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.
Pelargonin.
This pigment, named by Willstatter and Bolton (Annalen, 1915, 408, 42) after the pelargonium from which it was isolated by them, was the first anthocyan to be obtained in a crystalline condition (A. B. Griffiths, Chem. News, 1903, 249; Ber., 36, 3959). Griffiths suggested for it the formula C15H10O6, described a potassium salt and an acetyl derivative, and gave data concerning absorption spectrum and specific rotation, but his descriptions are very superficial, and in the light of more recent work these do not appear to be reliable. In 1905, H. Molisch (Bot. Ztg., 1905, 145) examined the pelargonium pigment from a botanical standpoint, and showed how readily this pigment could be obtained in a crystalline condition. A slightly modified form of his process forms a useful experiment for lecture demonstration. The work of Molisch caused Grafe to attempt the chemical investigation of the pelargonium pigment (Sitzber. d. K. K. A. Wiss., Wien, 1911, 765). He succeeded in obtaining the pigment in the crystalline condition by the use of glacial acetic acid as solvent, by preparation of the lead salt and liberating the free pigment from this, or by the dialysis of a solution of the pigment. Besides the crystalline product, he isolated a second substance which was amorphous in character, and employing 25 kg. of fresh petals he obtained 10 gr. of the crystalline pigment and 15 gr. of the amorphous substance. Grafe described the crystalline compound as very unstable and deliquescent, proposed for it the formula C18H26O13, and considered that it was a tribasic acid having two hydroxy and two carbonyl groups within the molecule, and was a sugar-free compound. To the amorphous substance he gave the formula C23H44O20, and showed it to be a glucoside as he obtained glucose from it by hydrolysis; he does not appear to have examined the resulting sugar-free compound. Grafe suggested that the two pigments obtained by him were related to each other, and might be formed according to the following scheme: C24H44O20 + H2O → C6H12O6 + (C18H34O15) (hypothetical product), then (C18H34O15) - 4H2O + O2 → C18H26O13. Grafe described various reactions and also the decomposition products resulting from melting the pigments with caustic potash, but his results have not been confirmed by later work. Willstatter and Bolton (loc. cit.), who have thoroughly investigated the pigment of the pelargonium, state, in regard to Grafe's results, that the anthocyan is quite a stable compound, gives no ferric chloride reaction, and with sodium carbonate yields a violet coloration. It contains neither carbonyl nor carboxyl groups, and does not yield protocatechuic acid nor catecol when decomposed by caustic potash.
According to Willstatter and Bolton, pelargonin is the only pigment present in the scarlet Pelargonium zonale (Meteor), in the petals of which it occurs to the extent of 6.6-7.1 per cent, of their dry weight. These authors also show that it occurs in other flowers e.g. pink corn-flower, cactus dahlia (Annalen, 1915, 408, 149).
For the preparation of the pure pigment, as chloride, Willstatter and Bolton extract the flowers either with glacial acetic acid or 96 per cent, alcohol the latter gives more complete extraction and a purer product, and was the solvent used by Griffiths. Using acetic acid they proceeded thus: 3 kg. fresh petals were allowed to stand some time in 4 litres of glacial acetic acid, filtered, and, as the extraction was not complete even after long standing, the residue was re-extracted with a further 2 litres of acid and again filtered. The extracts were united and had a deep red colour and faint green fluorescence. To each 500 c.c. of the extract 50 c.c. of alcoholic hydrochloric acid (as low as 2 per cent. HCl may be used, but preferably, somewhat stronger, even to 20 per cent. HCl, as precipitation is then better), then 1 litre of ether was added which caused the precipitation of the pigment, this being complete in a few hours. A further 100-200 c.c. of ether added to the filtrate leaving to stand for a few days precipitated a further quantity of pigment. The major portion of the colouring matter was thus obtained, the first fraction being the purer; it was not found advantageous to work up the further filtrates. 570 gr. petals yielded 6-7 gr. of crude chloride (38.4 per cent, pure), i.e. 63 per cent, of the total pigment contained in the petals.
By using in place of acetic acid 96 per cent, alcohol the pigment was much more completely extracted from the petals, so much so, that a re-extraction was deemed to be unnecessary, the residue from the first extraction being merely washed on the filter with fresh alcohol, and a purer crude product was subsequently obtained. The alcoholic extract and the petals lying therein became decolorised after standing a few days, owing to pseudo-base formation, but the change had no detrimental effect on the preparation as the addition of a little hydrochloric acid, even cold, reproduced the colour, whereas if ether were added a violet precipitate of the colour base separated. For the preparation of pelargonin chlorides Willstatter and Bolton took i litre of the alcoholic extract and added 20 c.c. of alcoholic hydrochloric acid (20 per cent. HCl), followed by 2 litres of ether, whereby a carmine-red flocculent precipitate was thrown down. The precipitate filtered readily and the filtrate contained but traces of the colouring matter.
The crude chloride obtained by either of the above methods was purified by fractional precipitation from solution in methyl-alcoholic hydrochloric acid by ether, or better, by recrystallisation from methyl-alcoholic acid, or a mixture of methyl alcohol, water, and hydrochloric acid in the following way. The impure product was dissolved by boiling for a short time in 2 per cent, methyl-alcoholic hydrochloric acid, filtered warm, to remove insoluble impurities, and the filtrate mixed with 1/4 - 1/5 of its volume of 10 per cent. aq. HCl. (In this solution the glucoside is not hydrolysed, even on long standing.) On cooling, fine long thin needles of the chloride separated, and after filtration a further almost equally pure crop was obtained by careful slow evaporation of the methyl alcohol from the mother liquor. The recrystallisation was repeated several times if the pigment was desired in a state of complete purity.
Pelargonin chloride has the composition C27H31O15Cl, and forms a hydrate C27H31O15Cl, 4H2O, which loses its water in a vacuum desiccator at room temperature, and crystallises in long thin red needles; the anhydrous salt softens at 175°C. and melts with decomposition at 180°C.
In cold dilute hydrochloric acid (1-2 per cent. HCl) the crystalline salt is difficultly, and in 5 per cent. HCl very difficultly soluble, but is easily soluble in these acids if hot, giving orange-red solutions ; hydrolysis takes place, however, if the solutions in any but very weak acids are warmed.
In water the salt is appreciably soluble giving an orange-coloured solution, but the colour becomes violet as the result of hydrolytic dissociation, and then, even when cold, gradually becomes colourless owing to the formation of the pseudo-base. The alcoholic solutions are more stable; in cold methyl alcohol the salt is appreciably soluble, and readily soluble in the warm solvent from which it crystallises on cooling - the yellow-red solution has a characteristic greenishyellow fluorescence; in ethyl alcohol the compound is less soluble, and if it be heated with a small quantity of this the normal chloride is converted into a basic salt.
In respect of its distribution between aqueous acid and amyl alcohol pelargonin behaves as a normal diglucoside. The hydrolysis of pelargonin yields either pelargonidin, the sugar-free pigment, if the hydrolysis be carried to completion, or pelargonenin, an intermediate monoglucoside, if the hydrolysis be carried out carefully under prescribed conditions.
With reagents the following changes may be noted: lead acetate, sodium acetate, or calcium carbonate cause the precipitation of the violet colour base when added to a solution of the chloride; ferric chloride, or alum, produces no colour reaction; sodium carbonate gives a violet colour (cf. pelargonidin), which gradually changes to greenish-red then to yellow sodium or ammonium hydrate gives the same reaction; in alcoholic solution the addition of sodium hydrate gives a fine blue colour, which passes to violet on dilution with water; sodium bisulphite forms a colourless easily soluble addition product; Fehling's solution, even warm, is only reduced to a very small extent.
Pelargonin chloride is optically active, and Willstatter and Bolton give the following values:
[a]D = - 291°, [M]D = - 1837°; & [a]614 = - 180°, [M]614 = - 1133°.
The absorption spectrum has also been investigated by them, and is described as consisting of one broad band passing much less into the green than that of cyanin, ending very indistinctly at that end, whilst the violet region is darkened, particularly on the edge of the visible portion. They give the following data:
1 mol. in 2500 litres... 2-5 mm. layer | 536... 530 - 458.
0.5 per cent. aq. HCl.... 5.0 " " | 547... 542 - 447.
Pelargonin basic chloride (C27H31O15Cl)2.(C27H30O15) is prepared by boiling 1 gr. pelargonin chloride with 250-300 c.c. of 96 per cent, alcohol for a few minutes. Some of the product remains undissolved, though converted into violet flakes, but from the orangered filtrate crystals separate on cooling, which consist of small deep violet needles possessing metallic lustre. The crystals are of a hydrate (C27H31O15Cl)2(C27H30O15), 12H2O, which loses its water (not alcohol) of crystallisation in a vacuum desiccator, leaving the anhydrous compound consisting of 1 mol. base and 2 mols. chloride.
Pelargonin acetate (C27H31O15)2.(C27H30O15.C2H4O2 ) is prepared by dissolving pelargonin base (see below) in boiling 90 per cent, acetic acid, and allowing the filtered solution to cool when the acetate separates in fine red needles. It also separates from a glacial acetic acid extract of pelargonium petals (in which pelargonin is probably present originally as tartrate). In a vacuum desiccator the crystals lose 9.33 per cent, of their weight, probably of acetic acid, and the dry product gives analytical figures which agree with the above formula. Doubtless the crystalline substance obtained by Grafe consisted of this acetate in an impure state.
Pelargonin base, C27H30O15. This compound is prepared by treatment of the chloride with a small quantity of water, or by addition of sodium acetate to a weakly acid solution of the pigment. It has not been obtained crystalline, but for analysis was merely washed free of chloride by means of water. It is difficultly soluble in water, the solution becoming gradually decolorised owing to pseudo-base formation; in alcohol it is very difficultly soluble, whereas in alkalis it is readily soluble with the formation of an unstable violet solution.
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