Pelargonidin.(CHAPTER VIII. Pyran Group.)

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.

Pelargonidin occurs in the form of glucosides in various flowers. It was first isolated from the scarlet Pelargonium zonale (Meteor) hence its name; more recently also from the purple-red summer aster (Callistephus chinensis, Nees, syn. Aster chinensis, L.), the scarlet salvia (Salvia coccinea, L., and Salvia splendent, Sello.), and the rose-coloured corn-flower; whilst Willstatter and Bolton (Annalen, 1916, 412, 136), as the result of qualitative tests, conclude that the scarlet-red gladiolas also owe their colour to a pelargonidin derivative, and that traces are present in other gladiolas and in Zinnia elegans (Jacq.), of which the chief pigments are derivatives of cyanidin. Pelargonidin has also been prepared synthetically by Willstatter and Zechmeister, by the demethylation of the product obtained by the reaction of anisyl magnesium bromide on 3:5:7: trimethox-coumarin (Sitzber. d. K. Preuss. Akad. d. Wiss., 1914, 886).

Pelargonidin forms a crystalline chloride which has the composition C₁₅H₁₁O₅Cl, and the structure [KUVA PUUTTUU]

This yields a hydrate C₁₅H₁₁O₅Cl, H₂O, which does not lose its water of crystallisation on drying in air, vacuum exiccator, or even in high vacuum at 50° C, but does so completely in high vacuum at 105° C. By treatment with hot water, preferably in presence of a trace of sodium bicarbonate, the chloride yields a pseudo-base which crystallises in colourless four-sided prisms; this product when dry has the composition C₁₅H₁₂O₆.

Pelargonidin chloride is readily prepared from any of its naturally occurring glucosides by hydrolysis with hydrochloric acid. This change takes place slowly in the cold if concentrated acid is used, but for the preparation of pelargonidin the glucoside is preferably boiled for three minutes with 20 per cent, hydrochloric acid ('2 gr. glucoside with 15 c.c. acid), the resulting product cooled to 65°C. (if further cooled the substance is contaminated by a by-product mentioned below), the crystalline chloride filtered off, washed with cold 20 per cent, hydrochloric acid and dried.

In this process it has been observed that whilst the pelargonidin chloride separates in brown-yellow leaflets, there is always some 5 per cent, of another product present in the reaction mixture, and this separates in needles. This product closely resembles pelargonidin, both in properties and composition, and can be prepared from it by the action of concentrated hydrochloric acid, but the reverse change has not yet been accomplished. It follows from this that long boiling in the preparation of pelargonidin from its glucosides is disadvantageous.

Pelargonidin chloride crystallises in three forms which are described by Willstatter and Bolton (Annalen, 1915, 408, 42) thus:
(i) Long, not quite rectangular, red tablets somewhat resembling crystals of xanthophyll.
(ii) Short red-brown, usually straight cut, four-sided prisms. These separate from hot dilute acid.
(iii) Sharply formed swallow-tail twin crystals, in form resembling carotin, but yellow-brown by transmitted light when viewed under the microscope. These are obtained by precipitation with concentrated hydrochloric acid.

Pelargonidin chloride is much more soluble in acids than is the cyanidin salt, being described as difficultly soluble in cold dilute hydrochloric or sulphuric acid, though fairly easily soluble when the acids are warm, forming solutions which are orange-red; from the solution in sulphuric acid the sulphate crystallises in needles on cooling. In methyl or ethyl alcohol the chloride is very easily soluble, yielding solutions that have a violet tinge, but show no fluorescence (cf. pelargonin); these solutions are not precipitated by the addition of water (cf. cyanidin). An aqueous acid solution of the chloride when shaken with amyl alcohol gives up all the pigment to the alcoholic layer, and if the red alcoholic solution thus produced is shaken with an aqueous solution of an alkali acetate it becomes violet, whereas if shaken with aqueous sodium carbonate, the colour turns to a fine pure blue and passes completely to the aqueous layer. These changes are the same as those observed with cyanidin chloride.

With reagents, the following are the most important reactions. Lead acetate, when added to an alcoholic solution, produces a blue precipitate; ferric chloride to an aqueous solution gives no characteristic coloration, and to an alcoholic solution only a brown-red tint (difference from cyanidin); Fehling's solution is noticeably reduced if warm.

When heated in a melting-point tube, pelargonidin chloride becomes darker, but does not melt below 350° C.

The chloride dissolves in water without separation of violet flocks (cf. cyanidin), forming a red solution which on warming (in the cold, if very dilute) becomes colourless as a result of the formation of the pseudo-base; if warmed with acids the colour is regained. The distribution number with respect to amyl alcohol is normal for a non-glucoside anthocyan = 100.

Willstatter and Bolton (loc. cit.) state that the absorption spectrum of this salt consists of two bands, one covering the spectrum from yellow to blue, and another in the violet; both have edges that are badly defined. Thus far the presence of a band in the violet has not been observed in any other anthocyan, but they propose to reinvestigate this point photographically. Measurements given are:
Thickness of solution:... 2,5 mm.... 5 mm.... 10 mm.
Band I... 579---569-491---483... 588---576-471... 586---583-
Band II... 448-442 ... 448-... -

The action of caustic potash upon pelargonidin chloride has been studied, and it has been found that whilst 60 per cent. KOH, even at temperatures not above 100° C., yielded the phenolic decomposition product phloroglucinol it required much more concentrated caustic potash and higher temperatures to allow of the isolation of the acid decomposition product (p-hydroxy-benzoic acid). Thus 0,5 gr. of pelargonidin chloride were heated with 10 gr. KOH and 3 gr. water to 220° C., for two to three minutes; the product, after acidification, was extracted with ether and the ethereal extract shaken with sodium bicarbonate which leaves phloroglucinol in the ether from which it was isolated and identified; the aqueous layer yielded p-hydroxy-benzoic acid and a trace of protocatechuic acid. This production of protocatechuic acid resembles its formation from apigenin (Perkin, Chem. Soc. Trans., 1897, 805).

The Ψ Base, C₁₅H₁₂O₆. When pelargonidin chloride is heated with water (0,.5 gr. with 600 c.c.) a pseudo-base is formed. The chloride dissolves, then the solution becomes decolorised the addition of a small quantity of bicarbonate of soda (0,13 gr.) facilitates complete decolorisation and the base can be obtained from this solution by the addition of salt followed by the extraction of the product with ether. It is finally recrystallised from water at 50°C., and in this way the base separates in the form of four-sided prisms.

The base is very easily soluble in alcohol, ether, or hot water, less soluble in cold water, and insoluble in benzol. If a cold aqueous solution be acidified with hydrochloric acid it slowly deposits crystals of the chloride; if the solution is hot, the chloride is very rapidly formed. The base gives a bright yellow coloration on addition of sodium carbonate. No definite melting-point can be observed when the pure substance is heated, the crystals turn red, then very gradually soften till a dark violet oil is produced; melting not taking place below 350°C.