The Chemistry of Dyestuff. Dyestuffs. XVII. Stilbene, Pyrazolone and Thiazol 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)
Belonging to the azo class, or closely related to it, are the small groups of stilbene, pyrazolone and thiazol dyestuffs.

Stilbene Dyestuffs. This group comprises a number of yellow and orange dyestuffs dyeing cotton direct. On account of the methods of synthesis and constitution of these dyestuffs they are conveniently grouped together. They have the common property of giving shades fast to washing and alkali and often also to light and acids. The constitution of these bodies has only recently been fully elucidated. The methods used by Green in this task may be classed as (1) oxidation of the dyestuffs with permanganate to benzaldehyde sulphonic acids, (2) characterisation of oxidation products and (3) reduction of dyestuffs to diamido-stilbene disulphonic acid with titanous chloride.

In 1883 Walther obtained Sun Yellow (G.), also known as Direct Yellow R, Curcumine S and by many other names, by action of hot caustic soda solution on p-nitrotoluene-o-sulphonic acid. By oxidation of this dyestuff with hypochlorite a more greenish yellow Mikado Yellow (L.) is obtained, while alkaline reduction gives Mikado Orange (L.) and further reduction a leuco compound (obtainable from any of the abovementioned dyestuffs) which reoxidises in air to orange. Still further reduction gives diamidostilbene disulphonic acid. Stilbene Yellows 8G and G (B.) and Stilbene Orange 4R (Cl.) are obtained by alkaline reduction, e.g., with glucose and caustic soda, of dinitrostilbene disulphonic acid, during which successive reduction and condensation give the above dyestuffs, and subsequently further reduction gives a leuco orange, and ultimately diamidostilbene disulphonic acid. Of the identity of the corresponding Mikado (L.) dyestuffs and these Stilbene dyestuffs there is now no doubt.

The alkaline condensation of p-nitro toluene sulphonic acid is largely influenced by the ortho electro negative substituent, and takes place in the following stages: [-]

Oxidation of this red intermediate compound gives dinitrodi benzyl disulphonic acid, from which Mikado Yellow (L.) may be obtained by heating with caustic soda solution. Rapid condensation, however, gives a blue intermediate body thus: [-]

Oxidation of this body readily gives dinitrostilbene disulphonic acid. In the ordinary preparation of Direct Yellow R or G (K.), the presence of aldehydic bodies formed simultaneously causes reduction, and by removal of an atom of oxygen two molecules of the dinitroso compound condense to give the dyestuff [-]

This body on oxidation gives Mikado Yellow, identical with the Stilbene Yellow 8G obtained from dinitrostilbene disulphonic acid by alkaline reduction: [-]

Further reduction and condensation of the latter body give by a reverse change Stilbene Yellow G or Direct Yellow R, This azo-azoxy dyestuff by reduction gives Mikado Orange or Stilbene Orange 4R (CJ.), a disazo compound in which by further [-] reduction the azo groups become converted into hydrazo groups NH NH , thus giving the leuco body which can reoxidise to orange in the air, while further reduction, as is usual with azo dyes, causes a dissolution of the azo groups to form amido groups, giving thus two molecules of colourless diamidostilbene disulphonic acid.

Another dyestuff of this series is Diphenylcitronine G (G.), obtained by alkaline condensation of dinitrodibenzyl disulphonic acid or dinitrostilbene disulphonic acid with two molecules of aniline, giving a disazo compound of the type N N.C6H6 Similarl} 7 , condensation of the intermediate dinitrosostilbene disulphonic acid above mentioned with p-phenylene diamine gives Polychromine B (G.), a brown cotton dyestuff.

Curcuphenine (CL), Chlorophenine and Diphenyl Fast Yellow (G.) are yellow stilbene dyestuffs obtained by condensations with dehydrothiotoluidine sulphonic acid.

Pyrazolone Dyestuffs. This group of dyestuffs, each of which contains the pyrazolone ring, have as [-] chromophore the azo group N=N.They belong to various dyeing classes, but are mostly yellows of good fastness to light, and are rather expensive products compared with many azo dyes.

The first to be discovered was Tartrazine, by Ziegler in 1884. It was obtained by the action of two molecules phenylhydrazine p-sulphonic acid on dioxytartaric acid in warm solution. Two molecules of water are first split off, giving a diphenyl-hydrazone thus: [-]

A further splitting off of one molecule of water, thus: [-]

results in the formation of a pyrazolone dyestuff [-]

An apparently more economical synthesis, involving the use of only one molecule of the expensive phenylhydrazine sulphonic acid, has since been accomplished. Oxal-acetic ester, condensed with phenylhydrazine sulphonic acid, gives a pyrazolone derivative with splitting off of water and alcohol, thus: [-]

The second stage, i.e., the splitting off of alcohol, takes place easily, especially on heating. The sodium salt of the body so formed, treated with diazo sulphanilic acid or its anhydride C6H4.N2.SO3 , gives an ester of Tartrazine, which is converted into the dyestuff itself by alkaline saponification.


The azo dyestuff represented by this formula is apparently identical with the dyestuff as previously represented by a hydrazone structure. No conclusive evidence has been obtained which justifies the exclusion of either of these formulae. The case in point is an apt illustration of the inadequacy of simple benzcnoid formulae for representation of dyestuffs without the assumption of tautomeric isomerism, which is part of the quinonoid theory, previously discussed.

Other pyrazolone dyestuffs are obtained by coupling diazo compounds with pyrazolone derivatives. For example, two direct cotton dyes, namely Dianil Yellow R (M.) from Primuline and l-phenyl-3-methyl-5-pyrazolone, and Dianil Yellow 2R (M.), by using instead of the latter body its para-sulphonic acid. In each case azo compounds are formed by coupling in position four of the pyrazolone ring.

Eriochrome Red B (G.) is an acid mordant dye obtained by coupling l-amido-2-naphthol-4-sulphonic acid with phenylmethylpyrazolone.

Flavazine L & S (M.), Fast Light Yellow G, 2G & 3G (By.), Hydrazine Yellow SO (G.E.), Xylene Yellow 3G, and Xylene Light Yellow 2G & R (S.), are acid dyes fast to light, and are all obtained from simple diazobenzene derivatives and sulphophenylpyrazolones.

Thiazol dyestuffs. These dyestuffs have as chromophore the thiazol ring most simply illustrated in benzothiazol, itself colourless.


With the introduction of auxochrome groups into thiazols a yellow colour is obtained, as in dehydrothiotoluidine.

The thiazol dyestuffs are made by methods similar to that of Green, who discovered Primuline in 1887, i.e., by heating p-toluidine or its homologues with sulphur. According to the amount of sulphur used, the temperature and duration, different mixtures of thiazol bases are produced. Thus dehydrothiotoluidine is obtained in largest quantity by heating two molecules of base with four atoms of sulphur up to 200°C.


A considerable amount of primuline base is formed at the same time. The two bases can be separated by the difference of their solubility in alcohol, but they are used commercially as sulphonic acids, obtained by treatment of the powdered mixed bases with strongly fuming sulphuric acid, stirring well and cooling with cold water>pipes and jacket. When complete, the acid mixture is run into water, and the sulphonic acid which precipitates from the acid solution is filtered off and washed. By treatment with strong ammonia, e.g., while still in the filter press, the ammonium salt of dehydrothiotoluidine sulphonic acid is not much dissolved, while that of Primuline readily dissolves. By adding salt to this solution, the commercial dye can be salted out as sodium salt.

In the manufacture of Primuline the melt is made in an iron pot with a long iron pipe sloping somewhat to the horizontal, this acting as air condenser for the toluidine. The sulphuretted hydrogen evolved may be piped back and burnt under the pot, the heat thus produced being sufficient to carry the reaction through once it is started. The temperature may be taken as high as 280°C. When the production of sulphuretted hydrogen ceases, the melt is run out, or blown out through a syphon tube with compressed air on a stone flag, where it sets to a hard yellowish brown mass.

Primuline contains both a diand tri-thiazol derivative:

In sulphonating, the acid group enters the ring containing the NH2 group.

Primuline dyes cotton a pure yellow shade, which is not fast to light. However, Green introduced the method of diazotisation on the fibre, and development with phenols or amines giving azo dyes of valuable properties, e.g., with β-naphthol, Primuline Red.

Thioflavine S (C.) is a methylated Primuline, and the replacement of NH2 by N(CH3) 2 renders the product less sensitive, although fastness to light still remains poor.

Thioflavine T (C.), also known as Methylene Yellow H (M.), Rhoduline Yellow T (By.), etc., is obtained from dehydrothiotoluidine by heating in an autoclave to 170°C. with methyl alcohol and sulphuric or hydrochloric acid. A basic ammonium substitution product is thereby formed, put out in commercial form as hydrochloride.


Thioflavine T is a pure greenish yellow, and is one of the most used basic yellows.

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