Synthetic Tannins | Page 5

[Footnote:
_Ibid._, 1890, v. 192.] later determined the molecular weight of tannin
in aqueous solution as 1104, in acetic acid solution as 1113-1322,
Krafft [Footnote: _Ber._, 1899, 32, 1613.] as 1587-1626 in aqueous
solution. Walden [Footnote: _Ibid._, 1898, 3167.] determined the
molecular weight of tannin-schuchardt as 1350-1560, tannin-merck as
753-763, digallic acid as 307-316 (calculated 322). Feist [Footnote:
_Chem. Ztg._, 1908, 918.] determined the molecular weight of tannin
as 615 and one of his own preparation as 746, Turkish tannin as 521
and Chinese tannin as 899. In this connection it should be noted that the
calculated molecular weight of pentagalloyl glucose, which in E.
Fischer's opinion forms a substantial part of the tannin molecule, is 940,
but Fischer also thinks that this compound possesses a much higher

molecular weight.
STRUCTURE OF TANNIN--The oldest structural formula of tannin is
Schiff's digallic acid formula:--[Footnote 1: Ber., 1871, 4, 231.]
---------CO.O.---------- ^ ^ OH | | | | HO | | OH HOOC | | OH V V OH
A drawback to the acceptance of this formula is the absence of an
asymmetrical C-atom; the formula, therefore, does not explain the
optical activity exhibited by tannin. Schiff attempted to overcome this
difficulty by adopting a diagonal structural formula, but even when
adopting Clauss' diagonal formula for benzene the optical activity of a
number of other compounds depends upon the existence of the
asymmetrical C-atom. Biginelli [Footnote 2: _Gazz chim. Ital_., 1909,
39, 268.] also opposed the digallic acid formula, and supported his
view by referring to the arsenic compounds obtained by him on heating
arsenic acid and gallic acid, instead of obtaining digallic acid. Walden,
[Footnote 3: Ber., 1898, 31, 3168.] on the other hand, found, on
analysing the digallic acid thus prepared, only slight traces of arsenic
and, by the elementary analysis, obtained figures closely corresponding
to those of digallic acid.
Bottinger [Footnote 4: Ibid., 1884, 17, 1476.] prepared the so-called
_[Greek: b]_-digallic acid by heating ethyl gallate with pyroracemic
acid and sulphuric acid and proposed the so-called ketone-tannin
formula:--
HO_____OH ______OH HO{_____}--------CO--------{______}OH
COOH OH
Schiff completed this formula by a diagonal, so as to explain the optical
activity observed--
HO OH ______OH HO{_____}--------CO--------{______}OH COOH
OH [Diagonal bond between HO and COOH on left.]
The ketone formula was corroborated by Nierenstein, [Footnote: _Ber._
1905, 38, 3641.] who distilled tannin with zinc dust and obtained

diphenylmethane (smell of benzene) and a crystalline product, M.P.
7O°-71° C. (M.P. of diphenyl = 71° C.). König and Kostanecki
[Footnote: _Ibid._, 1906, 39, 4027.] sought to find the constitution of
the tannins in the leuco-compounds of the oxyketones, to which
catechin belongs. Nierenstein (see above), however, emphasises that
the high molecular weight and the optical activity speak against the
digallic acid formula, but in favour of this are the following points: (1)
the decomposition of tannin with the formation of gallic acid; (2) the
decomposition of methylotannin with the formation of di- and trimethyl
esters of gallic acid; and (3) the production of diphenylmethane on
distillation with zinc dust. The latter reaction especially illustrates the
analogous formation of fluorene from compounds of the type--
--CO.O ^ ______ ^ | | | | | | | | V V
Nierenstein gave the name "Tannophor" to the mother-substance of
tannin, phenylbenzoate, C_6H_5-COO-C_6H_5.
Dekker [Footnote: "De Looistoffen," vol. ii, p. 30 (1908).] was,
however, unable to detect diphenylmethane on distilling with zinc dust,
and did, therefore, not accept Nierenstein's views. In proposing the
formula--
O || HO ^ _ __C | | | | | }O | | | __OH | |____|_C_/ \OH HO V \__/ OH
OH OH
Dekker [Footnote: _Ber._, 1906, 34, 2497.] was enabled to account for
most of the details in the behaviour of tannin, viz.: (1) the empirical
constitution, C_14H_10O_9; (2) the almost complete hydrolysis into
gallic acid (the dotted line indicates the decomposition of the molecule
into 2 molecules gallic acid by taking up water); (3) the formation of
diphenylmethane as a result of distillation with zinc dust; and (4) the
electrical non-conductivity. Since tannin on acetylating yields a
considerable amount of triacetylgallic acid, it should, according to
Dekker, contain at least six acetylisable hydroxyls.
Nierenstein [Footnote: _Chem. Ztg._, 1906, 31, 880.] objected to this
formula on account of its containing seven hydroxyl groups, whereas

Dekker found six, Nierenstein five, and Herzig still fewer hydroxyl
groups. The formula would also favour the conception of tinctorial
properties which could hardly be ascribed to tannin. Lloyd [Footnote:
Chemical News, 1908, 97, 133.] proposed a very intricate formula
containing three digallic acid groups joined into one six-ring system,
which would then explain the optical activity; it would, on the other
hand, also require an inactive cis-form.
Iljin [Footnote: _Jour. of the Russian phys. chem. Soc._, 1908, 39, 470.]
prepared two phenylhydrazine derivatives of tannin (C_74 H_58 N_8
O_30 and