Synthetic Tannins | Page 8

latter resembles tannin; it is therefore probable that the leucodigallic acid part of the tannin molecule imparts this characteristic property to tannin.
---CO.O--- ^ ^ | | | | HO V OH COOH V OH OH OH [Illustration: Digallic Acid becomes...]
---CO.O--- ^ ^ OH | | | | HO V OH COOH V OH OH OH [Illustration: Luteic Acid becomes...]
---CO.O--- ^ ^ OH | | | | HO V --O.CO-- V OH OH OH [Illustration: Ellagic Acid becomes...]
COOH COOH ^ _______ ^ | | | | HO V ---O--- V OH OH OH [Illustration: Purpuro Tannin.]
3. Ellagic Acid
Ellagic acid was discovered in 1831 by Braconnot, who named it "acide ellagique." Its presence in the vegetable kingdom was not quite comprehended for some time, and Nierenstein [Footnote: _Chem. Ztg._, 1909, 87.] was the first to prepare this substance from algarobilla, dividivi, oak bark, pomegranate, myrabolarms, and valonea. The acid is obtained by precipitating it with water from a hot alcoholic extraction of the plants referred to, and recrystallising the precipitate from hot alcohol. Another method of preparation consists in boiling the disintegrated plants with dilute hydrochloric acid, washing the residue, and extracting with hot alcohol, from which the acid will then crystallise. According to Lowe, [Footnote: _Zeits. f. analyt. Chem._, 1875, 35.] it may be obtained from dividivi, an aqueous extract of which is heated to 110�� C. in a tube closed at both ends, when crystalline ellagic acid is deposited. Heinemann [Footnote: Ger. Pat., 137,033 and 137,934.] obtained ellagic acid by simply boiling repeatedly aqueous tannin solutions.
Lowe [Footnote: _Jour. f. prakt. Chem._, 1868, 103, 464.] first synthesised ellagic acid by heating gallic acid with arsenic acid or silver oxide. Herzig [Footnote: Monatshefte fur Chemie, 1908, 29, 263.] states that ellagic acid is deposited when air is conducted through a mixture of the ethyl or methyl ester of gallic acid and ammonia. Perkin [Footnote: _Proc. Chem. Soc._, 1905, 21, 212.] obtained a substance very similar to ellagic acid by electrolysis of gallic acid in sulphuric acid solution; on oxidising gallic acid in concentrated sulphuric acid solution, Perkin and Nierenstein [Footnote: _Ibid._, 1905, 21, 185.] obtained flavellagic acid. Ellagic acid is also obtained by heating luteic acid in a 10 per cent. soda solution.
Ellagic acid thus prepared crystallises with 2 molecules of water as yellow micro-crystalline rhombic prisms or prismatic needles. The crystals lose this water when heated to 100�� C., and it is possible that it is water of constitution, in which case the substance would be hexoxydiphenylcarboxylic acid, and the substance left after drying at 100�� C., the dilactone.[Footnote: _Arch. d. Pharm_., 1907, 244, 575.] Ellagic acid is slightly soluble in water, alcohol, and ether, but is easily soluble in caustic potash. With concentrated nitric acid the product assumes a red colour, which appears to be due to the presence of impurities; ellagic acid is commercially known as "alizarin yellow."
The constitution of ellagic acid was uncertain for a long time, and different structural formulae were proposed which more or less corresponded to its properties. The most satisfactory structural formula was proposed by Graebe--[Footnote: _Chem. Ztg_., 1903, 129.]
---CO.O--- ^ -------- ^ OH | | | | HO V --O.CO-- V OH OH
This would represent a tetroxydiphenylmethylolide.
The probability of the correctness of this formula is supported by the possibility of the following derivatives: monomethylellagic acid, C'14H'6O'7(O.CH'3); dimethylellagic acid, C'14H'4O'6(O.CH'3)'2; tetramethylellagic acid, C'14H'2O'4(O.CH'3)'4; phenylhydrazinellagic acid, C'14H'6O'8.N'2H'3C'6H'5.
By the electrolytic reduction of ellagic acid, hexoxydiphenyl, (OH)'3C'6H'2-C'6H'2(OH)'3, is obtained; the ordinary methods of reduction yield leucoellagic acid, C'14H'10O'8, which crystallises in small sharp needles, melting with decomposition at 294��-295�� C. Leucoellagic acid is soluble in ethyl and methyl alcohols, and in glacial acetic acid, insoluble in chloroform, benzene, toluene, carbon tetrachloride, and petrol ether; it gives a bluish-green colour with ferric chloride which quickly turns black. Leucoellagic acid is soluble in alkalies, the solution assuming a deep-red coloration; it reduces silver nitrate in the cold, but is not adsorbed by mordanted cotton cloth, in which respect it differs from ellagic acid.[Footnote: Liebig's Ann., 1912, 394, 249.
ELLAGITANNIC ACID, C'26H'28'O'10-3H'2O, is closely related to ellagic acid; the former consists of faintly yellow needles, M.P. 329��-336��C. It is soluble in water, precipitates gelatine, and is adsorbed by hide powder. It occurs with gallic acid, tannin, and ellagic acid in dividivi, myrabolams, algarobilla, and chestnut wood extracts.
Other bodies of this class include:--
METELLAGIC ACID, Cl_4H_6O_5, derived from methoxybenzoic acid, and recrystallised from acetic acid, forms small crystalline needles, M.P. 273��-276�� C., and yields fluorene on distillation with zinc dust.
----CO.O---- ^ ---------- ^ | | | | V ---O.CO--- V OH
FLAVELLAGIC ACID, C_14H_6O_9, is obtained by the oxidation of gallic acid with concentrated sulphuric acid and potassium persulphate. It crystallises from pyridine in prismatic needles melting above 360�� C. Distillation with zinc dust yields fluorene (see above)--
----CO.O---- ^ ---------- ^ OH | |