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.
Glucosides of cyanidin occur in a variety of flowers and fruits. It was first isolated by Willstatter and Everest, from the corn-flower (Centaurea cyanus), and was the first sugar-free anthocyan to be obtained in a state of purity; it has since been obtained from the petals of Rosa gallica, peony, chrysanthemum (Chrysanthemum indicum, Linn.), dahlia (deep-red), aster (Callistephus chinensin, Nees, syn. Aster chinensis, Linn.), poppy (Papaver rhoeas, Linn.), and from the fruit of the cherry (Prunus avium, Linn.), cranberry (Vaccinum vitis idaea), and sloe (Prunus spinosa, Linn.), whilst, as the result of qualitative tests, Willstatter and Bolton (Annalen, 1916, 412, 138) consider that cyanidin derivatives are present in the flowers of Zinnia degans, Jacq., Hybrid Gladiolas, Gaillardia bicolor, Hook., Helenium autumnale, Linn., Tulipa gesneriana, Linn., Tropceolum majus, Linn., and the berries of the red currant (Ribes rubrum, Linn.), raspberry, and mountain ash. In some of these cases they consider that the cyanidin derivative is accompanied by compounds derived from pelargonidin.
Cyanidin.
Cyanidin is most readily isolated in the form of its chloride which has the composition C15H11O6Cl, and the structure [KUVA PUUTTUU] and which consists when in the crystalline form of the hydrate C15H11O6Cl, H2O, from which the water of crystallisation is not lost by drying in air, in a vacuum desiccator, or in high vacuum at 50°C., but is completely given up in high vacuum at 105°C. Both the colour base and the pseudo-base of cyanidin have been prepared in crystalline condition. The anhydrous chloride does not melt at 300°C., whereas th#t dried at 50°C. in high vacuum melts at once if dipped in a bath at 220°C. If put into a bath at 200°C., and the temperature is then raised, it does not melt.
Cyanidin chloride is readily prepared from any of the naturally occurring glucosides by hydrolysis with hydrochloric acid, or from the glucoside (peonin) or its methyl ether (peonidin) by hydrolysis and demethylation with hydriodic acid. Hydrolysis of the glucosides is best carried out by boiling with 20 per cent, hydrochloric acid for three minutes, when the product separates in long red-brown needles which have a metallic lustre, the separation being almost complete on cooling. The salt may be recrystallised by solution in ethyl alcohol, addition of half its volume of 7 per cent, hydrochloric acid (aqueous) and slow evaporation of the alcohol. When powdered it has a brown-red colour and marks paper a deep violet, and is very soluble in methyl or ethyl alcohol, giving fine violet-red solutions. When an alcoholic solution is mixed with double its volume of water and warmed to about 85°C., the colour gradually changes till finally only a pale brown-yellow remains. This, which is due to pseudobase formation, only takes place very slowly in the cold; addition of acid to the decolorised solution does not reproduce the colour immediately, but on warming it returns quantitatively.
In dilute hydrochloric acid the salt is very difficultly soluble, even hot, and a solution in 20 per cent. HCl contains only o-ooi per cent, of the pigment; it is fairly soluble in 7 per cent, sulphuric acid, and on cooling crystals of the sulphate separate. With perchloric acid a well-crystallised salt is formed which is very soluble in ethyl alcohol and separates on evaporation of the solvent in large deep-red prisms.
This colouring matter, like other anthocyanidins, is quantitatively extracted from an aqueous acid solution by shaking with amyl alcohol. The amyl alcoholic extract when shaken with sodium acetate becomes violet, whilst if shaken with aqueous sodium carbonate the colour changes to blue and passes to the aqueous layer. The fine red solution of the pigment in moist amyl alcohol is not very stable, becoming yellow-brown when left for a few days.
An aqueous, or dilute alcoholic acid solution of the chloride on addition of sodium carbonate becomes violet, then blue if any pseudo-base is present it changes to green; addition of potassium acetate to an alcoholic solution produces a violet-blue precipitate; lead acetate gives an amorphous blue precipitate. Ferric chloride with an alcoholic solution of the pigment gives a pure blue, whilst if added to an aqueous solution it produces a violet coloration; excess of the reagent causes rapid oxidation and the colour changes to yellow. When the chloride is boiled with hydriodic acid it passes into solution and on cooling the iodide crystallises out. Zeisel estimations have shown the absence of methoxy groups in the compound.
Willstatter and Everest found that careful oxidation with hydrogen- peroxide converted cyanidin chloride into a yellow crystalline compound that had properties resembling those of a flavonol. When decomposed by means of melted alkali, cyanidin chloride yields phloroglurinol and protocatechuic acid.
The absorption spectrum was examined by Willstatter and Everest, who described it as consisting of one broad band reaching from the yellow to blue, the edges being ill-defined. The following data were given by them:
1 mol. in 2500 litres
layer 2.5 mm. | 577 - 520... 507
layer 5.0 mm. | 583-486
layer 10.0 mm. | 589-474
Cyanidin Colour Base.
This was obtained as a flocculent precipitate by Willstatter and Everest, and later in a crystalline condition by Willstatter and Nolan (Annalen, 1915, 408, 13), by mixing a hot concentrated solution of cyanidin chloride in alcohol with twice its volume of water. When finely powdered the violet base thus obtained is fairly soluble in alcohol and more so in pyridine, but its solution in the latter becomes rapidly decolorised.
Cyanidin Pseudo-Base C15H12O7.
This substance can be obtained in the form of its crystalline hydrate C15H12O7, H2O. 0.5 gr. cyanidin chloride is dissolved in 820 c.c. of alcohol and twice the volume of water added. The liquid becomes violet-red, but practically no precipitate of the colour base is formed, and when heated gently for half an hour the colour changes to pale yellow-red, after which the alcohol is rapidly distilled off, in vacuo, on a water-bath, the solution saturated with ammonium sulphate and extracted with ether. The ethereal solution of the base is decolorised with animal charcoal and concentrated, after which some 0.35 gr. of the base separates as colourless lens-shaped crystals. The same form is said to be obtained by precipitation of a solution of the base in glacial acetic acid by means of benzol.
The base is readily soluble in water, more so when warm than when cold, but is not easily crystallised from water, as when a concentrated solution is allowed to stand some of the violet colour base is produced. It is easily soluble in alcohol, acetone, or glacial acetic acid, but insoluble in benzol. Sodium carbonate dissolves it with a yellow coloration, whilst treatment of the base with hot or cold 20 per cent, hydrochloric acid slowly produces crystalline cyanidin chloride.
The water of crystallisation in the crystalline hydrate is very difficult to remove; it is not affected by drying in a vacuum desiccator and only commences to come off in high vacuum at about 80°C., dehydration not being complete below 130°C. At this temperature conversion of the pseudo-base to the violet colour base has commenced and the dry product is deep violet in colour.
Derivatives of Cyanidin.
- 1. Methyl ether, and its glucoside, see Peonidin.
- 2. Monosaccharides: Asterin, Chrysanthemin, and Idaein.
- 3. Disaccharides
- (a) Diglucosides: Cyanin and Mekocyanin.
- (b) Rhamno-glucosides: Keracyanin and Prunicyanin.
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