Synthetic Tannins | Page 8

active components can only be brought about through the
_d_- or _l_-hexacarbethoxyleucodigallic acid on introducing the latter
into a 1 per cent. pyridine solution and heating to 45°-50° C., whereby
the _d_- or _l_-acid is formed accompanied by a strong evolution of
carbon dioxide.

Hydrolysis of leucogallic acid yields gallic acid and gallic aldehyde;
oxidation by means of hydrogen peroxide yields ellagic acid and luteic
acid, and oxidation with potassium persulphate and sulphuric acid, in
acetic acid solution, yields purpurotannin (see below) [Footnote:
Liebig's Ann., 1912, 386, 318.].
Another distinct difference between digallic acid and leucodigallic acid
is the fact that the formaldehyde condensation product of the former
resembles gallic acid, whereas that of the 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