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.
Cyanin, the colouring matter of the corn-flower (blue and deep purple), Rosa gallica, deep red dahlia, etc., has been the subject of several investigations, of which the earliest was carried out by F. S. Morot in 1849 (Ann. d. sciences nat, (3), 13, 160 (1849-50)). The work of Morot is of interest in that he attempted to prepare the pigment by precipitation from aqueous solution by means of alcohol, a process which formed a portion of the method of purification whereby Willstatter and Everest obtained the pure pigment. In addition to the work of Morot, that of Fremy and Cloëz (J. pr. Chem., 1854, 62, 269) should be mentioned, as they included the pigment of the corn-flower in their investigation of the colouring matters of flowers. They gave to it, in common with the other pigments examined, the name cyanin, and this was adopted by Willstatter and Everest for the corn-flower pigment when, in 1913 (Annalen, 401, 189), they succeeded in preparing it in a pure and crystalline condition, a result that paved the way for the great advance in our knowledge of the anthocyan pigments that has since been achieved. Although this colouring matter was first obtained from the petals of the blue corn-flower, Centaurea cyanus, it is much more readily prepared from Rosa gallica or deep red dahlia petals.
The isolation of cyanin from blue corn-flowers is a long and tedious process, consisting of repeated, rapid fractional precipitation of an aqueous extract of the petals by means of alcohol, followed by conversion of the partially purified blue pigment into the chloride (red) and slow fractional precipitation of its solution in alcoholic hydrochloric acid by means of ether. The amorphous, but almost pure product is then crystallised by solution in alcohol, addition of aqueous hydrochloric acid, and slow evaporation of the alcohol. Although it was not found possible to obtain the blue pigment itself in a pure condition, fractional precipitation yielded it so sufficiently to prove that before conversion into the chloride, it was present as a potassium salt, and that in this form it occurs in the blue corn-flower.
Much simpler methods are available for its isolation from other flowers rose, dahlia, or deep bordeaux coloured corn-flower; but all efforts to improve the method used by Willstatter and Everest for its preparation from the blue corn-flower have hitherto failed. The flowers mentioned contain: corn-flower (blue), about 0.75 per cent, (deep violet-blue), 3.6 per cent., (deep bordeaux), 13-14 per cent.; dahlia (deep red), about 20 per cent.; rosa gallica, 2 per cent., respectively, of their dry weight of cyanin. Willstatter and Mallison consider the petals of the deep red dahlia to be the best source of the pigment.
In 1878 Senier (J. Pharm., (3), 7, 650 (1878)), made an attempt to isolate the pigment of the rose, using the lead salt method, but failed, and ascribed his failure to the fact that the precipitation of the lead salt was not sufficiently specific. In 1905 Molisch (Bot. Ztg., 157) succeeded in preparing this colouring matter in microcrystalline form in the same way that he had obtained crystals of the pelargonium pigment, but it remained for Willstatter and Nolan (Annalen, 1915, 408, 1) to obtain it pure and crystalline in quantity, and to establish its identity with the cyanin previously isolated by Willstatter and Everest from the corn-flower. From 1 kg. of dry petals (commercial, rosa gallica) they were able to obtain 7 gr. of pure crystalline cyanin chloride. Their process consisted in extracting the petals with methyl alcoholic hydrochloric acid (about 2 per cent. HCl) ethyl alcohol is not so satisfactory (1 kg. to 3 litres) filtering after some sixteen hours, washing the residue with further solvent (1 per cent. HCl, 5 c.c.) several times to complete the extraction, and precipitating the filtrate (5 litres) by mixing it with two and a half times its volume of ether, when the pigment separates as a gummy mass. The best method of obtaining the pure pigment from this crude product (amount from 1 kg. of petals taken at a time) consists in allowing it, without drying, to stand for twenty -four hours with alcoholic hydrochloric acid, or better, with methyl alcohol (200 c.c.) and glacial acetic acid (140 c.c.), when the impurities present become hydrolysed or acetylated, but the cyanin remains unchanged as a deep brown micro-crystalline residue. The product is dissolved in boiling water (700 c.c.), mixed with an equal volume of 3 per cent, ethyl alcoholic hydrochloric acid and allowed to cool, when cyanin chloride separates in fine glistening crystals (about 6.5 gr.), and a further crop may be obtained from the mother liquors.
From the deep red dahlia flowers cyanin chloride is readily prepared in quantity as described by Willstatter and Mallison (Annalen, 1915, 408, 147), (note scarlet-red dahlia flowers yield pelargonin), in that an extract of the fresh flowers (700 gr.) in glacial acetic acid is first mixed with methyl alcoholic hydrochloric acid, then precipitated with ether (one and a half volumes), which produces a flocculent deposit that congeals together, and when dried (15 gr.) is 76 per cent. pure. This on solution in cold 7 per cent, hydrochloric acid (300 c.c.), filtration and standing, yields the chloride in a crystalline condition, which may be recrystallised in the same way described as the product obtained from rose petals. The yield from 700 gr. fresh petals is about 7.4 gr. pure crystalline chloride (air-dried), i.e. 50 per cent, of the total amount present in the petals.
To obtain cyanin chloride from deep bordeaux coloured cornflowers Willstatter and Mallison extracted these with glacial acetic acid, and added ether, when an oily product was deposited, which when dissolved in a little 7 per cent, hydrochloric acid after a day, separated in a granular condition. The product washed with hydrochloric acid was recrystallised from warm 3 per cent, aqueous hydrochloric acid when it separated in glistening rhombic-shaped tablets, which appear violet under the microscope, and have a melting-point 198°C.
Cyanin chloride, C27H31O16Cl, crystallises in the form of red-brown rhombic leaflets which contain 2½ molecules of water of crystallisation (C27H31O16Cl.2½H2O), of which some ¾H2O remains in the product when dried in high vacuum at 50°C., but all is lost when dried in this way at 105 C.
The air-dried crystalline chloride is almost insoluble in cold water, but dissolves very readily at 90°C. By the addition of an equal volume of 3 per cent, ethyl alcoholic hydrochloric acid to the solution, the salt is almost completely precipitated, separating as aggregates of leaflets with a golden reflex. The salt is only slowly and difficultly soluble in cold ethyl alcohol (100 c.c. at 19°C. dissolve 0.053 gr. of crystals from the rose, 0.059 gr. crystals from the corn-flower), very difficultly soluble in acetone and chloroform and insoluble in benzene. In dilute hydrochloric acid it is but little soluble, as the following figures indicate:
100 c.c. 1 p.c. HCl at 20°C. diss. 0.015 gr. air-dried crysts. from rose.
100 c.c. 1 p.c. HCl at 20°C. diss. 0.015 gr. air-dried crysts. from corn-flower.
100 c.c. 1½ p.c. HCl at 20°C. diss. 0.0053 gr. air-dried crysts. from rose.
100 c.c. 1½ p.c. HCl at 20°C. diss. 0.0055 gr. air-dried crysts. from corn-flower. and these figures have been used by Willstatter and Nolan (Annalen, 1915, 408, 1) as evidence of the identity of the pigment from the corn-flower and rose. In 7 per cent, sulphuric acid the crystalline chloride is fairly readily soluble, but on standing, the sulphate always crystallises out; thus a solution of 0.01 gr. of chloride in 30 c.c. deposits this in thin deep red needles.
Dilute aqueous, or alcoholic, solutions of the salt become decolorised as the result of pseudo-base formation, but addition of acid to the decolorised solution causes quantitative return of the colour. The decolorised solution gives a yellow colour on addition of sodium carbonate and a green precipitate with lead acetate; excess of sodium carbonate or caustic soda changes it to a yellow which does not become red on addition of acid.
The following reactions of cyanin chloride have been noted:
Ferric chloride when added to an aqueous solution produces a fine violet coloration, whereas in alcoholic solution a beautiful intense blue results; with excess of the reagent the colour rapidly fades, passing finally to yellow; lead acetate completely precipitates the colour from its solutions producing violet-blue flakes which have a coppercoloured reflex. When the lead salt thus prepared is treated in the presence of water with carbon dioxide, or with a limited quantity of sulphuretted hydrogen, a blue solution results, and appears to contain a primary lead salt which can be obtained from the solution by precipitation with alcohol. With alkalis (sodium carbonate or caustic soda) an acid solution of cyanin chloride passes through violet to a fine pure blue, whereas with calcium carbonate the reaction passes only to the violet stage, the violet solutions thus formed becoming rapidly decolorised owing to pseudo-base formation. Zinc and dilute acetic acid (or HCl, or H2SO4) reduce the salt with formation of a colourless leuco compound from which the colour may be regenerated by air oxidation; sodium bisulphite produces a colourless soluble compound which is decomposed by addition of acid with liberation of the pigment; Fehling's solution, even hot, is not reduced by cyanin chloride.
The absorption spectrum was described by Willstatter and Everest as consisting of one broad band covering a large portion of the green and blue. A solution of 1 mol. in 2500 litres gave the following figures:
Thickness of layer 2-5 mm... 549.540-467.462
Thickness of layer 5.0 mm... 561.553-448
Cyanin chloride is optically active, and Willstatter and Nolan obtained the subjoined values when using white light (1000 c.p. ½ watt Osram lamp): Pigment from corn-flower [a] = - 225° and - 251°
Pigment from rosa gallica [a] = - 249 and° - 267°
They consider the most probable value to be [a] = - 258° (±10°).
The distribution number of cyanin chloride (1.8) is normal for a diglucoside anthocyan, and on hydrolysis the salt yields cyanidin chloride (1 mol.) and glucose (2 mols.).
Cyanin picrate is very readily soluble in water and is difficult to prepare, but may be obtained by suspending cyanin chloride in a little water and adding a saturated solution of picric acid in the same solvent. The pigment goes into solution, and on standing for several days a fiocculent precipitate separates which consists of fine red needles.
In dilute aqueous solution the picrate readily undergoes isomerisation, the change being complete on warming; in alcohol it is readily soluble without undergoing change.
Cyanin potassium salt.
This compound, which is the actual blue colouring matter of the wild corn-flower, was obtained by Willstatter and Everest in a state of considerable purity, and finally in a crystalline condition by slow evaporation of a concentrated solution in 20 per cent, aqueous sodium chloride, after this had been purified by dialysis. The cyanin salt separated in fine deep blue six-sided tablets, but could not be completely freed from sodium chloride as small quantities only were available. On the other hand, it was necessary to use solutions in strong sodium chloride (or nitrate) solution for the dialysis, as otherwise the whole of the cyanin salt became isomerised before purification was complete.
The salt is very soluble in water, giving beautiful blue solutions, which, however, become decolorised on standing. The decolorisation is greatly retarded by the addition of various inorganic salts, and solutions containing 20 per cent, of common salt can be kept for months without decolorisation occurring. Addition of acid to the blue solution, or to the colourless one produced from it by standing, gives a red coloration. The addition of sodium carbonate to a blue solution causes no immediate change of colour, but if the liquid has stood before the sodium carbonate is added, a blue-green to green-blue colour results, whereas addition of the carbonate to a solution that has become colourless by standing produces a yellow colour. The salt is insoluble in alcohol, ether, and benzene, but fairly readily soluble in ethylene glycol. The colour is stable to light.
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