Quebracho Colorado(CHAPTER XIII. Tannins.)

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.

The Quebracho Colorado are anacardinaceous trees belonging to the genus Quebrachia, growing in the northern part of the Argentine Republic, the wood of which constitutes the well-known tannin substance "quebracho". It is imported into this country in the form of logs and is employed for tanning, either in the chipped condition or in the form of extract. Their wood is extremely hard, as the name "quebracho" (axe-breaker) denotes, and its specific gravity varies from 1.27-1.38.

Jean (Bull. Soc. chim., 1880, 33, 6) found that it contained 15.7 per cent, of a tannic acid, not identical with that of oak bark or chestnut wood, whereas Procter ("Leather Manufacture," 1903, 269) estimates it to contain about 20 per cent, of a tannin yielding reds (phlobaphene), and containing catechol and phloroglucinol nuclei. This tannin is somewhat sparingly soluble in water and can only be used in weak liquors, but gives a firm reddish leather.

In order to isolate the tannin, Strauss and Geschwender (Zeitsch. angew. Chem., 1906, 19, 1121) extract the bark first with chloroform and then with alcohol. Addition of water to the alcoholic extract causes the separation of phlobaphenes, and from the clear liquid concentrated in a vacuum the tannin is precipitated by lead acetate, the lead salt being collected, suspended in water and decomposed with sulphuretted hydrogen. The resulting solution is evaporated to dryness, the residue dissolved in alcohol and poured into ether. Thus obtained it consists of a light flaky mass, which is hygroscopic and becomes sticky on -exposure to moist air.

According to Arata (Jean, Bull. Soc. chim., 1879, 306) quebracho tannin, C₂₆H₂₄O₁₀, gives catechol on dry distillation, with nitric acid oxalic and picric acids, by fusion with alkali phloroglucinol and protocatechuic acid, whereas, by the latter method, Nierenstein isolated also hydroquinone and resorcinol (Collegium, 1905, 65). According to the latter author, the quebracho Colorado probably contains three tannins. By treating a cold aqueous extract of the quebracho Colorado with bromine, Böttinger (Ber., 17, 1123) obtained a reddish-yellow compound containing 42.1-44.5 per cent, of bromine. Nierenstein, who isolated the tannin according to Trimble's method ("The Tannins "), treated the solution with lead acetate, filtered, and on adding bromine to the clear liquid obtained a precipitate of monobromoquebrachotannin, C₁₆H₁₄BrO₈, which consists of a cinnabar red powder, and on digestion with alcoholic potash gives isovanillic acid and monobromoquebrachylic acid needles, melting-point 119-120°.

Strauss and Geschwender (loc. cit.) consider that quebracho tannin is identical with malettotannin, and with the tannin from cinchona bark, and ascribe to it the formula [C₄₁H₄₄O₁₈(OMe)₂]₂. With a mixture of acetic anhydride and acetic acid an acetyl compound, (C₃₀H₂₂O₁₁Ac₆)₂, colourless powder, is produced, and a corresponding benzoyl derivative, (C₃₀H22O₁₁Bz₆)₂, can also be prepared.

Quebracho phlobaphene, on distillation with zinc-dust, yields anthracene (Nierenstein, Ber., 1907, 40, 4575).

According to Arnaudon the wood contains a colouring matter which gives a fine yellow dye, and this has been examined by Perkin and Gunnell (Chem. Soc. Trans., 1896, 69, 1304), and found to be identical with fisetin, the colouring matter of young fustic. This compound, which appears to exist in the wood as glucoside, gives on fusion with alkali protocatechuic acid and resorcinol, and may account for the appearance of the latter phenol among the hydrolytic products of the crude tannin itself. According to Perkin and Gunnell, when an extract of the quebracho colorado is digested with boiling dilute acid a small quantity of ellagic acid is obtained.

In addition to the tannins above described, the quebracho Colorado is the source of the so-called "quebracho resin," which collects as a thickened juice in the crevices of the tree. It has been examined by Arata (Chem. Soc. Abstr., 1878, 984), who states that it is easily soluble in alcohol or ethyl acetate, insoluble in benzene. By fusion with alkali it gives protocatechuic acid (?) and phloroglucin, and by the action of nitric acid, oxalic and picric acids are produced.

A considerable amount of the tannin contained by the quebracho colorado is of a sparingly soluble nature, and is deposited to some extent from a hot aqueous extract on cooling. This product may be rendered soluble by treatment with alkalis or alkaline sulphites, and a large quantity of the so-called "soluble" quebracho extracts are prepared by heating the material in closed vessels with bisulphites, sulphites, sulphides, or even caustic alkalis (Lepetit, Dollfus and Gansser, Eng. Pat., 1896, 8582; cf. Procter, "Leather Manufacture," 338).