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.
Tannin, Tannic acid, Gallotannin, Gallotannic acid is found to the largest extent in galls which arise from the puncture of insects of the genus Cynips on the leaves and buds of various species of oak, more especially the Quercus lusitanica (Lam.), and on a species of sumach, Rhus semilata (Murr.). Aleppo galls, derived from the young shoot of the oak and which are the best.variety of oak gall on the market, contain 50-60 per cent, of gallotannic acid, whereas Chinese galls from sumach yield as much as 70 per cent. In smaller amount it occurs also in numerous plants, and is probably the main tannin of various sumachs, of valonia, divi-divi, and algarobilla.
The methods employed for the isolation of gallotannic acid from galls are more or less modifications of that of Pelouze, which consists in extracting finely powdered galls with commercial ether. The extract separates into two layers, the upper consisting of an ethereal solution of gallic acid, wax, and resinous substances, whereas the lower represents a concentrated solution of gallotannic acid which, on evaporation, remains as a porous mass. In place of the ether a mixture of 75 per cent, commercial ether and 25 per cent, alcohol can be employed. The method of Leconnet (Annalen, 1836, 18, 179) consists in stirring powdered galls into ether until a thin cream results, pressing the mixture and repeating the operation until tannin is no longer removed. According to Domine (J. Pharm. Chim., 1844, [3], 5, 231), it is advantageous to allow the powdered galls before extraction to remain for some time in a moist atmosphere, and Pelouze's process witji this modification was adopted by the British and United States pharmacopœias.
For the manufacture on the large scale, Chinese or Japanese galls are preferably employed owing to their richness in tannin. The finely powdered material is stirred with sufficient water at 50-60° to form a concentrated aqueous extract, and after filtration the clear liquid is agitated with one-fourth of its volume of ether until an emulsion results. After standing for several days, the upper ethereal liquid which has separated is removed, and the lower layer, which contains all the tannin matter, is run into a still and the ether which is present recovered. After cooling the syrupy liquid is. spread out on sheets of tin, and heated by means of a steam coil, when the gallotannic rapidly puffs up and dries.
Thus prepared the commercial tannin contains some quantity of gallic acid, plant wax, glucose and other impurities; to remove these the material may be washed with ether, or the aqueous solution shaken with ether, or the aqueous liquid fractionally precipitated with common salt, the precipitate dissolved in ethyl acetate, and the tannin recovered by evaporation under reduced pressure. Trimble ("The Tannins," 85) treats a 5 per cent, solution of the tannin with 10 per cent, lead acetate, drop by drop, until the precipitate, at first yellow coloured, ceases; to be granular and is colourless. This is collected, the filtrate agitated two or three times with ethyl acetate and the extract evaporated. The colourless product, which still contains gallic acid, is redissolved in water, and the solution, after agitation with ether, evaporated under reduced pressure. Finally, the residue dissolved in ether by the aid of a little water is again brought rapidly to dryness under reduced pressure, and thus obtained is colourless and gives no reactions for gallic acid or glucose. With the object of preparing a homogeneous product, Walden (Ber., 1898, 30, 3154) nas employed dialysis, and also the precipitation of a solution of the purest commercial tannin in ethyl acetate with benzene. Rosenheim and Schidrowitz (Chem. Soc. Trans., 1898, 73, 882) point out the extreme difficulty of removing the last traces of gallic acid from the tannin and employ a mixture of ether and acetone for this purpose. Paniker and Stiasny (Chem. Soc. Trans., 1911, 99, 1821) state, however, that the process of Rosenheim and Schidrowitz only partially removes gallic acid. The method suggested by Perkin, which consists in neutralising a solution of the tannin with sodium bicarbonate, extracting the mixture with ethyl acetate, and subsequently precipitating the substance from the extract with benzene, gives a product free from even traces of gallic acid (see also Iljin, Ber., 1909, 42, 1731). Fischer and Freudenberg (Ber., 1912, 45, 919) find the addition of dilute sodium hydroxide to the tannin solution until faintly alkaline before extraction with ethyl acetate is similarly beneficial.
Gallotannic acid as found in commerce consists of an amorphous powder possessing a faint yellow colour, although when exhaustively purified it is colourless. It is readily soluble in water and alcohol, more sparingly in ethyl acetate, insoluble in pure ether, chloroform, or benzene. With solutions of ferric salts, gallotannic acid gives a bluishblack coloration or precipitate, according to the concentration, whereas ferrous salts give with strong solutions only a white precipitate which gradually turns blue in the air. Many metallic salts give precipitates with the tannin, those of lead and tin being colourless, whereas the copper and silver compounds possess a brown tint. Cold alkaline solutions absorb oxygen from the air and darken in colour with production of the so-called metagallic acid. When boiled hydrolysis occurs, gallic acid being formed. Gallotannic acid precipitates most alkaloids and gives precipitates with albumen and gelatin, the latter, according to Trunkel (Zeitsch. Biochem., 1910, 26, 458), in quantitative amount.
Analyses of gallotannin by Berzelius, Pelouze, Mulder, Bijlert, Strecker, Gautier, Trimble, Dekker, and Walden have been in fairly close agreement, varying from about C = 51.5 to C = 52.3; H = 3.7 to H = 4.1 per cent. Iljin (Ber., 1909, 42, 1735) has suggested higher numbers (C = 54.13; H = 3.22), the correctness of which he has again emphasised (J. pr. Chem., 1910, [ii.], 82, 422; cf. Nierenstein, ibid.) 1909, 42, 3552). Mulder (J. pr. Chem., 1849, 48, 90) was the first to assign to gallotannin the formula C14H10O9, and this was subsequently adopted by Schiff (Ber., 4, 231), and until recently generally accepted as correct. Very numerous salts of gallotannic acid have been described which are in fair agreement with this view, of which ammonium tannate, NH4C14H9O9, potassium tannate, KC14H9O9, sodium tannate, NaC14H9O9, and barium tannate, Ba(Cl4H8O9) 2 (Buchner, Annalen, 53, 361) may be given as examples. For a complete list reference should be made to Beilstein (1896, ii., 1926).
Gallotannic acid isolated from plants is said to contain free glucose which is difficult to eliminate. Strecker (Annalen, 90, 340) indicated the possible existence of a glucoside of the formula C27H22O17, although Schiff (ibid., 170), while agreeing that unaltered tannin is probably a glucoside of digallic acid, preferred the formula
C34H28O22 ( = C6H12O6 + 2C14H10O9 - 2H20)
This corresponds to a yield of 23. per cent, of glucose; whereas in natural tannin about 22 per cent, is said to have been detected.
When gallotannic acid is heated at from 160-215°, water, carbon dioxide, and pyrogallol are evolved, and a dark coloured non-volatile substance known as metagallic acid is produced. According to Trimble (loc. cit.) the best yields of pyrogallol are obtained by raising the temperature slowly to 215° and then keeping it between 190-210° for half an hour. Digested with boiling dilute mineral acids, gallic acid is produced, and Wetherill, who employed for this purpose 50 grams of tannic acid and 500 c.c. of sulphuric acid (1 vol. acid + 4 of water), obtained a yield of 87.4 per cent, of gallic acid. Knop (Annalen, 170, 44) states he obtained 95 per cent., and Stenhouse the theoretical amount, whereas Trimble (loc. cit.) considers that when pure gallotannic acid is heated with a 2 per cent, solution of absolute hydrochloric acid, gallic acid only is produced.
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A number of other synthetic products of this nature have also been described by Fischer (cf. Ber., 1914, 47, 2485).
For further references to gallotannin see Harnack (Arch. Pharm., 1896, 234, 537), colour reaction; Ljubavin (J. Russ. Phys. Chem. Soc., 1901, 33, 680), tannin and tartar emetic; Thibault (Bull. Soc. Chim., 1903, (iii.), 29, 745), tannin and bismuth; Vigneron (J. Pharm. Chim., 1906, (vi.), 23, 469), iodotannin; Farbewerke vorm. Meister, Lucius, and Brüning (D.R.P. 173729), mixed anhydrides of tannic and cinnamic acids; Biginelli (Gazzetta, 1907, 37, ii., 205; ibid., 1903, 38, i, 559), tannates of quinine; Hildebrandt (D.R.P. 188318), tannin and formaldehyde; Francis and Nierenstein (Collegium, 1911, 335), action of benzoyl chloride and potassium cyanide on benzoyl-hydroxybenzoic acids and on acetylated hydroxybenzoylhydroxybenzoic acids; Nierenstein (Die Gerbstoffe).
Chestnut tannin.
Chestnut tannin has been examined by Nass (Inaugural Diss., 1884, Dorpat, Russia) and by Trimble ("The Tannins," ii., 119). According to the latter author it is probably identical with ordinary gallotannin (see Chestnut extract).
Chebulinic add or Eutannin.
This tannin was isolated by Fridolin from myrobalans, Terminalia chebula (Retz.), which also contain an ellagitannin. It crystallises in rhombic prisms, is sparingly soluble in cold water, gives with ferric chloride a blue-black precipitate, and by heating with water is converted into gallic acid and a new tannin.
Thorns (Chem. Zentr., 1906, i., 1829; Apotheker Zeit., 1906, 21, 354) has found that commercial eutannin is identical with chebulic acid, C28H22O19. It consists of small colourless needles, containing water of crystallisation, reacts acid to litmus paper, decomposes at 234°, and has [a]D initially +61,7°, gradually rising to +66,9°. An ennea-acetyl and methyl derivative are described, the latter giving trimethylgallic acid by the action of sodium hydroxide solution. With water at 100-150° the tannin yields gallic acid and eutannin hydrate, C28H24O20, a colourless powder decomposing at 200-210°. When eutannin is dissolved in cold sodium hydroxide, the solution acidified with acetic acid, and then treated with lead acetate solution, the resulting precipitate, after decomposition with sulphuretted hydrogen, gives gallic acid, and a tannin C14H16O12 or C14H14O11, which consists of a yellow powder, having [a]D+26° at 15°, and giving a blue coloration with ferric chloride. To chebulinic acid the following constitution is assigned: [KUVA PUUTTUU] According, however, to Fischer and Freudenberg (Ber., 1912, 45, 915), when hydrolysed chebulinic acid gives also dextrose.
Hamamelitannin.
This compound, one of the few gallotannins as yet isolated in a crystalline condition, occurs in the bark of the Hamamelis virginiana (Linn.), a tree, 10 to 12 feet high, common in North America. The bark, previously extracted with light petroleum to remove plant wax, is exhausted with ether-alcohol (5:1), the solution evaporated, the residue dissolved in a little alcohol and treated with ether to precipitate certain impurities. Evaporation of the ethereal liquid gives a product, a hot aqueous solution of which, after treatment with alumina and animal charcoal, deposits, on cooling, the substance in the form of small colourless needles. From a dilute aqueous solution, hamamelitannin, C14H14O9, crystallises with 5H2O, but deposited from strong solutions the crystals contain 2½H2O. The air-dried substance melts at 115-117°, although when dried at 100 the melting-point is 203°. Hydrolysed with boiling dilute sulphuric acid, gallic acid only was produced, and the presence of a sugar could not be detected. Hamamelitannin is dextro-rotatory, [a]D=+35,43°. According to Fischer and Freudenberg (Ber., 1912, 46, 2712), however, this tannin probably contains a sugar nucleus.
Benzoylhamamelitannin, C14H9O9(C7H5O)5, is a yellow powder which melts at about 125-132° (Gruthner, Arch. Pharm., 236, 303).
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Oak wood tannin is probably a member of this group, but is described under the heading of Phlobatannins.
Sumach tannin (see Sumach).
Diprotocatechuic acid is prepared by coupling monocarbomethoxyprotocatechuic acid with dicarbomethoxyprotocatechuyl chloride in alkaline solution and subsequently hydrolysing the product. It possesses the constitution (OH)2C6H3.CO.O.C6H3(OH)COOH, and consists of fine needles which begin to sinter at 230° and melt with decomposition at 237-239°. It is much more sparingly soluble in water than protocatechuic acid, gives with ferric chloride a bluishgreen coloration, and possesses tanning properties (Fischer and Freudenberg, Annalen, 1911, 384, 2, 238).
Di-β-resorcylic acid, prepared from β-resorcylic acid by the same general method, (OH)2C6H3.CO.O.C6H3(OH)COOH, forms small microscopic needles, melting at 215° (corr.), sparingly soluble in water. It gives with ferric chloride a violet-red coloration and behaves as a tannin.
Digentisic acid, (OH)2C6H3.CO.O.C6H3(OH)COOH, crystallises in fine needles, melting-point 208-209° (corr.), sparingly soluble in water. The aqueous solution precipitates gelatin and gives a blue coloration with ferric chloride.
Fischer and Hoesch (loc. cit., 224) and Fischer and Lepsius (loc. cit., 224) have prepared numerous other acids of this type, but it is not stated as yet if these are to be regarded as tannins. Lecanoric acid, ramalic acid, evernic acid, and no doubt other lichen acids (loc. cit.) which structurally belong to this group may, it is possible, also possess tanning property.
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