Cochineal(CHAPTER I. The Anthraquinone 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.

This important natural dyestuff, which, in its native country, Mexico, was used as a dye and cultivated by artificial means at a remote period of history, was for a long time considered to be of vegetable origin. Cochineal, however, consists of the dried body of an insect, the Coccus cacti, which lives upon a species of cactus (the Nopalea coccinellifera (S.-Dyck) or Nopal], a plant which is found in the wild condition, but which, for the sake of the insect, is cultivated in gardens which are termed Nopaleries. The collection of the insects takes place before the commencement of the rainy season, and they are then brushed either into straw baskets or into basins of tinned iron. A number of insects are left upon each plant, and a new generation is produced, which is again gathered at a suitable period. The insects are killed by immersion in boiling water, or are enclosed in a linen bag and placed in an oven; by the latter process the peculiar white down covering the insect is preserved, but in the former case is lost.

In Mexico and Central America two varieties of cochineal are known the home-grown, or fine cochineal (grana fina), and the wild, or forest cochineal (grana silvestra) The former is more .valuable than the latter, and is richer in colouring matter. Since 1830 the cultivation of cochineal was introduced into Spain and the Canary Islands, Algeria, and Java, but the most productive of these newer plantations were those of Java. Since the discovery of the coal-tar colours, the consumption of cochineal as a dye-stuff has gradually decreased, and at the present time it is only employed in a minor degree. According to Liebermann, cochineal contains about 10 per cent, of colouring matter (Ber, 18, 19).

Carminic acid, the colouring matter of cochineal, was first isolated by Pelletier and Caventou (Ann. Chim. Phys., (2), 8, 250), and was subsequently examined by Preisser (Annalen, 52, 375; J. Pharm. Chim., (3), 5, 191) and Arppe (Annalen, 55, 101); but Warren de la Rue (ibid., 64, 1) was the first to isolate this substance in a pure condition, and described it as a purple-brown mass, which, on grinding, yields a bright-red powder, easily soluble in water and alcohol, but not in ether. The mean of his analyses gave 54.13 per cent, of carbon, 4.62 per cent, of hydrogen, and 41.25 per cent, of oxygen (by difference), and from these figures he deduced the formula C₁₄H₇O₈ or (C₁₄H₇O₈)₂.

Schützenberger (Ann. Chim. Phys., (3), 54, 52), on the other hand, was the first chemist who succeeded in obtaining carminic acid in a crystalline condition. He precipitated the colouring matter, from an aqueous cochineal extract, in the form of its lead compound, suspended this in water, and decomposed it with sulphuretted hydrogen. The carminic acid thus liberated dissolved in the water and was recovered from this solution by evaporation at a low temperature. The product, dissolved in alcohol, was treated with ether to precipitate certain impurities, and the liquid partially evaporated, when, on cooling, a crystalline mass was obtained, which Schützenberger considered to consist of two substances, carminic acid, C₉H₈O₅, and oxycarminic acid, C₉H₈O₆, the latter being distinguished by its solubility in ether.

Schaller (J., 1864, 410), who prepared carminic acid by the same method, assigned to it, however, the formula C₉H₈O₆.

The work of Hlasiwetz and Grabowski (Annalen, 141, 329) indicated that carminic acid was a glucoside which could be decomposed into a sugar and a new colouring matter, carmine red:
C₁₇H₁₈O₁₀ + 2H₂O = C₁₁H₁₂O₇ + C₆H₁₀O₅
Carminic acid. Carmine red. Sugar. but according to Liebermann (Ber., 18, 1969; Will and Leymann, ibid., 1 8, 318; and Von Miller and Rohde, ibid., 26, 2647), this is incorrect.

Coccinin, according to Hlasiwetz and Grabowski, is produced when carminic acid is fused with caustic potash. It crystallises from alcohol in straw-yellow needles or leaflets, dissolves in alkalis with a yellow colour, which, by air oxidation, develops first a green, then violet, and, finally, a purple tint. The analyses of this substance were in agreement with the formula C₁₄H₁₂O₅.

Ruficoccin. - By heating carminic acid with sulphuric acid to 130-140°C., Liebermann and van Dorp (Annalen, 163, 105) obtained a new colouring matter ruficoccin, C₁₆H₁₀O₆, and this consisted of a bright-red powder, sparingly soluble in hot water and ether, with a greenish-yellow fluorescence. On distillation with zinc-dust, it yielded a colourless crystalline hydrocarbon, C₁₆H₁₂, melting-point 183-188°C, from which, by oxidation, a quinone melting at 250°C. could be produced.

Fürth, somewhat later (Ber., 16, 2169), prepared the same hydrocarbon by the distillation of both cochineal carmine and coccinin with zinc-dust.

Ruficarmine, C₁₆H₁₂O₆, can be obtained, according to Liebermann and van Dorp, by heating carminic acid with water in a sealed tube at 200°C. It consists of a carmine-red powder, easily soluble in alcohol.

In view of the uncertainty existing as to the percentage composition of carminic acid, Schunck and Marschlewski (Ber., 27, 2980) submitted this substance to an elaborate process of purification, and, using in their operations as low a temperature as possible, obtained a product which crystallised from alcohol in red prismatic needles. Their analyses agreed closely with that required by the formula C₁₁H₁₂O₆, and the percentage composition approximately with the figures given by Warren de la Rue, and also by Schützenberger.

On the other hand, analyses by Miller and Rohde (Ber., 30, 1762) pointed to the formula C₁₂H₁₁O₇ or (C₁₂H₁₁O₇)₂, but according to the more recent work of Liebermann, Horing and Wiedermann (Ber., 1900, 33, 149), it now appears that the correct formula for carminic acid is C₂₂H₂₂O₁₃.

The most simple method of purification of carminic acid is that devised by Miller and Rohde. A solution of the crude colouring matter in five times its weight of water is diluted with four times its volume of acetic acid. The filtered liquid, on standing over sulphuric acid, gradually deposits the carminic acid in a crystalline condition.

Carminic acid crystallises in red prisms, easily soluble in water and alcohol, with a purple-red colour. It possesses no melting-point, but darkens at 130°, and at 250° becomes quite black.

By the action of alcoholic potassium acetate, carminic acid (Perkin and Wilson, Chem. Soc. Trans., 1903, 83, 139) yields two potassium salts, viz.:
Monopotassium carminate, C₂₂H₂₁O₁₃K, which is red coloured; and Dipotassium carminate, C₂₂H₂₀O₁₃K₂, soluble in water with a violet-red coloration.

Hexabenzoylcarminic acid, C₂₂H₁₆O₁₃(C₇H₅O)₆, obtained by digesting carminic acid with benzoyl chloride (Liebermann, Horing and Wiedermann), is an orange-coloured powder, easily soluble in benzene.

Octacetylcarminic acid, C₂₂H₁₄O₁₃(C₂H₃O)₈, crystallises in goldenyellow needles, melting-point 155-165°, and is readily prepared by the action of acetic anhydride in presence of zinc chloride or sulphuric acid on carminic acid (Miller and Rohde).