The Chemistry of Dyestuff. Dyestuffs. XIX. Xanthene 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 name of xanthene is applied to the anhydride of o-dioxy-diphenylmethane.

By oxidation it gives xanthone which by reduction gives a xanthhydrol which forms xanthonium salts with acids, the constitution of which demands the assumption of tetravalent oxygen (oxonium theory).

The xanthene dyestuffs offer great analogy with the di- and triphenylmethanes of which they are really derivatives. Thus the general method of preparation is to first prepare an ortho dioxy-p-disubstituted dior triphenylmethane, to dehydrate forming the oxygen bridge, and to oxidise the leuco compound to give the dyestuff.

Fluorescein or Uranin, a phthalein, is obtained by heating resorcin with phthalic anhydride and conversion of the product into the sodium salt. The condensation takes place in two stages. First:

This body then by dehydration gives a xanthene:
which is colourless until treated with alkali when it forms a yellow dye soluble in water, showing a strong green fluorescence. The coloured sodium salt is represented by a quiuonoid formula, namely

The effect of alkalis on the lactone ring is similar to that in the case of phenolphthalein, and it has also been shown by Nietzki that isomeric coloured and colourless ethers exist. The dyestuff finds practically no use as such, but is important for preparation of the Eosines.

With the xanthene and phenylxanthene dyestuffs some doubt exists as to whether ortho or para quinone structure is more suitable. The oxonium structure finds growing favour.

The xanthenes may be subdivided into pyrones or pyronines and phthaleins. Closely allied to the latter are the small groups of succineins and anthraphthaleins.

Pyronine Dyestuffs. A small class possessing the pyrone ring.

They are red basic dyestuffs of minor importance obtained by formaldehyde condensation of substituted metamidophenols, e.g., dimethyl-m-amidophenol gives
which on heating with sulphuric acid is dehydrated, giving a leuco compound
which by oxidation gives Pyronine G

Acridine Red B, 2B, 3B (L.) is obtained by oxidation of this body with permanganate when each -N(CH3)2 becomes oxidised to NH(CH3).

Succinein Dyestuffs. Rhodamine S is the only important dye of this class; it is obtained by heating dimethyl-m-amidophenol with succinic anhydride.

A para quinone formula may also be written.

Phthalein Dyestuffs. - The amidophthaleins or Rhodamines are bluish red basic dyes, mostly exhibiting fluorescence. They also have faintly acid properties in virtue of the possession of the phthalein carboxyl group, e.g., they do not precipitate so readily with tannin as most basic dyes. Acid dyes of this series are obtained by sulphonation.

The dyes of this class are derivatives of fluorane.

Rhodamine B (B. and other makers) is obtained by condensation of phthalic anhydride with diethyl-mamidophenol on heating.

Rhodamine G is obtained from the above dyestuff" by heating with aniline hydrochloride when one ethyl group is lost, and the shade becomes less bluish.

Rhodamine 3B is obtained from Rhodamine B by ester ification with alcohol and HCl or by acting on the base with ethyl chloride. The shade is bluer than that of the B brand, and the fastness is increased by the aid to stability afforded by ester formation, whicli prevents ready return to the colourless lactone type. These ethylated Rhodamines are also known as Anisolines (Mon.) and are more basic than Rhodamines, having the property also of dyeing cotton direct.

Fast Acid Violet B (M.) (Violamine B) is obtained by the action of aniline or other primary aromatic amine on the di-chloride of Fluorescein. The reaction product is then sulphonated, giving

When o-toluidine is used similarly, Fast Acid Violet A2R (M.) (Violamine R) is obtained.

Fast Acid Blue R (M.) is obtained when p-phenetidine replaces aniline above.

Fast Acid Eosine G (M.) is obtained by sulphonation of Rhodamine.

Intermediate between the amidoand oxy-phthaleins come amido oxy-phthaleins, partaking to some extent both of the character of Rhodamine and Fluorescein, being obtained by a combination of the methods of synthesis for these dyestuffs. These have been called Rhodoles but do not merit discussion here.

The oxy-phthaleins are derivatives of Fluorescein already described.

Chrysolin is obtained by condensation of resorcin, phthalic anhydride and benzyl chloride in presence of sulphuric acid.

It is brown in solution with a green fluorescence. Halogenation of Fluorescein gives Eosines, red dyestuffs of greater fastness, being very fast to alkalis but only moderately to light. Fluorescence is still exhibited to a smaller degree.

The Eosines are weakly acid dyes and are applied from weakly acid baths to wooj and silk.

Bromination of Fluorescein in aqueous or alcoholic solution and conversion of the tetra-brom-fluorescein into an alkali salt, gives Eosine or Eosine G (B.) (many other brands). In aqueous solution bromination is done with hypobromite.

(A para quinone formula may also be written.)

The constitution of Eosine has been established from the following evidence. On heating with caustic soda it is broken down, giving dibroinresorcin and dibromdioxybenzoyl-benzoic acid.


The free benzoic acid derivative by dehydration, e.g., with sulphuric acid, gives dibrom-xanthopurpurin according to a general reaction for preparation of anthraquinone derivatives.


Further, if the free benzoic acid compound be heated, the original Eosine is formed along with phthalic acid. The original Eosine is also re-formed when dibromresorcin is heated with phthalic acid. The constitution of Eosine is therefore established as a symmetrically brominated body shown in the above ortho-quinonoid formula or in the following p-quinone structure.

Methylation of Eosine gives Eosine spirit soluble, i.e., instead of C6H4-CO2K above, -C6H4-CO2-CH8. It is also somewhat soluble in water as is Eosine S (B.), a similar product obtained by ethylation with alcohol and sulphuric acid.

Eosine BN (B.) is obtained by nitration of dibromfluorescein. This dye is one of the fastest of its class to light and milling. It gives bluish red shades and has only a weak fluorescence.

By introducing iodine into Fluorescein, homologous dyestuffs of a more bluish character than Eosine are obtained, i.e., Erythrosines. The preparation is similar to that of Eosine.

Di-iodo-fluorescein is known as Erythrosine extra yellowish (B.) and tetra-iodo-fluorescein as the extra bluish brand.

By use of dichlor-phthalic acid instead of phthalic acid in manufacture of Fluorescein, a dichlor-fluorescein is obtained, which on bromination gives Phloxine.

Rose Bengale is the tetra-iodo derivative of the same dichlor-fluorescein.

Other brands of Phloxine and Rose Bengale are obtained from tetra-chlor-fluorescein derived from tetrachlor-phthalic acid.

The Cyanosines are spirit soluble bodies used in lake manufacture and are obtained by alkylation of Phloxines (compare Eosine spirit soluble).

The only important mordant dye of the phthalein class is Gallein or Alizarin Violet. It is obtained by heating phthalic anhydride with gallic acid or pyrogallol; in either case the product is the same since C0 2 is split off from gallic acid during the heating at 190°-200°C.

By heating Gallein with sulphuric acid to 200°C. a further molecule of water is split off giving Coerulein (B.), a green mordant dye put out as a paste, also as a water-soluble powder in the form of bisulphite compound C20H10O6 + NaHSO3, Coerulein S.

It will be seen that Coerulein is a derivative of anthraquinoue, and it has recently been found to have vat-dyeing properties. Its commercial importance however is dependent on its use as a mordant dye.

Coerulein B, BR, BW, BWR (M.) are obtained by heating Fluorescein with excess sulphuric acid.

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