20.12.16

The Chemistry of Dyestuff. Dyestuffs. XVI. Azo Dyestuffs.

A Manual for Students of Chemistry and Dyeing
By
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)
Azo dyes contain the chromophore group - N = N -.(The azo group in the formulae under this section is expressed by the common contraction -N2-.) This large class of dyes has been obtained by coupling diazo compounds with phenols or amines or suitable derivatives. Coupling depends on the removal of an atom of hydrogen from the ring by formation of free acid with the acid radicle of a diazo salt, whereby the diazo body joins on to the ring, giving quantitath e substitution by formation of an azo compound. E.g., diazobenzene chloride and β-naphthol give Sudan I thus:

[-]

To obtain ready coupling, the hydrochloric acid must be removed, and this is done by coupling in presence of caustic soda, sodium carbonate or sodium acetate. Usually the diazo compound is in solution, and it is desirable that the other component, phenol or amine, should also be in solution. It is usual to dissolve phenolic compounds with the aid of a theoretical amount of caustic soda, and then add sodium carbonate in amount sufficient to unite with the whole of the hydrochloric acid set free in coupling. On account of the liability, especially the case with simpler diazo bodies, to form anti-diazo compounds which couple with great difficulty, an excess of strong alkali is to be avoided.

Coupling with amines is generally done in acid solution, as acid is usually required to dissolve these bodies. Such coupling is usually more difficult than with phenolic compounds, and it is often the case that two couplings are necessary to obtain a certain disazo dye, i.e., one containing two azo groups, one coupling being in a phenolic substituted ring, and the other in an amino substituted ring. The latter being more difficult must be done first, as increased complexity or molecular weight, such as would be caused by coupling first on the phenolic side, renders it all the more difficult. In certain cases, e.g., the phenylene and tolylene diamines, where the amine is easily soluble in water alone, coupling may be done in neutral or even alkaline solution, the latter being commonly used in works practice. In other cases non-aqueous solvents are used, e.g., in coupling m-sulphanilic acid on to diphenylamine, the latter is dissolved in alcohol, giving Metanil Yellow. Sodium acetate is often added during coupling of amines to remove the free hydrochloric acid that is used to dissolve these components, the free hydrochloric acid in solution being replaced by acetic acid, and at the same time sodium chloride is formed. Commercial dyestuffs are produced containing not only one or two azo groups but also trisazo dyes containing three, and tetrakisazo dyes containing four azo groups. Thus the following series of couplings may take place with aniline as the first component (the body diazotised) and resorcin as second component:

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With each increase in the number of azo groups the shade is deepened, and also coupling becomes more difficult. These are the main reasons why coupling is rarely carried on to give tetrakisazo bodies, and practically never to give pentazo dyes. Also a diminished affinity for fibres often becomes evident in tetrakisazo dyes, especially with direct cotton dyes.

Most influential in coupling is the character and position of groups already substituted in the ring to be coupled with a diazo body. Coupling cannot take place in an unsubstituted aromatic nucleus. Thus in the case of Sudan I, quoted above, coupling is possible only in the nucleus containing the hydroxy group. As has been already indicated, hydroxy or amido groups or substituted amido groups, e.g., N(CH3)2, are required in the nucleus before coupling can take place. Where this primary condition is fulfilled, other groups, the presence of which alone will not induce coupling, may now distinctly favour it; notably electronegative groups, e.g., halogens, nitro, methyl, etc., groups in amines are favourable to coupling. Thus aniline as second component couples with some difficulty, toluidines couple more readily, with less tendency to form diazoamido compounds, while xylidines couple, giving azo compounds quite easily, as do also naphthylamines. The stability of diazo compounds is also increased by the presence of electro-negative substituents; e.g., diazotised p-nitraniline is more stable than diazobenzene. The stability of these bodies is important as regards time and temperature in coupling. Diazobenzene must be coupled in ice-cooled solution, but diazotised p-nitraniline may be coupled in ordinary cold solution, and certain other diazo compounds are stable in presence of hot water, e.g., diazo compound from o-anisidine. Generally, coupling is done in ordinary cold solution, ice being added where necessary.

The position in which coupling takes place in the ring of the second component must now be discussed. A general rule is that substitution by diazo groups takes place in the para position to the hydroxy or amido group where possible, and if prevented by the presence of another substituent, the ortho position to the auxochrome group is taken. Ortho-coupling most commonly occurs with phenols in alkaline solution. The example of diazobenzene chloride and resorcin given above may be referred to here also in illustration. Special modifications of the above rule must now be stated.

In the class of benzene derivatives, orthoand parasubstituted di-amidoand di-oxy-benzenes do not couple with diazo bodies under ordinary conditions, and in practice only meta derivatives of this type are in use as second components, e.g., resorcin, m-phenylene diamine, m-tolylene diamine, etc. Derivatives of aniline with substituents in the para position, e.g., sulphanilic acid, also refuse to couple with diazo compounds.

In the naphthalene series other modifications prevail. Coupling with α-naphthol or α-naphthylamine occurs in the para position to the auxochrome. Where the para position is already substituted, or if already coupled, the ortho position is assumed. Also it is found that if the para position be actually free to couple of itself, the presence of a substituent group in either adjacent position will exert steric hindrance, and prevent coupling at this point. For example:

(i) α-naphthol behaves thus: [-] which is quite in accordance with the general rule first stated.

(ii) α-naphthol-4-sulphonic acid couples with diazo bodies to give ortho azo compounds: [-]

(iii) If the 1.5-naphthol sulphonic acid be employed, coupling can only take place in the ortho position to the oxy group: [-]

Steric Hindrance of the peri substituent, i.e., the sulphonic group, here prevents position 4 being filled.

(iv) Again where position 3 from the auxochrome is filled as in [-] coupling takes place in the ortho position to the oxy group. Thus with diazotised m-xylidine Palatine Scarlet (B.) is obtained:

[-]

In the case of β-oxy and amido naphthalene and their derivatives, coupling always takes place in the adjacent a position. E.g., aniline when diazotised couples with β-naphthol in the position shown by the formula for Sudan I. (See page 136.) Or again, if diazotised sulphanilic acid be coupled on to β-naphthol, Orange II is obtained:

Coupling with di-amidoand oxy-naphthalenes obeys two rules: (i) 1.2 and 2.1 amido-naphthols cannot be coupled; (ii) where the two auxochrome groups are in different nuclei, coupling may take place in each nucleus under the conditions laid down above.

For example, Palatine Black (B.) is obtained from H-acid by coupling on diazotised sulphanilic acid, first in acid solution (being the more difficult coupling), followed by coupling diazotised α-naphthylamine to the monazo dye thus prepared, after making the solution alkaline.

[-]

In certain cases where coupling is difficult, copper salts are added to assist the operation.

Beside the effect of the number of azo groups on the shade, the nature, position and number of auxochrome groups also influences colour.

Bluer or deeper shades are obtained from hydroxy groups than from amido groups, while methoxy and ethoxy groups tend to give brilliant shades, especially in certain positions, e.g., ortho to the chromophore group -N:N-.

Thus aniline, coupled with 1.4-naphthol sulphonic acid, gives an orange dye not in use now, while o-anisidine, in place of aniline, give, Azoeosine G (By.) (see p. 140), which dyes bright bluish red shades. The fastness of dyes to light is always important, and the use of chlorinated derivatives almost invariably increases resistance to fading, and frequently also to other agencies, e.g., oxidation, alkalies and acids. Thus dichlorobenzidine 4s used instead of benzidine, e.g., in certain Dianol Reds (Lev.), and sulphonic acids are used as second components, with dianisidine as first, in the Chlorazol Blues (R.H.). Fastness to acids and alkalies is largely dependent upon the ability ot such reagents to attack auxochrome groups. Thus the entrance of sodium into a hydroxy group usually modifies the colour and fastness. This occurs more easily where the auxochrome oxy group is para to the azo group, and ortho-oxy-azo dyes are much more valuable on account of offering greater resistance to alkalies. Where however coupling takes place in more than one stage, e.g., in making disazo and trisazo dyes, if it is desired to diazotise an amido azo compound for this purpose, such compounds, with the amido group ortho to the azo group, e.g., amido azo derivatives of β-naphthylamine, offer great resistance to diazotisation, while other amido groups may be readily diazotised. Hence to obtain successive coupling with amines and rediazotisation of the amido azo compounds formed, p-amido-azo compounds are desirable, as they allow of ready diazotisation, e.g., α-naphthylamine and its derivatives are frequently used on this account, as in the case of Naphthylamine Black D (C.). This dyestuff is obtained by coupling diazotised α-naphthylamine 3.6-disulphonic acid to α-naphthylamine, the resulting p-amido-azo compound being rediazotised and again coupled with α-naphthylamine.

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General Methods for Manufacture of Azo Dyes. (Figures XI and XII, Appendix.) Simple amines are dissolved as hydrochlorides in water, and diazotised by rapid addition of sodium nitrite in slight excess of the theoretical amount, to avoid possible formation of diazo amido bodies. The. calculated quantity of hydrochloric acid, necessary to decompose the nitrite, is previously added to the amine solution. The reactions are done in wooden vats with rotating paddle stirrers (occasionally shovel stirrers). The large coupling vat is on the ground, and supported on a platform above are the diazotising vat and the vat containing the acid or alkaline solution of the second component to be coupled. A small nitrite vessel is placed above the diazotising vat, and pipe connections are fitted from vessel to vessel, leading down ultimately to the coupling vat. The nitrite pipe is carried to the bottom of the diazotising vat to avoid loss by nitrous fumes. The amido-sulphonic acids are often mixed with nitrite solution and diazotised by running hydrochloric acid into the mixture. The formation of dyestuff in the coupling vat varies in period; in the case of simpler bodies it often takes place at once, in other cases several days' stirring is required for complete coupling. For convenience in working, the contents of the coupling vat may be passed on to a tank or "monteju," from which it may be pumped or blown with compressed air to the filter press.

Usually, after coupling, azo dyes are largely in solution, and salt is added where necessary to salt out or precipitate the dye. Filtration is accomplished by means of a filter press, for a detailed description of which some manual of chemical engineering may be consulted. It consists, however, essentially of a number of plates of wood or metal, with distance frames and pieces of cloth between, the whole when screwed up forming a horizontal block of separate chambers, into which the liquor containing precipitate can be forced through tunnels running through the block. The precipitate is retained in the chambers, while clear filtrate passes out through the cloth layer on each side to an outlet tunnel. For acid liquors the press is constructed of wood. Iron is used for neutral and alkaline liquors. Filter cloth materials consist of stout-ribbed cotton for general use where neutral or alkaline liquors are in question; flannel for acids, and camel's hair cloth for strong acids. Certain dye manufacturers prefer small presses (4 feet length), others use much larger ones. After filtering, the press is unscrewed, the press cakes turned out of the cloths on to trays, and dried in vacuum ovens at 75°C. This later method gives better products than the old stove drying. So-called "concentrated brands" of azo dyes are obtained by the use of hydraulic pressure on the filter cakes, thus squeezing out a good deal of salt.

Finally the dyes are ground in a ball or roller mill, this being an iron cylinder with a screw lid, which is revolved so as to cause enclosed iron balls or rollers to pound the dry cakes of dye put inside to powder. Although so strictly a scientific process, the various batches of an azo dye are never quite the same strength, and a suitably varied amount of common salt or sodium sulphate is generally ground with the dye, in order to turn out a standard strength of product. It is common also to grind a little soda with dyes of an acid character, to facilitate solution. More serious adulteration may readily be practised in the grinding of dyes than is justified by the necessity of turning out a standard product.

Occasionally metallic salts or mordanting substances may be ground in with dyes to give some special properties when dyed, e.g., ortho-oxy-azo dyes (see p. 154). The mixing of dyes to obtain some special shade or mixture, as for union dyeing, is also frequently done in the grinding.

General Properties of Azo Dyes. Azo dyes may be of various dyeing classes, e.g., basic, acid, mordant, and direct cotton dyes, but in all cases on reduction are decolorised by splitting up at the azo groups, giving amido compounds thus:
R-N:N-R'+4H=R-NH2+R'-NH2.

Reduction methods of both qualitative and quantitative analysis are based on this reaction, which takes place usually in hot aqueous solution with one of the following reducing agents: zinc dust and acid or alkali, titanous or stannous chloride and hydrochloric acid, hydrosulphite compounds, etc. Such reduction methods are used for stripping azo dyes from dyed cloth, and for discharge styles in calico printing. The colours of azo dyes in strong sulphuric acid solution vary, and are often characteristic.


Insoluble Azo Dyes. - Certain of these, oxy-azo compounds, are made for use as spirit colours, e.g., the Sudans.

Brilliant Lake Red R (M.), obtained from aniline and /9-oxynaphthoic acid, is an example of azo dyes used only for lake manufacture. Many water soluble dyes are obtained as lakes, however, by conversion into sparingly soluble salts, such as calcium and barium salts.

There is also a small class of insoluble azo dyes produced on the fibre, of which the chief is p-nitraniline red or Para-Red. Cotton is steeped or padded in sodium β-naphtholate solution, and, after squeezing out evenly, dried. It is now treated in a 1 per cent, solution of diazotised p-nitraniline, when coupling takes place, the insoluble azo dye formed on the fibre being firmly fixed there. It is much used for cheap imitation
[-]
Turkey Reds. β-naphthol is almost exclusively used as second component (Naphthol AS (G.E.), the anilide of β-oxynaphthoic acid, offers certain advantages over β-naphthol as a "grounding" or "prepare"), but a variety of diazo compounds are in use as well as p-nitraniline. The most important of these, and the respective colours of the dyes obtained when coupled with β-naphthol are as follows:
p-nitraniline → bright red.
mor o-nitraniline → orange.
α-naphthylamine → claret.
benzidine → puce.
dianisidine → blue.
o-anisidine, chloranisidine, or p-nitroanisidine → reds.

Azophor Red PN, Nitrazol C, Nitrosamine Red, Azogen Red and Benzonitrol are pastes containing diazotised p-nitraniline rendered stable, e.g., with strong acids.


Basic Azo Dyes. - Of the basic class only a few are in use. To obtain the azo bases in a water soluble form they are converted into hydrochlorides.

Aniline Yellow or p-amidoazobenzene C6H5.N2.C6H4.NH2, and its homologue p-amido-azo -toluene are used for colouring fats (e.g., butter substitutes) and varnishes.

Chrysoidine is a well-known yellow basic dye obtained from diazobenzene chloride and m-phenylene diamine by coupling and crystallising the hydrochloride from solution in hot water.

[-]

Strongly basic dyes suitable for leather are obtained by use of strongly basic derivatives, e.g., (i) p-amidobenzyldimethylamine coupled with β-naphthol is the main constituent of Tannin Orange R (C.)
[-]

(ii) m-amido-phenyl-trimethylamine coupled with resorcin gives Azophosphine GO (M.)

[-]

The Janus dyes (M.) are strongly basic dyes dyeing cotton direct. Janus Red B is obtained by diazotising m-amido-phenyl-trimethylamine, coupling with m-toluidine and again coupling the amido-azo compound thus formed with β-naphthol.

Bismarck Brown is the hydrochloride of the disazo base obtained by treatment of metaphenylene diamine with nitrous acid in aqueous solution. The calculated amount of nitrite is used to get one-third of the diamine completely diazotised in both amido groups. Coupling takes place with the remaining diamine to give the dye, which mainly consists of the disazo compound shown.

[-]

It is still largely used on animal fibres, vegetable fibres, and leather. Other brands are made from m-tolylene diamine. Other names for these products are Vesuvine (B.), Manchester Brown, Leather Brown, Phenylene Brown, etc.

By diazotisation of Safranine (see p. 231) in one amido group and coupling, strongly basic monoazo dyes are obtained. Among others are the following, the second component used along with diazotised Safranine being given in each case:

phenol - Diazine Black (K.).
β-naphthol Indoine Blue, Janus Blue (M.), etc.
dimethylaniline - Diazine Green (K.), Janus Green (M.) (many other names).


Mordant Azo Dyes. - In this section only the simple mordant dyes incapable of application by other methods are considered. The acid mordant dyes, a much more numerous and important series of azo compounds, are dealt with later (see p. 153). The required grouping in the mordant dyes is generally obtained by use of salicylic acid as an end component. Alizarin 2G (M.) is obtained by coupling with diazotised m-nitraniline.

[-]

It is also sold as Alizarine Yellow in paste, Mordant Yellow (B.), Anthracene Yellow 2 G (C.), etc.

Alizarin Yellow R (M.), also sold under many other titles, is obtained by use of p-nitraniline.

[-]

Other mordant azo dyes andthe components used in their manufacture are:

Prage Alizarin Yellow G (Ki.) from metanitraniline and β-resorcylic acid.

Azogallein (G.) from p-amido dimethylaniline and pyrogallol. Deep violet on chrome mordant.

Azochromine (G.) from p-amidophenol and pyrogallol. Brown on chrome mordant.

Diamond Flavin G (By.), obtained by coupling tetrazotised benzidine with one molecule of salicylic acid and boiling to decompose the other diazo group.

[-] Yellow on chrome mordant.

Mordant Yellow GRO (B.) is obtained as for Diamond Flavin by treatment with bisulphite instead of merely boiling with water.

This last body is water soluble; the others are scarcely to be reckoned soluble, and are usually sold as pastes. They are dyed in a manner similar to Alizarin and other simple mordant dyes, and give shades of almost similar excellent fastness.

Anthracene Yellow C (C.), also of this class, is obtained from thioaniline and two molecules of salicylic acid.


Acid Azo Dyes. - The commercial dyes of this class are very numerous, including every shade, and frequently attaining to a considerable degree of fastness. They are considerably used in wool dyeing, and to a less extent in silk dyeing. The number and variety is so large that only a small selection of well-known products and their components can be given here.


Monazo Acid Dyestuffs.
Colour.

Orange. Orange G
aniline → 2-naphthol-6.8-disulphonic acid.

Orange red. Ponceau G (B., M., etc.)
Orange R (R. H.)
aniline → 2-naphthol-3.6-disulphonic acid.

Bright bluish scarlet. Ponceau R, 2 R G, etc. (Xylidine Scarlet.)
xylidine → 2-naphthol-3.6-disulphonic acid.
(m- or mixed xylidines according to brands.)

Bluish red. Azo Acid Red B (M.), Lanafuchsin 6 B, etc. (C.), Sorbine Red 2 B, G (B.), etc.
acetyl p-phenylene diamine → 1-naphthol-3.6-disulphonic acid.

Bordeaux. Fast Red B or Bordeaux B, etc.
α-naphthylamine → 2-naphthol-3.6-disulphonic acid.

Bright red. Crystal Scarlet or Ponceau 6 R, etc.
α-naphthylamine → 2-naphthol-6.8-disulphonic acid.

Golden yellow. Metanil Yellow (various brands)
m-sulphanilic acid diphenylamine.

Orange. Orange IV.
p-sulphanilic acid → diphenylamine.

Orange. Orange I.
p-sulphanilic acid , α-naphthol

Bluish red. Azofuchsin G (By.),
p-sulphanilic acid → 1.8-dioxynaphthalene-4-sulphonic acid.

Brown. Naphthylamine Brown (B.)
naphthionic acid → α-naphthol.

Red. Fast Red A
naphthionic acid → β-naphthol.

Red. Amaranth (C.)
naphthionic acid → 2-naphthol-3.6-disulphonic acid.

Red. Azorubine (various brands)
naphthionic acid → α-naphthol-4-sulphonic acid.

Violet Lanacyl Violet B (C.)
l-amido-8-naphthol-3.6-disulphonic acid (H acid) → ethyl-α-naphthylamine.

Blue. Lanacyl Blue 2B (C.)
l-amido-8-naphthol-3. 6-disulphonic acid (H acid) → l-amido-5-naphthol (coupled in alkaline solution).

---

The student should write full formulae for each of the above dyestuffs, attending carefully to the rules for coupling laid down previously. The effect of different groups on shade may then be noted. Metanil Yellow is rather sensitive to acids, turning red. It is much used in the paper industry. Where detailed information as to fastness, levelling properties, etc., is available, from the makers' pattern cards for example, some comparison of these properties should also be made. See also Azoeosine G, Cochineal Scarlet G, Palatine Scarlet, Orange II.

The direction of the arrows indicates the order of coupling.

Colour.

Brown. Fast Brown (By.)
naphthionic acid → resorcin ← naphthionic acid.

Blue Black. Naphthol Blue Black S (C.) (many other brands)
[-]

Red. Cloth Red G (By.)
amidoazobenzene → l-naphthol-4-sulphonic acid.

Red. Brilliant Crocein M (C.) (other brands)
amidoazobenzene → 2-naphthol-6.8-disulphonic acid.

Scarlet Red. Crocein Scarlet 3B (By.)
amidoazobenzene-4-sulphonic acid → 2-naphthol-8-sulphonic acid.

Black. Coomassie Wool Black S (Lev.)
p-phenylene diamine → α-naphthylamine → 2-naphthol-3.6-disulphonic acid.

Red. Milling Red G (C.)
2-naphthol-6-sulphoriic acid ← thioaniline → 2-naphthol-6-sulphonic acid.

See also Palatine Black and Naphthylamine Black D. Many commercial acid blacks are mixtures of this latter dyestuff and Naphthol Blue Blacks.

Very few trisazo and tetrakisazo acid dyes are made commercially, and therefore these classes do not merit treatment here.


Azo-acid Mordant Dyestuffs. - There is nothing to prevent an acid dyestuff also possessing mordant dyeing properties, in virtue of the presence of the usual mordant groups, in addition to acid groups. Such a dyestuff offers many advantages, its dual nature allowing of simple dyeing from an acid bath, with subsequent mordanting in a separate or the same bath, or in certain cases dyeing with the mordant present from the start. The dyeing operation is thereby greatly shortened, brighter shades are often obtained, and by the last method matching to shade is rendered easier. In addition to the usual mordant groupings, i.e., ortho-dioxy or ortho-oxy carboxyl groups, it has been found that in many cases ortho-oxy -azo compounds have mordant dyeing properties. Hence o-amidophenols are largely used as first components, and also their derivatives, such as sulphonic acids, nitro, and chlor derivatives. Such bodies are usually susceptible to oxidation, giving quinone compounds readily, and ordinary treatment with sodium nitrite and hydrochloric acid is not suitable for diazotising them. It has been found (Sandmeyer) that diazotisation is almost normal in presence of a copper salt, e.g., CuSO4, and in absence of mineral acid. The addition of zinc sulphate (K.), excess sodium chloride (B.), and organic acids (W.t. M.) are other methods of rendering diazotisation easy.

Coupling of ortho diazo phenols is often very slow and difficult..This is accelerated by working in strong solutions with strong alkalis such as caustic soda or lime. Acetylation of certain amido groups may be necessary in the second component to protect them in such coupling. Another method of expediting coupling (Ber. & 0.) is to condense the amidophenol with a sulphochloride, e.g., p-toluene sulphochloride thus: H2N-R-OSO2-C6H4CH3. The diazo body obtained from such a sulphonic ether readily couples, and the sulph-aryl group is then removed by saponification with hot dilute alkalis.

If in a dyestuff the azo group is ortho to a substituted chlorine atom, or even in the case of naphthalene bodies to a sulphonic acid group, these electro-negative groups are rendered easy of removal with weak alkali or even podium acetate in aqueous solution, being thereby replaced by the hydroxy group (B., 1901, pat.). This method of obtaining a mordant grouping does not involve the diazotisation of amido phenols. Thus 2.6-diamido-l-chlorobenzene-4-sulphonic acid may be tetrazotised or diazotised in both amido groups, and on treatment of the tetrazo body with sodium carbonate, the chlorine atom is replaced by a phenolic group.

[-]

The tetrazo-oxy-sulphonic acid may be used for coupling.

In all cases mordanting causes more or less change in the shades given by these dyes although in certain cases the deepening may be small. In others the change in colour is considerable, e.g., Chromotrope 2R (M.) dyes wool red from an acid bath but on chroming subsequently becomes blue black. This dye is obtained by coupling diazobenzene with chromotropic acid.
[-]

Chromotrope 2B (M.) is obtained similarly by using p-nitraniline, 6B (M.) p-amido-acetanilide, 10B (M.) anaphthylamine, 8B (M.) naphthionic acid, etc. all coupled with chromotropic acid (one molecule of each).

Other important azo acid-mordant dyes with the components from which they are derived are the following:

Metachrome Bordeaux B paste (Ber.)
picramic acid → m-phenylene diamine.

Chromazone Red A (G.)
p-amidobenzaldehyde → 1.8-dioxynaphthalene-3.6-disulphonic acid.

Acid Alizarin Brown B (M.), Palatine Chrome Brown W (B.)
o-amidophenol-p-sulphonic acid → m-phenylene diamine.

Acid Alizarin Violet N (M.), Palatine Chrome Violet (B.)
o-amidophenol-p-sulphonic acid → β-naphthol

Diamond Black PV (By.)
o-amidophenol-p-sulphonic acid → 1.5-dioxynaphthalene.

Acid Alizarin Black R (M.)
6-nitro-2-ainido-l-phenol-4-sulphonic acid → β-naphthol.

Palatine Chrome Black 6B (B.), also Salicine Black U (K.), Eriochrome Blue Black R (G.), Acid Alizarine Blue Black A (M.), Diamond Blue Black EB (By.), and other brands.

l-amido-2-naphthol-4-sulphonic acid → β-naphthol.

It is prepared by diazotisation of the first component by means of zinc nitrite with subsequent coupling in strong alkaline solution (coupling in presence of lime is often used for this class of body). An alternative method relies on preparing the first component from l-amido-2.4-disulphonic acid by diazotisation and treatment with caustic soda whereby the 2-sulphonic acid group is replaced by hydroxyl.

Acid Alizarin Red B (M.)
anthranilic acid → 2-naphthol-3.6-disulphonic acid.

Anthracene Acid Brown G (C.)
sulphanilic acid → salicylic acid ← sulphanilic.

Cloth Red B (By.)
amidoazotoluene → l-haphthol-4-sulphonic acid.

Acid Alizarin Black SE paste (M.)
β-naphthol ← 2. 6-diamido-l-phenol-4-sulphonic acid → β-naphthol.

Anthracene Red (By.)
salicylic acid o-nitrobenzidine → l-naphthol-4-sulphonic acid.

The Mercerol dyes (R.H.) belong to this class as do also the Erachromes (Lev.). The former contain their own mordant and dye like acid dyes. The Erganon dyes (B.), a new series used in calico printing, are obtained as soluble chrome compounds. They are fixed on the fibre by steaming or alkaline treatment when an insoluble chrome lake is fixed. The fastness is excellent.


Direct Cotton Dyes of the Azo Class. This class is a very large one and new members are still being added. Until Congo Red was discovered in 1883 by Böttiger no azo dyes were known which would dye cotton direct from aqueous solution. Benzidine and its homologues, when coupled to give dyestuffs, art found also to give cotton dyeing properties. The following diamines are used for this purpose:
benzidine [-]
tolidine [-]
dianisidine [-]
diphenetidine [-]
dichlorbenzidine [-]

All the above ortho-substituted benzidine homologues have the property of furnishing azo dyes dyeing cotton direct, while meta-substituted derivatives have not this property or only to a much less extent. However, cotton dyeing properties are obtained where substitution gives a linkage in the meta position thus:
benzidine sulphone [-]
diamido-carbazol [-]
also diamido fluorene.

Besides the above diphenyl derivatives other paradiamines also give direct cotton-dyeing properties. Thus:
p-phenylene diamine [-]
diamido stilbene [-]
Also 4.4'-diamido-stilbene-2.2'-disulphonic acid.
p, p'-diamido diphenylamine [-]
p, p'-diamido-diphenyl-urea [-]

Also a similar derivative of thio-urea CS(NH2)2.

Certain other para-diamines do not give cottondyeing properties to a derived azo dye, e.g., p, p'-diamidodibenzyl.
Naphthalene diamine-1.5 and also its sulphonic acids like the above para-diamines give cotton-dyeing properties when diazotised and coupled to form azo dyes. Certain thiazol bases also give azo dyes having direct cotton-dyeing properties. (See p. 162.) On the other hand some substances which can be used as second components, namely, to which diazo bodies may be coupled, can also give direct dyeing properties to derived azo dyes.

Thus amido-naphthol sulphonic acid J is largely used for the above reason.

There are certain peculiarities about J-acid and its azo derivatives. Thus coupled with diazobenzene derivatives cotton-dyeing properties are absent, but with diazonaphthalene derivatives are strongly marked even with some mon-azo dyes. By arylor acyl-substitution of the amido group still more affinity for cotton may be obtained, e.g., phenyl J-acid obtained by boiling with bisulphite and aniline.

The glycine derivative of J-acid obtained by treatment with chloracetic acid and other related derivatives are important (Lev.).

Again, the urea condensation product of this acid obtained by treatment with phosgene forms the second component of certain direct cotton dyes obtained by coupling on to it various diazo compounds.
[-]

The Benzo Fast Scarlets (By.) are obtained from this body by further coupling, e.g., with sulphanilic acid, etc., ortho to the oxy groups.

Azidine Fast Scarlets (J.) are obtained from a still more complex urea derivative from l-methyl-2.6-diainido benzene-4-sulphonic acid and 2 mols. J-acid. Toluidine or naphthylamine is coupled on this urea compound to give the dyes.

Dyes of this type are of good fastness to acids.

Certain strongly basic dyes possess direct cottondyeing properties and to this class belong the Janus dyes (M.), Indoine Blue and a few others. Such dyestuffs are faster however w r hen applied with a tannin mordant or "back-tanned" after dyeing.

The direct cotton dyes dye from a neutral bath as salts, apparently the whole dyestufF molecule being taken up by the fibre, hence they are also known as salt dyes or substantive dyes. Many of them are also used on wool on account of their fastness to washing, rubbing and perspiration. The most marked characteristics of the class are a general tendency to stain white cotton effectthreads when soaped or milled, and a frequent sensitiveness to acids.

It has been found that by after-treatment, subsequent to dyeing, many of these dyes can be greatly improved in fastness. Certain of them are greatly improved towards washing by a weak chrome bath after dyeing, and others again towards light by a similar treatment with copper sulphate. Dyes with known mordant groupings invariably respond to such after-treatments as these, but in other cases also an improvement can frequently be distinguished. The presence of a hydroxyl group ortho to the azo group has been assigned as responsible for coppering taking place. After-treatment with formaldehyde is now extensively used for certain cotton dyes and great fastness to washing and milling is obtained in many cases. The action of formaldehyde is not yet clear but in many cases it seems to be connected with the presence of diamines or resorcin as an end component, although all such dyes are not susceptible to after-treatment. In the case of dyes having a resorcin component, Baekeland's theory of the formation of synthetic resins by action of formaldehyde on phenols is applicable. Probably a benzyl alcohol is first formed by the action of formaldehyde, this product next losing water in a condensation with another molecule of dye, the process repeating itself until a very stable complex is obtained. Special series of direct cotton dyes of this type have recently been put out, notably the Vulcan dyestuffs (Lev.), Diamine Aldehyde (C.), Formal (G.), Benzoform (By.), Naphtoform (K.), etc.

Other after-treatments include the use of "Solidogen" (M.), which is the methyl amido benzyl-p-toluide obtained by condensing formaldehyde with ortho- and para-toluidine, and also the use of sodium thiosulphate (H.) which increases light-fastness.

"Coupling" of the dye on the fibre after dyeing with diazotised p-nitraniline is sometimes employed. A new dye is thereby produced in situ which possesses a modified shade and fastness. The structure of the original dye must of course permit of coupling taking place readily.

In other cases "developing" is used, i.e., after dyeing, the goods are treated with nitrous acid to diazotise amido groups in such dyes, which can then be further "developed" by a bath of β-naphthol forming a more complex azo dye. In this way more complex bodies can actually be produced on the fibre than could satisfactorily be dyed after preparation in substance.

The Diazo series (By.) merits special mention in this connection. Certain direct cotton dyes offer unique advantages for application to wool when dyed as salt, acid or mordant dyes. In some cases they are put out commercially only as wool dyes.


Monazo Direct Cotton Dyes.

These are derived from thiazol bases, e.g., dehydrothio-p-toluidine diazotised and coupled with 1 -naphthol-3.8-disulphonic acid gives Erika 2GN (Ber.), a red dyestuff.
[-]

The same first component with l-naphthol-4.8-disulphonic acid gives Geranine 2BG (By.), with l-naphthol-8-chlor-3.6-disulphonic acid Diamine Rose R extra, etc. (C.).

The homologous dehydrothiometaxylidine is used in Erika B extra and G extra (Ber.).

Other dyes are obtained by the use of Primuline sulphonic acid (see p. 179), thus Dianil Yellow 3G (M.) is obtained from this body, or dehydrothiotoluidine sulphonic acid,, by diazotisation and coupling with acetoacetic ether:
[-]

Other dyes of this class with the second component required are as follows:
P = Primuline sulphonic acid,
D = dehydrothiotoluidine sulphonic acid.
Oriol Yellow (G.) }
Cotton Yellow R (B.)} P (or D) → salicylic acid.

Clayton Yellow (CL), Thiazol Yellow (Ber., By.). Titan Yellow G (R.H.), etc.
P (or D) → P (or D).

Rosophenine 10B (CL), Thiazine Red R (B.)
D → l-naphthol-4-sulphonic acid.

Titan Pink 3B (R.H.)
D → 2-naphthol-6-sulphonic acid.

Thiazine Red G (B.)
P → 2-naphthol-6-sulphonic acid.

The above dyes are in general of good fastness to acids, especially the reds which are also fast to alkalis and soap in a moderate test.

Chloramine Yellow (By.), Chlorophenine (CL) is obtained by oxidation of 2 mols, of dehydrothiotoluidine sulphonic acid with hypochlorite. Four atoms of hydrogen are removed from the two amido groups giving an azo dye dyeing cotton very fast yellow shades.
[-]


Disazo Direct Cotton Dyes.

Diaminogen Blue 2B (C.), Diazanil Blue 2B (M.) 1.4-naphthalene diamine-7-sulphonic acid → α-naphthylamine → 2-naphthol-6-sulphonic acid.
Dyes a deep blue, by diazotisation on the fibre and developing with β-naphthol gives a fast indigo shade.

Toluylene Yellow (G.E.)
l-toluylene-2.6-diamine-4-sulphonic acid
→ 6-nitro-l.3-phenylene diamine
→ 6-nitro-l.3-phenylene diamine
The same diazo compound with only one molecule of m-phenylene diamine gives Toluylene Brown G (G.E.).

Diphenyl Fast Black (G.)
p.p'-diamidoditolylamine
→6-toluylene-1.3-diamine
→7-amido-1-naphthol-3-sulphonic acid (alkaline coupling).

Benzo Fast Red 2BT, (By.)
p.p'-diamidodipnenylurea
→ 7-amido-l-naphthol-3-sulphonic acid
→ ditto
(acid or neutral coupling).

Hessian Purple N (By., etc.)
4.4-diamidostilbene-2.2 -disulphonic acid
→ β-naphthylamine
→ Ditto.

Brilliant Yellow (By., etc.)
4.4'-diamidostilbene-2.2'-disulphonic acid
→ phenol
→ ditto.

Chrysophenine G, Pyramine Yellow G (B.), Sultan Yellow G (R.H.), etc., the most used of all cotton yellows is obtained from Brilliant Yellow above, by ethylation, e.g., with ethyl bromide, chloride, or sulphate.
[-]

This makes a dyestuff which is not sensitive to alkali, as is the case with Brilliant Yellow.

St Denis Red (P.), Rosophenine 4B (Cl.) obtained from diamidoazoxytoluene and Neville and Winther's acid has the formula:
[-]

It is dyed from alkaline baths and on washing the true red shade develops and is very fast.

Congo Red, the first direct cotton dye, is no longer used extensively. It serves as an indicator for mineral acids which turn it blue. It has the following formula [-]

By use of tolidine instead of benzidine the superior Benzopurpurine 4B was obtained which is still extensively employed.

Other dyes of the benzidine type possessing special interest are detailed below. To some extent the shade can be predicted from the formulae of these dyestuffs, for example, Chrysamine G (By., etc.) is yellow,
benzinide
→ salicylic acid
→ ditto,
while by using one molecule of salicylic and one of naphthionic acid a shade mid-way between Congo Red and Chrysamine G is obtained, i.e., Benzo Orange R (By.)
benzidine
→ 1-naphthylamine-4-sulphonic acid
→ salicylic acid.

Benzidine and its homologues diazotise under ordinary conditions in both amido groups simultaneously. Coupling of the tetrazo bodies thus formed however takes place in two distinct stages, one diazo group coupling immediately and the other only after some time, e.g., in the manufacture of Benzopurpurine 4B the first stage is over in a few minutes oat mechanical stirring and gradual addition of the sodium carbonate necessary for the second stage is continued for at least two days.

Another method (Badische) of obtaining disazo dyes of the benzidine type is to oxidise two simple azo compounds to get a diphenyl linkage. This method has little practical importance owing to the risk of destroying other groups.
Diazo Black B (By.)
benzidine
→ l-naphthylamine-5-sulphonic acid
→ ditto.

It is diazotised on the fibre and developed with β-naphthol for blue-black shades or m-phenylene diamine for brownish-blacks, or a combination of the two may be used.

Diamine Scarlet B (C.); the dyestuffis obtained from
benzidine
→ 2-naphthol-6.8-disulphonic acid
→ phenol
by ethylation, which improves the alkali-fastness.

Diamine Black BH (C.) (many other brands), a developing cotton black of constitution
[-]

Diamine Blue 2B (C.) (many other brands)
benzidine
→ 1.8-amidonaphthol-3.6-disulphonic acid
→ ditto.

Diamine Fast Red P (C.) (many other brands)
benzidine
→ salicylic-acid
→2-amido-8-naphthol-6-sulphonic acid
(coupled first in acid solution).

Largely used on both wool and cotton, fastness improved in either case by after-treatment with bichrome or chromium fluoride.

Sulphonazurine D (By.), obtained from benzidinesulphone-disulphonic acid and two molecules of phenylα-naphthylamine, is used on both cotton and wool giving blue shades of good fastness to milling, alkalis and acids. It has the following formula:
[-]

Benzopurpurine 4B (many other brands), obtained from tolidiue and naphthionic acid, has the formula
[-]
The Benzopurpurines are all red dyestuffs.

Benzopurpurine 6B is isomeric with the above, being
tolidine
→ 1.5-naphthylamine sulphonic acid
→ ditto.

By diazotisation on the fibre and development with β-naphthol it couples further to give a fast black, Diazo Brilliant Black B and R (By.) is the same product applied by this method.

Benzopurpurine B,
tolidine
→ 2-naphthylamine-6-sulphonic acid
→ ditto.

Diamine Blue 3B (C.), obtained by alkaline coupling
tolidine
→ 1.8-amidonaphthol-3.6-disulphonic acid
→ditto.
it becomes redder with acids and violet with alkalis.

Benzopurpurine 10B, the bluest benzopurpurine red,
dianisidine
→ naphthionic acid (l-NH2-4-SO3H)
→ ditto.

The Chicago Blues (Ber.) are a series of blues of extraordinary brightness.

Chicago Blue 6B (Ber.) (other brands) is prepared from dianisidine and l-amino-8-naphthol-2.4-disulphonic acid. The advantage of using this acid instead of H-acid (1.8-3.6) is in its constitution prohibiting any coupling on the amido side occurring as a side reaction. If such mixed coupling occurs, even to a small extent, as is often the case, the greatest brilliancy of shade cannot be expected from such a mixed product.

[-]

Diamine Gold (C.), the di-ethoxy-azo dye obtained by ethylation of the product from
1.5-naphthalene diamine-3.7-sulphonic acid
→ phenol
→ phenol.

Diamine Catechine (C.) is obtained by dyeing the disazo dye represented here
1.5-naphthalene diamine-3.7-sulphonic acid
→ α-naphthylamine
→ α-naphthylamine;
the violet shade thus obtained is Naphthalene Violet (C.) and on diazotisation and developing with warm soda solution gives a butch-brown shade the aniido groups in the second component (2 mols.) being replaced by hydroxy groups.


Trisazo Direct Cotton Dyes.

These supply the deeper shades of blue, also browns, greens, greys and blacks. Important typical examples are the following:

Columbia Black FP (Ber.), Dianol Black FF (Lev.), etc.
[-]

Benzo Olive (By.).
benzidine
→ salicylic acid
→ α-naphthylamine → 1.8-amido-naphthol-3.6-disulphonic acid.

Benzo Grey S extra (By.) is obtained by the use of the 1.4-naphthol sulphonic acid instead of the H-amido naphthol acid in the preceding example.

Diamine Green B (C.). The first green direct cotton dyestuff.
benzidine
→ phenol
→ [-]

Obtained by coupling para-nitraniline on to H-acid in acid solution, then coupling tetrazotised benzidine first with this azo compound and finally with phenol.

When salicylic acid is used instead of phenol Diamine Green G (C.) is obtained.


Tetrakrisazo Direct Cotton Dyes.

Such of these dyes as are in use are mainly browns and the class has a limited commercial importance.

Benzo Brown G (By.) is obtained by coupling two molecules of sulphanilic acid on to one of Bismarck Brown.

Toluylene Brown (G.E.) is obtained similarly by using naphthionic acid instead of sulphanilic.


Azo dyes of the anthracene series present greater difficulties in manufacture than those above described, among other causes being the insolubility of many intermediate products of this class, slow diazotisation and coupling. At present no azo-anthracene derivatives are of commercial importance as dyestuffs, but it is probable that in the near future this will no longer be the case.

---

Additional information may be acquired from a "Summary of Recent Progress in Colouring Matters," L. E. Vlies, Journ. Soc. of Dyers, p. 316, 1913, and pp. 22, 29, 1914, Thorpe's Diet, of Applied Chemistry, section "Azo Dyestuffs," Schultz, Farbstoff-Tabellen, and the patent record of Friedlaender, as well as abstracts of patents in the technical journals.

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