Peonin.(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.

Peonin, the pigment of the deep violet-red peony, in which it occurs to the extent of 3-3½ per cent, of the dry weight of the petals, has been investigated by Willstatter and Nolan (Annalen, 1915, 408, 136), who obtained it, as chloride, in a beautifully crystalline condition.

For the isolation of the colouring matter the commercial, powdered, dry petals were mixed with 2 per cent, methyl alcoholic hydrochloric acid (2 litres for 1 kg. petals), the solution after standing for two hours being filtered and the residue on the filter washed with per cent, methyl alcoholic acid (1½ litres). To purify it, the syrup thus obtained was mixed with methyl alcoholic hydrochloric acid (300 c.c. containing 2.2 gr. HCl), stirred, and glacial acetic acid added (210 c.c.), the mixture being then allowed to stand for sixty hours, when the syrup had been converted into a powder. This was washed with 2 per cent, methyl alcoholic hydrochloric acid, then ethyl alcohol (containing a very small per cent. HCl), and dried. Many of the impurities present in the crude pigment were thereby rendered soluble, either by hydrolysis or acetylation, and were thus removed from the more insoluble colouring matter. Trie powder was further purified by repeated boiling (10 mins.) with 2 per cent, methyl alcoholic hydrochloric acid (twice with 150 c.c., followed by twice with 100 c.c.). Although some of the pigment passed into the filtrates, from which a portion only could be recovered, the bulk remained undissolved, and this was dissolved in boiling N/2HCl (800 c.c.), filtered, and the solution allowed to stand, when beautiful brown-red needles separated (6 gr.).

Peonin chloride, C₂₈H₃₃O₁₆Cl, readily crystallises from dilute hydrochloric acid (N/2) in the form of glistening brown-red needles (red-violet under microscope) which have the formula
C₂₈H₃₃O₁₆Cl, 5H₂
It melts, with decomposition, at 165°C. The hydrate loses most of its water of crystallisation in a vacuum desiccator at room temperature, and all when dried in high vacuum at 100°C.

The salt is very easily soluble in cold water, and the aqueous solutions very readily decolorise on account of pseudo-base formation; it is somewhat less soluble in absolute alcohol, the solutions in which also show decolorisation; it is insoluble in acetone. In cold dilute hydrochloric acid it is difficultly soluble 1 litre of 1 per cent. HCl dissolves 0,11 gr. at 20°C. whilst it is insoluble in cold, but easily soluble in hot 2N/HCl. In 7 per cent, sulphuric acid it is less soluble than is cyanin chloride, and a solution of 7 mg. in 25 c.c. rapidly deposits the greater part of the pigment in a crystalline form.

Peonin chloride closely resembles cyanin chloride in many of its reactions; pseudo-base formation proceeds with the same velocity in each case; sodium carbonate produces a pure blue colour when added to an acid solution; sodium acetate forms a fine violet, whilst caustic soda gives a blue that changes to green on standing. The ferric chloride reaction of peonin chloride is however quite different from that given by cyanin chloride, as but little colour change occurs when the reagent is added to an alcoholic solution of peonin chloride, though on addition of water a violet colour appears. The two colouring matters differ also in their solubility in water and in alcohol, and in the colour of their solutions in alcohol, peonin chloride being more red than cyanin chloride.

On hydrolysis the salt yields peonidin chloride (1 mol.) and glucose (2 mols.), and, like cyanin chloride, it is optically active. The following data have been recorded by Willstatter and Nolan:
Solution in 0.05 per cent. HCl, white light as for cyanin chloride [a] = - 191° (± 5°).

Peonin picrate may be prepared by the addition of an aqueous solution of picric acid to a suspension of the chloride in water. The chloride first dissolves, and, on standing, the picrate crystallises out in the form of red-brown (under the microscope red-violet) needles.