The Chemistry of Dyestuff. Dyestuffs. XVIII. Di- and Triphenylmethane 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)
The characteristic group which is also the chromo phore in these two series is

Oxidation of diphenylmethane H2 C(C6H5)2 gives benzophenone C6H5.CO).C6H5, which by reduction gives a secondary alcohol, i.e., diphenylcarbinol or benzhydrol C6H5 - CH(OH) - C6H5, which by still further reduction gives diphenylmethane again.

In the case of triphenylmethane HC(C6H5)3 oxidation gives the carbinol HOC(C6H5)3. These bodies are all colourless, and to obtain colour it is necessary to have auxochrome groups in the para position to the methane carbon in bodies of the carbinol type, which then by a quinonoid change may become transformed to coloured isomers. Corresponding colourless and coloured isomeric carbinol and quinonoid bodies respectively are known. The ordinary benzene ring theory can only supply formulae inadequate to express these differences. The assumption of quinonoid change in the ring has been most fruitful in explaining such difficulties by supplying adequate formulae. From the point of view of the quinonoid theory of colour, the chromophore in the series under consideration is
according to the auxochrome groups in the para position. In certain cases it has not yet been definitely decided in which of the benzene rings substituted in the methane group the quinone structure ought to be written.
The only important diphenylmethane dyestuff is Auramine O (other brands). It was first obtained by Kern and Caro in 1883 by heating tetramethyldiamidobenzophenone (Michler's ketone) at 150° to 160°C. with ammonium chloride, and also zinc chloride, as dehydrating agent. Apparently water is split off and an imido group combined thus:

Auramine base is colourless, but the hydrochloride shown above is a yellow basic dyestuff used in dyeing and printing on cotton (tannin mordanted), on wool and silk, and also for paper staining.

Auramine I, II, etc. are weaker brands liable to contain dextrine, which is the favourite adulterant for basic dyes.

The modern commercial synthesis of Auramine is by a different method. A melting-pot with upright pipe condenser (water-cooled) is heated in an oil bath, the contents of the pot are tetramethyldiamidodiphenylmethane, sulphur, ammonium chloride, and as diluent common salt. A stream of ammonia is passed during the course of the reaction. Ammonium sulphide is formed, the condensation giving the dyestuff as follows: [-]

The melt is powdered-up, extracted with water and the dyestuff salted out.

As respresented above Auramine is a ketone-imide, and it behaves in this way on boiling with acids or even alone, splitting off ammonia thus:
forming a colourless ketone. On this account the dye must not be applied above 60°-70°C.

Stock has shown that the phenyl derivative behaves as though a mono substituted amido group were present, namely,

On this basis the alternative quinonoid formula has been advanced.

Against this formula it may be urged that all known para quinonoid diphenylmethane derivatives are blue or violet.
Auramine G is obtained similarly by the initial use ot methyl ortho toluidine instead of dimethylaniline, the ultimate product being an ortho (to the substituted amido groups) dimethyl derivative of Auramine O.

Tripheny line thane dyestuffs are very numerous, including basic, acid, mordant, and even direct cotton dyes. They are systematically classified as amidoand oxytriphenylmethanes, and it is more instructive to deal with them in this way than by a dyeing classification such as was adopted for the azo dyes. The pyronines, phthaleins and rhodamines, subsequently dealt with, are also, strictly speaking, of the triphenylmethane class.

Triamidotriphenylmethane Dyestuffs. The oldest member of the triphenylmethane class is Magenta or Fuchsin. It is made commercially by heating a mixture of aniline with para and ortho toluidine in molecular proportions ("aniline for red") with an oxidising agent, e.g., arsenic acid or nitrobenzene with an iron salt as oxygen carrier. The latter agent is now preferred as giving a non-poisonous product. An excess of ortho toluidine is advantageous to the yield.

An enamelled iron melting-pot is used, heated by direct fire and fitted with stirring apparatus and in the lid a bent pipe and condenser, by which water and some excess oil, mainly o-toluidine and aniline, escapes. Apertures for removing test samples are fitted in the lid and a tube for running off the melt at the bottom.

Before heating the pot is charged with one part of "aniline for red," two parts of the dried hydrochloride of "anilinefor red" and half a part of nitrobenzene. This is mixed with about 2-3 per cent, of iron filings during the heating which is continued up to 190°C. Test samples are removed to follow the course of the reaction, which is continued 12 hours after reaching 160°C.

A current of steam is finally blown in to remove by distillation the remaining uncombined oils, leaving a pasty mass of rosaniline base.

The pasty mass is dissolved in boiling hydrochloric acid solution to which salt is added salting out the crude Magenta, other bases dissolving. The bronzy mass is purified by dissolving in water under pressure. The insoluble products are filtered off.

Chalk or sodium carbonate may be added to precipitate azine bases and the resulting solution contains only rosaniline and chrysaniline. This is treated with hydrochloric acid to reform rosaniline hydrochloride or Magenta, and salt is added to aid crystallisation.

Crystallisation is done in large wooden tanks, in which are placed bars of wood or frames to aid the deposition of the large crystals, which take two or three days to grow. The yield of large crystals is 40 50 per cent, on the oils used.

The arsenic acid method offers great similarity, but gives a rather less yield.

The impurities of the magenta melt include acridities, e.g., chrysaniline or Phosphine, azines, e.g., Indulines and Nigrosines, and many other bodies. These are to a large extent worked up into marketable products: Cerises (B.), etc.

Constitution of Magenta. Nitrobenzene itself does not enter into the composition of Magenta, as is proved by use of chlornitrobenzene, which does not give a chlorinated dyestuff. All higher homologues of p-toluidine, in which ineta and para positions are free, give dyes of the Magenta class.

By diazotisation of Magenta and boiling, an Aurine is obtained.

E. and O. Fischer in a classic investigation established the constitution of Magenta base or rosaniline base as a tri-p-amido-triphenylmethane.

The dyestuff obtained from p-toluidine and two molecules of aniline, C19H17N3.HCl, gave with alkalis the coloured base C19H17N3.H2O called pararosaniline, which by reduction gave a colourless or leuco base C19H19N3. This leuco compound was shown to be a triamido compound, i.e., C19H13(NH2)3 by diazotisation and boiling with alcohol, whereby triphenylme thane itself was obtained.

The usual Magenta obtained by oxidation of ptoluidiue, aniline and o-toluidine under similar treatment gave homologous products, resulting finally in diphenyltolylmethane.

The position of the substituent groups in Magenta was then determined by the inverse process of synthesis.

Nitration of triphenylmethane gave a trinitro derivative which on reduction gave a triamido compound identical with the leuco base, which by oxidation gave the coloured base of rosaniline, and this with hydrochloric acid gave the dyestuff itself.

The position of substituents was established first from the fact that para toluidine or a para homologue is essential to get a rosaniline by the oxidation process, hence one amido group in Magenta is 4 para to the methane carbon. This was also clear from the synthesis of Magenta by a condensation of p-amido-benzaldehyde with two molecules of aniline, heating with zinc chloride as dehydrating agent.

In a similar condensation, using benzaldehyde instead of the amido derivative, diamidotriphenylmethane was obtained, Avhich by diazotisation and boiling gave the corresponding di-oxy derivative.

Caustic potash fusion causes this di-oxy body to form the p-di-oxybenzophenone shown above. Hence it is deduced that the two amido groups supplied to rosaniline by aniline are also para to the methane carbon. The simplest rosaniline must be represented therefore as follows:

This p-rosaniline is present in commercial Magenta along with rosaniline, which is the tolyl homologiie:

On the basis of the quinonoid theory of colour Rosenstiehl's formula for Magenta has been almost universally abandoned, and it is no longer written [-]

Both coloured and colourless bases are known corresponding to the formulae [-]

Hanztsch has found the former to exist in solution as a true ammonium base, whereas the colourless carbinol base is not dissociated. Colourless salts of the carbinol base have been prepared which readily change to coloured quinonoid isomers.

On heating rosaniline base, it loses one molecule of water, giving an anhydro base

The formulae of rosaniline dyestuffs may be written as hydrochlorides of this base instead of chlorides of the quinonoid base.


Two general methods of synthesis of rosaniline dyestuffs have now been dealt with, i.e., (i) the oxidation process, (ii) the substituted-benzaldehyde synthesis, and the next to be dealt with is (iii) the formaldehyde or New Fuchsin process.

Formaldehyde and aniline in aqueous solution readily give anhydroformaldehyde-aniline (methylene aniline)
which by heating with an excess of aniline and aniline hydrochloride undergoes isomeric change to anhydrop-amidobenzyl alcohol, which readily condenses with another molecule of aniline on heating giving diamidodiphenylmethane.


By oxidation with nitrobenzene and ferric chloride as carrier, one molecule of aniline or o-toluidine may be condensed with this body to give homoor pararosaniline.

New Magenta obtained by this process has the formula [-]

Other syntheses are by use of p-nitrobenzaldehyde with aniline (2 mols.) followed by reduction, while it is easily seen how rosaniline may be obtained by use of para-nitrobenzyl chloride and other benzyl derivatives.

The quinonoid theory may be used to explain the separate stages in condensations of the type dealt with. For example, by successive oxidation from benzenoid to quinonoid types, by which ready condensation with another molecule of an amine occurs, causing a higher phenylated methane to be formed:
which again by oxidation gives a quinone derivative leading to a further addition of aniline, thus forming the leuco base of rosaniline.

Another means of synthesis is used commercially in production of triphenylmethane dyestuffs, (iv) the phosgene process. Phosgene readily condenses with tertiary amines, e.g., with dimethylaniline. The gas is passed into the dimethylaniline. (COC12 is made by passing a mixture of chlorine and carbon monoxide over a catalyst, such as animal charcoal.) In this way tetramethyldiamido-benzophenone is formed by splitting off HCl.

This type of benzophenone derivative on heating with another molecule of a tertiary amine in presence of phosphorus oxychloride, or by further treatment with phosgene, gives a tfiphenylmethane dyestuff, e.g., Crystal Violet.

(v) Another method of synthesis is by condensation of tetra-acyl-diamido-benzhydrols with tertiary bases. This is really associated with the New Fuchsin process, in which the oxidation is carried through direct from the diamido-diphenyl-methane. By method (v) this latter body is oxidised in the cold to the alcohol or benzhydrol with lead peroxide and hydrochloric acid, and the benzhydrol so obtained condensed with another molecule of an amine, e.g., by heating in glacial acetic acid solution.


Colour and General Properties of Triphenylmethane Dyestuffs. The unsubstituted tri-amido compounds are red, substitution in the benzene ring having little effect upon shade. Substitution in the amido group causes a deepening of colour, e.g., hexamethylrosaniline hydrochloride (Crystal Violet) is violet, while triphenylrosaniline hydrochloride is blue (Aniline Blue). Removal of one para amido group giving diamido-triphenylmethane dyestuffs causes a change of shade to green or greenish blues; compare Malachite Green, etc., described later.

As explained above, reduction causes formation of colourless leuco compounds, which by slow reoxidation in air, or quickly with an oxidising agent, give a return of colour. This is used as a test, employing hydrosulphite formaldehydes for reduction and very dilute persulphate for re-oxidation.

Oxidising agents readily destroy triphenylmethane dyes, ultimately giving quinone, and leuco compounds must be carefully oxidised to give dyes without waste, e.g. with theoretical amounts of lead peroxide. Closely connected with this question is that of the fastness of these dyes to light, which is not very great.

With excess of mineral acids, e.g., cold concentrated sulphuric acid, these dyestuffs give yellow solutions, which on dilution pass through mixed shades, finally giving the true colour on extreme dilution. This may be used as a confirmatory test in qualitative analysis.

Alkalis decolorise triphenylmethane dyestuffs, finally regenerating the carbinol bases with intermediate formation of a coloured ammonium quinonoid base (see above). The fastness to alkalis when dyed is somewhat limited for a similar reason.

Other important members of the triphenylmethane class are the following:

Methyl Violet - pentamethylrosaniline. [-]

This basic dyestuff is now obtained without the use of methyl iodide on rosaniline, by oxidation of dimethylaniline with air, using cupric chloride as carrier. One of the methyl groups leaves the nitrogen to give the central methane carbon. This probably involves intermediate formaldehyde formation and condensation to give the dyestuff.

The operation is carried out in large drums with loose lids to give free air exposure. Heating by means of a steam coil or jacket is used to start the reaction, which afterwards proceeds of itself.

Rotating stirrers give good mixing of the ingredients which include a large excess of common salt, also phenol or crude cresol, a small amount of water, and in some cases sand and an acid. The reaction giving the dyestuff takes place at 50-60°C., when the mass takes on a shining metallic appearance. Common salt is apparently not merely a diluent, but a direct aid to the activity of the copper salt. The reaction takes about eight hours to complete. The melt is then treated with water and is purified by adding milk of lime, which precipitates the dye base and copper hydrate. These are filtered off and treated with sulphuretted hydrogen, e.g., by adding sodium sulphide afterwards acidifying with hydrochloric acid. The copper is precipitated and the dye dissolves as hydrochloride. The Methyl Violet solution is filtered off and salted out to obtain a purified product of which the yield is about 85 per cent.

By action of benzyl chloride on Methyl Violet the NH(CH3) group is converted into N(CH3).CH2.C6H5, giving a bluer dyestuff, i.e., Methyl Violet 5B, 6B, and 7B, or Benzyl Violet. The reaction is done under a reflux condenser in alcoholic solution in presence of a slight excess of alkali, at the boil for six to eight hours.

Ethyl Violet is the ethyl analogue of Crystal Violet, and is obtained by (i) condensation of diethylaniline with tetraethyldiamidobenzophenone, or (ii) action of phosgene on diethylaniline (+ ZnCl2), or (iii) oxidation of diethylaniline and tetraethyldiamidodiphenylmethane (+ CuSO4).

Methyl Green. [-]

This strongly basic dye contains two substituted ammonium groups. It is obtained by treatment of Methyl Violet in amyl alcohol with methyl chloride.

Aniline Blue, Spirit Blue, Opal Blue obtained by heating rosaniline base with excess aniline and benzoic or acetic acid under a reflux condenser to 180°.

Phenylation takes place according to the amount of aniline (or toluidine) used, giving phenylated derivatives. The benzoic acid is recovered from the melt unchanged. Its use is to expedite the reaction, but in what way is unknown. The di- and tri-phenylated para rosanilines are the most common of the dyestuffs thus prepared.

Victoria Blue 4R (B.), (Ber.), etc. is naphthylpentamethylrosaniline obtained by condensing methylphenyl-α-naphthylamine with tetramethyldiamidobenzophenone chloride.

Acid Magenta is an acid dye obtained by sulphonation of Magenta with fuming sulphuric acid (20% SO3), 120°-170°C., which gives a mixture of the di- and trisulphonic acids of homoand para-rosaniline. The acid mixture is diluted, "limed out" x and converted into the sodium salt by treatment with sodium carbonate.

The coloured quinonoid inner anhydride is decolorised on treatment with dilute caustic soda, which converts it into the tri-sodium salt.

Acid Magenta is not very fast but is still extensively employed as a cheap easy levelling dye.

Acid Violet 4RS is the dimethyl derivative, i.e., with two NH(CH3) groups, of Acid Magenta.

The Acid Violets are a numerous and important class of which one or two examples will suffice.

Acid Violet 4BN (R), 6B (By.), N (M.), etc. are obtained by sulphonation of Benzyl Violet, whereby tri-sulphonic acid is obtained. One sulphonic acid group enters the benzyl nucleus, otherwise it is analogous to Acid Magenta.

Formyl Violet S4B (C.) (many other Acid Violet brands) is prepared by a formaldehyde condensation of two molecules of ethylbenzylaniline sulphonic acid, the benzhydrol derivative thus obtained being further condensed with diethylaniline and oxidised from the leuco compound to give the dyestuff.
It is a valuable acid dye for animal fibres.

Sulphonated Aniline Blues. By sulphonation of Spirit Blue mono-, di-, tri- or tetra-sulphonic acids may be obtained, soluble in water.

The mono-sulphonic acid is Alkali Blue (various brands), and this acid dye, insoluble as the free colour acid, is dyed from alkaline baths as the colourless salt, being developed blue on the fibre by a subsequent acid bath.

Soluble Blues and Water Blues, etc. are the mixed di- and tri-sulphonic acids and being more soluble can be dyed from acid baths.

These dyes are considerably used for cheap bright blues on animal fibres and tannin-mordanted vegetable fibres. The dyeing of the latter is dependent on the residual basic properties still potent in these acid dyes.

A somewhat similar type of body of unknown constitution is obtained by sulphonating the product from condensing diamido-diphenylmethane with rosaniline in presence of benzoic acid (R. H.). It dyes both animal fibres and unmordanted cotton. The most brilliant cotton blues have been obtained by dyeing a body of this type alone or in conjunction with benzidine derivatives. Thus the essential constituent of the Chlorazol Brilliant Blues (R.H.) is the di-sulphonate of tri-β-naphthyl-rosaniliue.

The Patent Blues and Cyanols are sulphonated diamido-oxy-triphenyl methanes and in both their structure and greenish shade show a closer resemblance to the Malachite Green class of dyestuffs than to Magenta, but are faster to alkalis than either. They are most extensively used, being easy levelling acid dyes of good general fastness.

The condensation product of m-nitrobenzaldehyde with two molecules of diethyl or ethylbenzyl aniline gives a meta-nitro triphenyl methane body. The nitro group is then reduced to an amido group, diazotised and boiled with dilute acid to replace it by OH. The resulting m-oxy substituted diamidotriphcnyl-methane is then disulphonated.

In preparing the Cyanols m-oxybenzaldehyde is used initially but otherwise resemble Patent Blue in their general preparation and properties.

Patent Blue A (M.) (carbinol formula): [-]

Chrome Violet in paste (By.) is a mordant dyestuff obtained by oxidation of the product from condensing salicylic acid with tetramethyl diamido benzhydrol.

Malachite Green is obtained by heating 24 hours at 100°C. under reflux condenser in an enamelled stirring pan a mixture of one molecule benzaldehyde and two molecules dimethylamline with a quantity of hydrochloric acid, insufficient to neutralise the base. (Zinc chloride was formerly used.) The mass is treated with caustic soda solution, the excess of base and benzaldehyde blown over with steam, and the oil remaining run into cold water where it solidifies while being well washed. It is then almost colourless and on oxidation in acid solution with the theoretical amount of lead peroxide the colourless leuco base gives the coloured carbinol. The lead is precipitated as sulphate, and the green solution of the coloured base as hydrochloride is decanted off, the dye salted out, or crystallised as a double chloride with 2ZnCl2. (Sometimes also it is prepared as oxalate with 3C2H2O4.) Further purification may be done by reprecipitation of the base with ammonia and retransformation into salt form.

1 It even pays in this trade to pack the crystals in cotton wool to preserve their shape intact. Particularly large crystals, measuring several inches, are made for export to the East1.

Malachite Green is a bluish green basic dye, dyeing silk, wool, jute and leather direct, and also tannin mordanted cotton. It is also used in paper staining and lake manufacture. The dye itself, and also when on the fibre, is rather sensitive to alkalis, being readily converted into the carbinol form. By introduction of an ortho-chlorine substituent in the unsubstituted benzene ring greater fastness to alkali is obtained, i.e., increased stability. (Compare the ortho SO3H group in Patent Blues and Cyanols.) Thus Setoglaucine O (G.) obtained by a similar condensation using o-chlorbenzaldehyde has improved properties. Similar dyestuffs to the latter are Setocyanine O (G.), Glacier Blue (J.), Night Green A (t. M.), Patent Green AGL (M.), Brilliant Milling Green B (C.), etc., the last three dyestuffs being sulphonated compounds, namely acid dyes.

Guinea Green B (Ber.) and Light Green SP (B.) also belong to this class. The latter is obtained by condensation of benzaldehyde with methylbenzylaniline (bluish brand) or ethylbenzylaniline (yellowish), trisulphonation of the product and oxidation. One sulphonic acid group enters each benzyl nucleus, the other enters the unsubstituted ring in the para position.

The Erioglaucines (G.) are dyes of closely similar constitution. Erioglaucine A (G.): [-]

Oxy Derivatives of Triphenylmethane. These furnish only a small number of commercial dyestuffs. The oldest is Aurin or Rosolic Acid (Yellow Corallin) discovered by Runge in 1834. It finds small use now, only for varnishes and in photography. It is obtained by heating ten parts of phenol with five parts of strong sulphuric acid and six parts oxalic acid at 120 130°C. The oxalic acid supplies the methane carbon atom and other condensations using CC14, or formaldehyde may be arranged to give rosolic acid. It is obtained pure by diazotising p-rosaniline and boiling.

Phenolphthalein is a well-known indicator giving bluish red coloration with alkalis, it finds some use also as a drug. The constitution of phenolphthalein has been elucidated with some difficulty and the subject has been closely identified with the discussion on the quinonoid theory of colour. The body is obtained by heating phthalic anhydride with zinc chloride or sulphuric acid as dehydrating agent.

It is dioxyphthalophenone, phthalophenone itself being obtained by condensation of phthalyl chloride with two molecules of benzene in presence of aluminium chloride. Although a triphenylmethane derivative, phenolphthalein is also a phthalein (see later), and these bodies are strongly coloured in alkaline solution, but are decolorised by the weakest acids, e.g., H2CO3. Reduction of phthaleins gives phthalines:

The lactone ring is also broken with salt formation when phthaleins are treated with alkali. Instant dehydration occurs however (compare Aurin) with formation of a quinone type, which is coloured.

Probably the free (OH) group in the above formula by replacement gives (ONa).

Excess of alkali decolorises phenolphthalein giving [-]

Neutralisation of the cooled solution with acetic acid gives quantitative removal of two Na atoms, the body still remaining colourless.

On heating or allowing to stand this solution again becomes coloured by dehydration and re-formation of quinone type.

Further evidence of the reality of this tautomerism has been found in the existence of two series of ethers of phenolphthalein, coloured and colourless according as they possess quinonoid or lactone structure.

Chrome Violet (G.) is the sodium salt of the tricarboxylic acid of Aurin, prepared by the action of formaldehyde on salicylic acid in strong sulphuric acid solution.

Other oxy-triphenylmethanes include the following acid mordant dyes, Eriochromazurol B (G.), Eriochromcyanine R (G.) and Chromazurol S (G.). These dyes, applied with chrome, are considerably used for commercial indigo-navy shades on wool fabrics. Eriochromcyanine R is obtained by condensation of benzaldehyde ortho-sulphonic acid with ortho-cresotinic acid.

A few naphthylphenylmethane dyesfruffs have attained commercial importance.

Victoria Blue B prepared by condensation of phenyl-α-naphthylamine with tetramethyldiamidobenzhydrol or tetramethyldiamidobenzophenone chloride.

Victoria Blue R is prepared similarly using ethylα-naphthylamine.

In spite of being of inferior fastness to light, the Victoria Blues are very considerably used, for the brightest blues. They retain their purity considerably when viewed in artificial light.

Night Blue is of the same class, being prepared from tetraethyldiamidobenzophenone chloride and ptolyl-α-naphthylamine.

Wool Green S (B.) is an acid dyestuff prepared by condensing β-naphthol with tetramethyldiamidobenzophenone chloride and sulphonating the product with fuming sulphuric acid.

Wool Green BS (By.) is obtained as above by use of G-salt instead of β-naphthol, and omitting also the final sulphonation.

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