Chrysanthemin.(CHAPTER VIII. Pyran Group. Cyanidin and its derivates.)

The Natural Organic Colouring Matters
By
Arthur George Perkin, F.R.S., F.R.S.E., F.I.C., professor of colour chemistry and dyeing in the University of Leeds
and
Arthur Ernest Everest, D.Sc., Ph.D., F.I.C., of the Wilton Research Laboratories; Late head of the Department of Coal-tar Colour Chemistry; Technical College, Huddersfield
Longmans, Green and Co.
39 Paternoster Row, London
Fourth Avenue & 30th Street, New York
Bombay, Calcutta, and Madras
1918

Kaikki kuvat (kemialliset kaavat) puuttuvat // None of the illustrations (of chemical formulas) included.

Chrysanthemin, the pigment of the deep-red chrysanthemum (Chrysanthemun indicum, Linn.), has been isolated and described by Willstatter and Bolton (Annalen, 1916, 412, 136), who used deepred flowers for their investigations (Ruby King), but also found that many scarlet and red garden kinds contained this pigment mixed with various carotinoids.

The pigment closely resembles Idaein (the cranberry pigment) in many of its properties, but the differences in solubility are staled to be sufficient to characterise it.

For the isolation of the colouring matter the petals (those used contained about 7 per cent, of their dry weight of chrysanthemin, other red sorts examined contained amounts varying from one-half to one-third only of this) were extracted with glacial acetic acid (fifteen times the weight of the petals) for three days and the residue on the filter washed with fresh glacial acetic acid (equal to weight of petals); under these conditions the extraction is said to be almost quantitative. On addition of ether (twice the volume) to the filtered extract a heavy red flocculent precipitate of the colouring matter as acetate is produced. (In this case, as in some few others, it is said, to be advantageous to precipitate the pigment as acetate rather then as chloride.)

The purification of the crude chrysanthemin thus prepared was carried out in either of two ways, one making use of the picrate and the other of the distribution between amyl alcohol and aqueous acid. The latter method is more or less generally applicable to monoglucosides of the series on account of their passing to a reasonable extent into the alcoholic layer, but chrysanthemin having a high distribution number for a monoglucoside may even be more advantageously purified by this method than other pigments of the group. On the other hand, even in this case, the very large bulk of solvents required to work up even small quantities of pigment make the method, though of great interest, useless for the purification of the colouring matter in quantity.

For the picrate process the crude acetate (from 2½ kg. of petals) was dissolved in 0.5 per cent, hydrochloric acid (400 c.c.) and twice extracted with amyl alcohol (200 c.c. each time) to remove any free cyanidin that may be present, and the residual traces of amyl alcohol were removed from the aqueous layer by shaking with ether. After this preliminary treatment the aqueous solution of the pigment was mixed with cold saturated picric acid solution (500 c.c.) and allowed to stand for a day when the crude picrate had separated as spherical aggregates (18 gr.). It was then dissolved in methyl alcohol (225 c.c.), filtered, and the solution mixed with 10 per cent, methyl alcoholic hydrochloric acid (40 c.c.), after which ether (ten times the volume) was added to reprecipitate the colouring matter which separates as a brown-red powder (9.7 gr. of 64 per cent, purity). After thrice purifying this product by slow precipitation from alcoholic solution by addition of aqueous hydrochloric acid (7 per cent.), the pigment was finally obtained as chloride in the form of fine microcrystals (prisms and long tablets, 4.5 gr.).

The purification by means of amyl alcohol consists in dissolving the crude product in 0.5 per cent, hydrochloric acid, extracting twice with half the volume of amyl alcohol, to remove any cyanidin, and rejecting these alcoholic extracts, then extracting further twenty times (each with one volume of amyl alcohol), partially drying the alcoholic extract and precipitating the pigment from it by means of light petroleum, mixing the syrup so obtained with methyl alcohol and completing the precipitation by means of ether. The further purification of the chloride thus obtained was similar to that obtained by the picrate process.

Chrysanthemin chloride, C₂₁H₂₁O₁₁Cl, forms a crystalline hydrate having the composition C₂₁H₂₁O₁₁Cl, 1½H₂O, which loses its water if dried in high vacuum at 100°C.; the crystals are red-violet pointed rhombic leaflets which have a fine metallic sheen. In a melting-point tube they begin to decompose at 205°C. and blacken, but show no melting-point.

Its acid aqueous, or alcoholic, solutions have the same colour as corresponding solutions of cyanin, idaein, or asterin, but their intensity is about twice that of cyanin solutions.

In water the salt is easily soluble, in methyl alcohol very easily, but in ethyl alcohol rather difficultly soluble, and but little more so when this is hot. In dilute hydrochloric acid it is much less soluble than idaein chloride, as the following figures given by Willstatter and Bolton show: at 17°C. 100 c.c. ½ per cent. HCl dissolves 0.0845 gr. chrysanthemin chloride, but dissolves 8.48 gr. idaein chloride; at 17°C. 100 c.c. 2 per cent. HCl dissolves 0-0604 gr. chrysanthemin chloride, but dissolves 0.7 gr. idaein chloride, whereas in hot ½ per cent, hydrochloric acid chrysanthemin chloride is very easily soluble. It is interesting to note that this salt readily forms super- saturated solutions, and if a concentrated aqueous solution is mixed with an equal volume of 4 per cent, hydrochloric acid, it may stand for weeks before any precipitate of pigment is formed.

In 7 per cent, sulphuric acid it is very easily soluble, much more readily than idaein chloride or cyanin chloride.

The following reactions have been recorded. On addition of sodium carbonate to an acid solution of the chloride a violet colour is produced, whereas caustic soda produces a pure blue; ferric chloride on addition to an alcoholic solution gives a blue colour which on dilution with water passes to violet; calcium chloride causes the precipitation of a blue calcium salt, whilst the lead salt if precipitated from alcoholic solution of the pigment is blue, and if from aqueous solution violet.

The distribution number of this salt is 19, and is therefore high as compared with the majority of monosaccharide anthocyans.

On hydrolysis chrysanthemin chloride yields cyanidin chloride (1 molecule) and glucose (1 molecule).

Chrysanthemin picrate was prepared by adding 10 c.c. of a saturated solution of picric acid to 20 c.c. of a solution of chrysanthemin chloride (0.3 gr.) and separated freely as thin red prisms which sinter at 155°C, and melt with decomposition at 165°C. The salt is very easily soluble in alcohol, but rather difficultly so in water; its dilute solutions are said to become decolorised even more readily than those of the chloride.