Synthetic Tannins | Page 3

the natural (true) tannins
probably belong to different groups of organic compounds, and with
our present-day scant knowledge of their chemistry, it is impossible to
classify them. One is, however, justified in assuming that both the
natural tannins and the related humic acids are ester-derivatives of
hydroxybenzoic acids. [Footnote: E. Fischer, _Ber._, 1913, 46, 3253.]
The production of synthetic tannins employs two quite distinct methods;
one is to synthesise the most simple tannin, viz., the tannic acid
contained in galls (tannin), or to build up substances similar in
character to the tannins, from hydroxybenzoic acids. The other, entirely
new way, is to produce chemical substances, which certainly have
nothing in common with the constitution of the natural tannins, but
which behave like true tannins in contact with animal pelt, and in
addition, since they can be manufactured on a commercial scale, are of
practical value.
Owing to the fact that, until recently, the constitution of tannin has
remained unknown, it is easy to comprehend that the efforts to
synthesise the latter substance, or compounds similar to it, have been
mainly attempted on similar lines. The oldest investigation in this
direction dates from H. Schiff,[Footnote: Liebig's _Ann._, 1873, 43,
170.] who prepared substances similar to tannin by dehydrating
hydroxybenzoic acids. By allowing phosphorus oxychloride to interact
with phenolsulphonic acid, he obtained a well-defined substance
possessing tanning properties, which he considered an esterified
phenolsulphonic acid anhydride, the composition of which he
determined as HO.C_6H_4.SO_2.O.C_6H_4HSO_3. It is, however,
probable that this substance is not homogeneous, but consists of a
mixture of higher condensation products.
Klepl [Footnote: _Jour. pr. Chem._, 1883, 28, 208.] obtained--by
simply heating _p_-hydroxybenzoic acid--a so-called di- and tridepside,
but this simple method is not applicable to many other hydroxybenzoic
acids, since these are decomposed by the high temperature required to
induce reaction.
Amongst other attempts to produce condensation products with

characteristics similar to those possessed by the tannins, those by
Gerhardt [Footnote: Liebig's Ann, 1853, 87, 159.] and Loewe [Footnote:
_Jahresh. f. Chem._, 1868, 559.] must be especially noted; they treated
gallic acid with phosphorus oxychloride or arsenic acid, and thereby
obtained amorphous compounds, exhibiting the reactions characteristic
of tanning substances. E. Fischer and Freudenberg, [Footnote: Liebig's
_Ann._, 372, 45.] by treating _p_-hydroxybenzoic acid in the same way,
succeeded in obtaining a didepside, and during the last years practically
only these two investigators have demonstrated the syntheses of these
depsides and produced high-molecular polydepsides.
At the same time researches were instituted with the object of
determining the constitution of tannin, and E. Fischer succeeded in
demonstrating its probable composition as being that of a glucoside
containing 5 molecules of digallic acid per 1 molecule of glucose.
This last-named class of synthetic tannins--which may be properly
termed "tanning matters" in contradistinction to the true
tannins--exhibit very distinct tanning character when brought in contact
with animal hide, but from the point of view of chemical constitution
have nothing in common with the natural tannins. Not only are they of
interest to the industry from a practical point of view; they have also
been examined very closely from a chemical standpoint.
It is, however, necessary to differentiate with great exactitude between
the conception of true tanning effect and pickling effect when
considering the action of chemical substances on pelt (i.e., animal hide,
treated with lime, depilated, and the surplus flesh removed). Whereas
any true tannage is characterised by the complete penetration of the
substance and its subsequent fixation by the pelt in such a way that a
thorough soaking and washing will not bring about a reconversion (of
the leather) to the pelt state; pickling, on the other hand, is only
characterised by the penetration of the substance in the pelt and fixation
to such an extent that a subsequent washing of the pickled pelt will
bring back the latter to a state closely approximating that of a true pelt.
Simple as such a differentiation appears, there are still a number of
cases occupying a position between the two referred to, and which we
may term _pseudo-tannage_. An example of the latter is formaldehyde
tannage; formaldehyde has for a long time been employed in
histological work for the purpose of hardening animal hide, by which it

is readily absorbed from solution whereby it hardens the hide without,
however, swelling it. A hide which has thus been treated with
formaldehyde absorbs the natural tannins with greater ease; this, on the
one hand, argues the probability of formaldehyde acting as a pickling
agent; on the other hand, it is also one of its characteristics that it will
either in neutral acid, [Footnote: R. Combret, Ger. Pat, 112, 183.] or,
still better, in alkaline [Footnote: J. Pullman, Ger. Pat, 111,408; Griffith,
_Lea. Tr. Rev._, 1908.] solution, convert pelt into leather. In a
formaldehyde-tanned leather, however, no trace of