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.
Glucosides of delphinidin have been isolated from various kinds of grapes, from the berries of Ampelopsis, of Vitis riparia, and of the bilberry (Vaccinium myrtillus, Linn.), also from the flowers of the hollyhock (Althae rosea), delphinium (Delphinium consolida, Linn.), petunia (Petunia hybrida hort.) wild mallow (Malva silvestris, Linn.), and viola (Viola tricolor}, but it is of interest to note that except in the case of the colouring matters of the delphinium and viola, all the pigments are glucosides of mono- or dimethyl ethers of delphinidin, and not of free delphinidin.
Delphinidin has been isolated in the form of its crystalline picrate, iodide, sulphate, and chloride, most frequently as chloride, and also as the colourless pseudo-base. The chloride has been obtained in no less than four distinct hydrated forms containing iH2O, i|H2O, 2H2O, and 4H2O respectively. The anhydrous chloride has been shown to possess the structure [KUVA PUUTTUU]
Delphinidin chloride has been prepared by Willstatter and his collaborators by the hydrolysis of delphinin and violanin. For this purpose these glucosides were first dissolved in hot dilute hydrochloric acid (0.4 gr. in 10 c.c. of 0.5 per cent. HCl), the solution when boiling mixed with concentrated hydrochloric acid (12 c.c.) and the mixture boiled for two minutes. On cooling in contact with ice, the chloride separated, and was purified by recrystallisation. This process, which differs somewhat from that used for the preparation of pelargonidin and cyanidin from their glucosides, was adopted to avoid a too prolonged action of the acid, whereby amorphous decomposition products are formed. Willstatter and Weil also prepared this compound by first forming the iodide by boiling with hydriodic acid as if for a Zeisel estimation, followed by conversion of that salt into the chloride by means of silver chloride in alcohol. It has also been obtained from the mono- and dimethyl ethers of delphinidin, resulting from the hydrolysis of their naturally occurring glucosides, by converting the delphinidin iodide, resulting from the Zeisel methoxy determinations of these, into the chloride as stated above.
A solution of the chloride in water readily passes through the violet colour base to the pseudo-base, a change that also takes place when warm water is added to an alcoholic solution of the chloride. A concentrated aqueous solution soon deposits the colour base as a flocculent violet precipitate.
Delphinidin chloride is easily soluble in methyl or ethyl alcohol, giving beautiful purple-coloured solutions; it is fairly soluble in amyl alcohol and passes completely from aqueous acid into amyl alcohol; it passes appreciably from an aqueous solution less so from dilute acid solution into ethyl acetate, and even ether will take a small fraction from an aqueous solution particularly on addition of calcium carbonate but dilute acid removes all from the ether.
Anhydrous delphinidin chloride does not melt below 350°C.
Willstatter and Weil describe the preparation and properties of the various hydrates of delphinidin chloride, viz.:-
C15H11C7Cl, H2O. This hydrate was formerly obtained by allowing a solution of the pigment in dilute hydrochloric acid to stand over concentrated hydrochloric acid, when as the acid became gradually concentrated, crystals of the compound were deposited. As this hydrate separated when the concentration was about 3-4 per cent. HCl, and even at 5 per cent. HCl a totally different hydrate commenced to separate, it was difficult to obtain a pure product in this way. Regular yields of pure product were obtained by dissolving 0.4 gr. of the pigment in 5 c.c. of 0.5 per cent. HCl, adding 1 c.c. of 20 per cent. HCl, and keeping for several days in a closed vessel to prevent evaporation, hence concentration in an ice room. The crystals thus produced were homogeneous, thin, sharply cut rhombic tablets of a deep violet colour; they lose all their water of crystallisation on drying in a vacuum desiccator at room temperature, and reabsorb the same amount on standing in the air. In 0.5 per cent. HCl the compound is very easily soluble, in 3-5 per cent, still easily soluble. An interesting change occurs when it is added to 5 per cent. HCl at first the solid readily dissolves, producing dark streaks in the liquid, soon, however, the liquid gets lighter, then no further solid dissolves and the liquid deposits fine brown needle-like crystals, and, in about half an hour, the liquid becomes colourless. The undissolved solid gradually changes in appearance and finally consists of jagged crystals resembling aggregates of thin prisms. A very similar transformation takes place when sulphuric acid is used, the compound first dissolving, and then being redeposited in the form of fine prisms.
C16H11O7Cl, 1½H2O separates from hydrochloric acid of more than 20 per cent, concentration, and may be prepared by dissolving 0-4 gr. of pigment in 10 c.c. water, and adding 15 c.c. of concentrated hydrochloric acid all at once whereupon an amorphous precipitate separates, which changes in the course of a few hours into small crystals. This hydrate loses all its water when dried in a vacuum desiccator at room temperature.
C16H11O7Cl, 2H2O is the hydrate that separates on adding aqueous acid (7 per cent, to 20 per cent. HCl) to an alcoholic solution of the pigment, and allowing the alcohol to evaporate. It forms aggregates of prismatic tablets which are difficultly soluble in 5 per cent. HCl, fairly soluble in hot but insoluble in cold 7 per cent, sulphuric acid. These lose all their water of crystallisation when dried in vacuum desiccator, and the product reabsorbs water when left in the air.
C16H11O7Cl, 4H2O separates from 5 per cent, hydrochloric acid and is conveniently obtained by dissolving 0.3 gr. of the pigment in 5 c.c. of 0.5 per cent. HCl and adding 1.5 c.c. of 20 per cent. HCl. Crystallisation commences after standing a few hours, and practically all the pigment separates in the form of fine prisms and needles which form a chocolate-brown mass with bronze reflex, which, on account of its woolly nature, is difficult to powder. The compound loses all of its water on drying in vacuum desiccator at room temperature.
Note. - As distinct from these hydrates, Willstatter and Mieg previously described crystals of delphinidin chloride which lost 4.6 per cent, water in vacuum desiccator, and a further 4.5 per cent, in high vacuum at 105°C. Willstatter and Weil state that this is not incorrect, as crystals are indeed frequently met with that do behave thus, but that they have not been able to ascertain the conditions necessary for their formation. Crystals that behave thus are usually needle-like in form.
Willstatter and Mieg record the following reactions of delphinidin chloride: An acidified solution on addition of sodium carbonate gives a violet, then blue coloration which is unstable; lead acetate produces a blue precipitate; sodium bisulphite forms a colourless, soluble product that is decomposed by dilute hydrochloric acid with liberation of the pigment; ferric chloride, on addition to an alcoholic solution, gives a fine intense and stable blue coloration, but on dilution with water this changes to an unstable violet; Fehling's solution is vigorously reduced, even when cold.
Zeisel estimations proved the absence of methoxy groups in the compound. When decomposed by means of 75 per cent, caustic potash at 250°C., delphinidin chloride is stated to yield phloroglucinol and pyrogallol, together with a small amount of gallic acid, but owing to the small quantities available, Willstatter and Mieg state that the identification of these products is incomplete.
The absorption spectrum of delphinidin chloride is described as consisting of one band in the yellow and green, which is fairly well defined, more so when long columns are examined than when shorter ones are used. The data given are:
Solution in ethyl alcohol, 1 mol. in 2500 litres
Column 2.5 mm. 595.. 590- -529. 512
Column 5-0 mm. 603.. 598-500.. 493
Solution in ethyl alcohol, 1 mol. in 12,500 litres
Column 2.5 mm. 581.. 546
Column 5.0 mm. 587.. 582... 550. 533
Delphinidin sulphate separates in the form of characteristic long prisms when solutions of the pigment in hot dilute sulphuric acid are allowed to cool.
Delphinidin iodide is deposited from a boiling mixture of phenol and hydriodic acid as brown glistening prisms and pointed leaflets. By solution in acidified alcohol and treatment with precipitated silver chloride and a trace of silver, it is converted into delphinidin chloride.
Delphinidin picrate crystallises in fine red-brown needles and prisms, and is difficultly soluble in water.
Delphinidin pseudo-base, C15H12O8. This compound, which is characterised by considerable crystallising power, was prepared by Willstatter and Mieg in the following way: 1 gr. of delphinidin chloride was warmed rapidly with ½ litre of water to the temperature of the water-bath. After quarter of an hour - when the liquid had already become much lighter - 2 gr. sodium phosphate (primary) were added and this brought the change rapidly to completion. The solution was cooled, saturated with ammonium sulphate, and extracted with ether; the ether extract, which contained a small quantity of unchanged substance, was washed with a few c.c. of dilute hydrochloric acid, then with water, dried over sodium sulphate, and evaporated in vacuo. The product thus obtained was slightly brownish in colour, but was readily recrystallised from ether, or from water, when the base separated in the form of colourless prisms. The substance must be protected from acid fumes, or from glass vessels possessing an alkaline reaction.
When heated to 120-130°C. the substance is rapidly coloured violet, whereas at higher temperatures it becomes darker and gradually sinters. It is easily soluble in ethyl alcohol, acetone, ethyl acetate, and glacial acetic acid; in ether it is somewhat more difficultly soluble, and insoluble in benzol; from solutions in glacial acetic acid, fine needle-like crystals separate on the addition of toluol. This pseudo-base is stable when in dry crystalline form, also in dilute aqueous, or ether, solution; in aqueous sodium carbonate it dissolves with a yellow colour, which is fairly stable. Mineral acids convert it into the coloured salt, a solution in cold 7 per cent, hydrochloric acid becoming intensely red in a few hours, whereas, if boiled, this change occurs in a few minutes.
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