by reduction to the leuco form and separation of the sodium salt from caustic soda solution, this being then well washed with alkaline bisulphite solution.

It would be termed as a leuco-azine N-dihydro-1.2-1'.2'-anthraquinone azine. It dyes bright blue shades from a dark blue vat.

Algol Blue K (By.) is the N-dimethyl derivative of the above compound.

Algol Blue 3G (By.) is 4.4'-dioxyindanthrene.

Indanthrene Yellow G. (B.) is obtained similarly to Indanthrene Blue R, except that the condensation temperature is much higher. It may also be obtained by heating β-amidoanthraquinone with antimony pentachloride in nitrobenzene solution.

It dyes bright yellow shades from a dark bluish violet vat.

Helindone Yellow 3GN (M.) is 2.2'-dianthraquinonyl urea.

Imides of the anthraquinone series form a large class of vat dyes of which the following are a few examples.

Algol Red B (By.) is β-anthraquinone-α-anthra-N-methylpyridonamine.

Indanthrene Bordeaux B (B.) is dichloro-di-aanthraquinonyl-2.7-diamido-anthraquinone.

Algol Bordeaux 3B (By.) is 4.4'-dimethoxy-di-α-anthraquinonyl-2.6-diamido-anthraquinone.

Vat dyes of the acridone series. - These dyestuffs are obtained by condensing chloro anthraquinone derivatives with anthranilic acid.

Indanthrene Red BN extra (B.) is an anthraquinonenaphthacridone.

Indanthrene Violet RN extra (B.) is an anthraquinonediacridone.

Sulphurised Vat Dyes. - These are obtained from anthracene and many of its derivatives or condensation products by heating with sulphur to a high temperature.

Indanthrene Olive G (B.) is produced by heating one part of anthracene (96 to 98 %) with three parts of sulphur to 250°C. until sulphuretted hydrogen is no longer given off. It gives a dark violet vat which dyes olive shades fast to light and washing.

Cibanone Blue 3G (S.C.I.) is a sulphur melt product of methylbenzanthrone.

Cibanone Black B (S.C.I.), Cibanone Green B, Cibanone Orange R, and Cibanone Yellow R (S.C.I.) are all vat dyes of this series.

By the condensation of indophenols of carbazol, or N-substi tuted carbazols with sodium poly sulphide, vat colours are obtained.

Hydron Blue R (C.) is obtained from the indopbenol of carbazol and sodium polysulphide. It is one of the vat dyes most used on cotton.

The addition of copper salts to the melt gives Indocarbon S (C.).

-(For reference:
Barnett, "Chemistry of Vat Dyes," J. Soc. Dyers, 1913, p. 183.
Vlies, "Recent Progress in Colouring Matters," J. Soc. Dyers, 1913, p. 316; 1914, pp. 22 and 29.)

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