Peonidin.(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.

This substance occurs in the form of its diglucoside (peonin) in the flowers of the deep violet-red peony, and has been obtained from it by Willstatter and Nolan, who found that the hydrolysis of the glucoside is best carried out by rapidly heating 0-5 gr. peonin chloride in a boiling solution with 75 c.c. of 20 per cent, hydrochloric acid, and when complete solution occurs, boiling for exactly five minutes, by which time the peonidin has almost completely separated as crystalline chloride, and is collected while hot, and washed with 20 per cent, hydrochloric acid. It was found that shorter hydrolysis tends to leave unchanged glucoside, whilst too prolonged action of the acid partially attacks the methoxy group.

Peonidin chloride, C₁₆H₁₃O₆Cl, forms a crystalline hydrate of the composition C₁₆H₁₃O₆Cl, H₂O, which consists of long red-brown needles similar in form to the crystals of cyanidin chloride, but differing from them in that the water of crystallisation in peonidin chloride is completely removed by drying in vacuum desiccator at room temperature.

The anhydrous chloride is considered by Willstatter and Nolan to have the structure or in that alkaline decomposition has shown that the methoxy group is attached to the hydroxyphenyl group. Further, as sodium carbonate gives a blue colour when added to an acid solution of the pigment - this they regard as evidence of a free OH in the pyrylium ring - and ferric chloride gives no colour reaction, they consider that the attachment of the methyl to one of the OH groups of the catechol residue is certain. Of the two formulæ, they regard (1) as more probably correct.

Peonidin chloride is fairly soluble in cold water, very soluble in hot; on boiling the solutions become decolorised, but addition of dilute acid causes the colour to return, slowly if cold, rapidly and quantitatively if hot. In alcohol it is very soluble, forming a fine violet-red coloured solution which differs from that of cyanidin chloride in that the addition of water causes the colour to change to brownish-red, without formation of any precipitate, whereas similar treatment of a cyanidin chloride solution causes the separation of a flocculent violet precipitate. In 1 per cent, hydrochloric acid the salt is very difficultly soluble cold, but easily soluble hot, and may readily be recrystallised from this medium; its solubility in 7 per cent, sulphuric acid is very similar, but in this case crystals of the sulphate separate from warm solutions on cooling.

Sodium carbonate, when added to an acid solution of peonidin chloride, produces a blue colour, whereas potassium acetate causes the colour of an alcoholic solution to become violet, and on diluting with water a red-brown precipitate forms, the liquid remaining violetred (cyanidin chloride solutions similarly treated yield a finely divided violet precipitate). Ferric chloride produces no characteristic colour on addition to an alcoholic solution, only a slight change to violetred being observable.

Zeisel estimations proved that peonidin chloride contains one methoxy group in the molecule, and when the finely crystalline iodide of the methoxy-free compound that separated from the liquors after the estimations was converted into the chloride (by treatment with precipitated silver chloride in alcoholic hydrochloric acid solution), the product was found to be identical in properties and composition with cyanidin chloride.

When peonidin chloride was subjected to decomposition by the action of caustic potash it was not possible to find any evidence of the production of a methyl ether of phloroglucinol, despite the fact that the reaction was carried out under very mild conditions, the only phenolic product present being phloroglucinol itself. On the other hand, by carefully regulating the conditions, an acid decomposition product was obtained that contained a methoxy group, though in the circumstances a large proportion was demethylated. Thus at 180-185°C. the acid product showed 8 per cent. MeO, at 215° only 3.5 per cent. MeO, and at 235° no MeO remained attached to the hydroxycarboxylic acid.

Peonidin sulphate is deposited as brown-red needles when a solution of the chloride in warm dilute (e.g. 7 per cent.) sulphuric acid is allowed to cool.