Violanin. (Rhamnoglucoside.)(CHAPTER VIII. Pyran Group. Delphinidin (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.

Violanin, the colouring matter of the blue-black pansy (Viola tricolor), has been isolated and examined by Willstatter and Weil (Annalen, 1916, 412, 178), whose investigations are, however, as yet incomplete. The petals used contained as much as 24 per cent, of their dry weight of violanin, being the richest in colour content of all the flowers of the anthocyan pigments which have as yet been examined. In order to prepare it the fresh petals were employed. Although glacial acetic acid was first used for extraction, it was found that 2 per cent, methyl alcoholic hydrochloric acid gave the best results, both as to completeness of extraction and purity of the crude product obtained. The petals (100 gr.) were extracted with this medium (1 litre), filtered after standing two hours, the residue washed with a smaller quantity (100 c.c.) of the solvent, and the filtrate mixed with two and a half times its volume of ether, whereby the pigment was precipitated as chloride (4-5 gr.). By dissolving (10 gr.) this in 0.01 per cent, hydrochloric acid (250 c.c.), adding alcohol (100 c.c.), then 4 per cent, hydrochloric acid (250 c.c.), and allowing to stand for several days in covered vessels, the salt crystallised out, and when purified formed six-sided, or tetrahedral tablets of a blue-violet colour, having a greenish metallic lustre. It is stated that in the recrystallisation, acetone can, with advantage, replace the alcohol. Violanin has also been isolated by Everest (Proc. Royal Soc.,1918, B) from dried purple-black violas (Sutton's "Black Knight "), and evidence was obtained that a glucoside of myricetin is also present in these flowers.

Violanin chloride, C₂₇H₃₁O₁₆Cl?), as crystalline hydrate, loses about 15-16 per cent, of water (= 6H₂O) when dried in a vacuum desiccator, and a further 2-5 per cent, in high vacuum at 105°C. When completely dehydrated it loses hydrochloric acid, as is the case also with delphinidin and petunidin chlorides, and although Willstatter and Weil following their usual practice in such cases calculated their analytical figures to the chlorine-free base in order to eliminate this difficulty, their results were not in satisfactory agreement with the formula C₂₇H₃₀O₁₆, which corresponds to that of simple rhamnoglucoside of delphinidin. Moreover, although they were able to show that on hydrolysis violanin chloride yielded delphinidin chloride (1 molecule), rhamnose, and glucose, they were not able to obtain figures sufficiently in agreement with the presence of i molecule each of the sugars, to satisfactorily settle the composition of this pigment. In view of the low rhamnose content found when estimated by Tollens' method, the pigment was carefully fractionally extracted from an aqueous acid solution by amyl alcohol, to find whether there was a mixture of rhamnoglucoside and diglucoside present, but the results proved that the substance was homogeneous.

Solutions of violanin chloride are bluer than those of malvin chloride, and closely resemble those of delphinin chloride, but they show decolorisation with formation of pseudo-base, which does not occur with solutions of delphinin chloride. Its solutions in water are bluish-red; in alcohol violet-red.

This salt is very soluble in acidified methyl alcohol, only fairly so in ethyl alcohol; in water it is difficultly soluble; in 0.15 per cent, hydrochloric acid it is very easily soluble, in 0.5 per cent, still fairly so, but as the per cent, of HCl increases, the solubility decreases, till in 1 per cent. HCl it is only little soluble, and in 5-12 per cent, almost insoluble. Curiously enough in 20 per cent, it is again appreciably soluble, and is easily so in concentrated hydrochloric acid. In dilute sulphuric acid (0.05-7 per cent, and higher) it is practically insoluble, and a warm, saturated solution deposits the colouring matter completely on cooling.

An acid solution turns blue when sodium carbonate, or caustic soda, is added, and its solution in aqueous sodium phosphate (ordinary) is likewise blue. When ferric chloride is added to an ethyl alcoholic solution, it gives a blue coloration.

Violanin chloride, like the rhamnoglucoside anthocyans, keracyanin and prunicyanin, behaves, in regard to distribution between amyl alcohol and dilute acid, very much more like a normal monoglucoside than as a diglucoside, and it is stated to have a distribution number approximating to that of a normal monoglucoside. That this is not due to admixture of monoglucoside was shown by Willstatter and Weil by fractional extraction with amyl alcohol, and quantitative hydrolysis of the fractions of the pigment thus obtained.

Violanin picrate has been prepared by dissolving violanin chloride in ten times its weight of hot 0,01 per cent, hydrochloric acid, and adding to the solution finely divided picric acid. On cooling, a mass of fine cherry-red needles separates, which, when dry, forms a copper-red mass. The picrate is fairly soluble in water.