Idaein.(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.

Idaein, the colouring matter of the cranberry (Vaccinium vitis idaea, Linn.), the berries of which contain about 0.035 per cent, of their weight of the pigment, has been subjected to examination by Willstatter and Mallison (Annalen, 1915, 408, 15), who succeeded in isolating it in a finely crystalline condition.

For the preparation of the pure colouring matter the skins of the berries only were made use of, as the major portion of the pigment occurs in them. 138 kg. of berries gave 34 kg. of skins (containing about 76 per cent, of water), and these were worked up in lots of about 11 kg. at a time, such a quantity being left with 17 litres of glacial acetic acid in a closed vessel for eight days in order to extract the pigment, and, after filtration, a further 8 litres was used to wash the residue. The filtrate (25 litres) was divided into portions and the pigment precipitated by the addition of ether (two and a half times the volume) and shaking for twenty minutes, after which the upper layer was decanted from the deep red syrup. This was dissolved in warm glacial acetic acid (2 litres) and the solution diluted with a further quantity of the acid (2 litres); after filtration, the solution thus obtained was fractionally precipitated by means of ether, the first oily precipitate being neglected, and the second fraction, which contains the bulk of the colouring matter, collected, washed with ether, and converted into picrate by dissolving in water (300 c.c.) and mixing the solution with a solution of picric acid (20 gr.) in warm water. The first flocculent precipitate that appeared whilst still hot was rapidly filtered off, and the solution on cooling then deposited a mixture of crystals of the colour picrate and potassium picrate. For preparation of the pure chloride the picrate was not further purified, but if the pure picrate was desired, the above product was dried, dissolved in warm methyl alcohol, in which potassium picrate is not readily soluble, and the solution precipitated by ether. It was then further purified by recrystallisation from water. For preparation of the chloride the impure picrate was dissolved in boiling methyl alcohol (150 c.c.) and such an excess of methyl alcoholic hydrochloric acid (30 c.c., 17 per cent. HCl) added that about 2 per cent, free hydrochloric acid remained in the solution, and the chloride then precipitated by addition of ether (1 litre), the precipitate being washed free from picric acid by means of ether. 10.7 kg. of skins yielded 1.6 gr. of idaein chloride, i.e. about 17 per cent, of the total pigment present in the original acetic acid extract. The product was recrystallised by dissolving in water (0.5 gr. in 50 c.c.), adding concentrated hydrochloric acid (10 c.c.), filtering off the small amount of flocculent precipitate, then adding ethyl alcohol (20 c.c.) and allowing the alcohol to evaporate off slowly, when the chloride separates in fine leaflets with a metallic lustre. A further recrystallisation was found to be advantageous.

Idaein chloride, C₂₁H₂₁O₁₁Cl, forms a crystalline hydrate having the composition C₂₁H₂₁O₁₁Cl, 2½H₂O, which is described as forming brown-red monoclinic prisms with a green metallic lustre. It loses all its water of crystallisation when dried in a vacuum desiccator at ordinary temperature, melts with frothing at 210°C, and differs considerably from cyanin in its solubility.

In water it is very easily soluble - 1 in 10 at 15°C. - yielding a deep brown-red solution which on dilution passes to orange-red - very dilute solutions show decolorisation, but do not appear to become completely decolorised, a pale rose colour remaining. In alcohol the salt is readily soluble producing a bluish-red solution; in hydrochloric acid of very low concentration (e.g. ½ per cent.) it is easily soluble, but in 6 per cent, it is almost insoluble; in sulphuric acid it is more soluble than in hydrochloric acid, 7 per cent, acid dissolves 0.03 gr. in 100 c.c. at 25°C., but warm solutions on cooling deposit crystals of the sulphate.

The following reactions have been recorded by Willstatter and Mallison. With sodium carbonate an acid solution of the salt passes to violet which is stable in excess of the reagent, whilst with caustic soda a blue colour is produced that rapidly passes to green, then to yellow; ferric chloride added to an alcoholic solution gives a blue coloration which on dilution passes to violet; alum produces a very stable violet colour when added to an aqueous solution, whilst bismuth nitrate gives a red-violet coloration. Zinc (or copper) acetate on addition to an alcoholic solution produces a blue coloration, whilst to an aqueous solution the addition of copper (or lead) acetate gives a red- violet precipitate; lead acetate added to a solution in alcohol gives a blue precipitate. Zinc and hydrochloric acid (or sodium hydrosulphite) decolorise a solution of the pigment, and treatment of the resulting solution with air, or hydrogen peroxide, causes the return of the colour.

Idaein chloride has a distribution number about 10, i.e. behaves as a normal monosaccharide anthocyan, and on hydrolysis it yields cyanidin chloride (1 molecule) and galactose (1 molecule). It will be noticed that in respect of the distribution number of saccharide pigments of this series containing it, galactose appears to resemble glucose in its effect, and differs from rhamnose.

Willstatter and Mallison describe the absorption spectrum as consisting of one broad band resembling that of cyanin, but reaching less far into the blue. They obtained the annexed results. Solution containing i molecule in 2500 litres of 7 per cent, sulphuric acid,
Thickness of layer 2-5 mm.... 556... 542-476
Thickness of layer 1.25 mm.... 546... 530-484

Idaein chloride is optically active, and the following measurements are on record:
Dilute solution in 0.5 per cent. HCl, monochromatic light
[a]_c = - 219° (±15°);
more concentrated solution, white light (1000 c.p. Osram globe 23 cms. away)
l = 0.5 [a] = - 378° (±5°); l = 1.0 [a] = - 342° (±5°).

Idaein picrate forms microscopic red needles. It is soluble in 30 parts of boiling water, very insoluble in cold water, the solutions being orange-red; it is soluble in ethyl alcohol yielding cherry-red solutions very dilute solutions in either solvent show loss of colour with formation of pseudo-base, unless excess of picric acid is present. If a drop of the aqueous solution be placed on filter paper it produces a spot with violet centre and yellow exterior, as the result of hydrolytic dissociation.

Idaein sulphate is described as forming bright brown rhombic crystals when a solution of the chloride in warm sulphuric acid (7 per cent.) is allowed to cool.