Synthetic Tannins
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Georg Grasser >> Synthetic Tannins
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SYNTHETIC TANNINS
THEIR SYNTHESIS, INDUSTRIAL
PRODUCTION AND APPLICATION
by
Georg Crasser, Dr. Phil., Ing.
Lecturer in Tanning Chemistry
at the German Technical College, Brunn
AUTHOR'S PREFACE
Whilst the synthesis of the natural tannins has been successfully
outlined by Emil Fischer, it has been left to the Chemical Industry,
notably the Badische Anilin und Soda-fabrik in
Ludwigshafen-on-the-Rhine, to discover the means of making possible the
production of the synthetic tannins.
The scientific results of Fischer's researches are to-day common
knowledge, and these, together with questions arising therefrom, will
only be lightly touched upon in the book herewith presented. Even an
attempt at enumerating the present synthetic tannins has so far not been
published, and I have therefore availed myself of the opportunity of
making a brief summary of them. My work at the B.A.S.F. deepened my
insight in this new field; ample opportunity of applying these synthetic
products in practice was given me when, as a result of the war, I was
appointed technical consultant to the Austrian Hide and Leather
Commission, and in this capacity was called upon to act as general
adviser to the trade. The ultimate object of my scientific researches
was then to investigate the chemistry of this particular field, and this
has led me to present a picture, complete as far as it goes, of this
branch of chemical technology.
The intention of the present volume is to communicate to the reader what
has so far been scientifically evolved and practically applied in this
field. First of all, however, it may illustrate the extreme importance
and the universal applicability of the synthetic tannins in the making
of leather. The modern leather industry cannot, to-day, be without these
important products, but also in those tanneries, where the synthetic
tannins have not so far been regarded as indispensable, their use is
strongly recommended. Just as in the case of the coal-tar dyes, the
synthetic tannins will make us independent of foreign supplies, and thus
keep within our own borders the vast sum of money required in former
days for the purchase of foreign tanning materials. May this book prove
the means of providing an incentive for a still wider application of the
synthetic tannins.
GRASSER.
GRAZ, _August_ 1920.
TRANSLATOR'S PREFACE
Doctor Grasser hardly needs an introduction to the leather trade of this
country in its scientific aspect, but if one be sought for, none could
serve the purpose better than a translation of the book herewith
presented to the British-speaking public.
Viewed with curiosity from their start, the synthetic tannins
needed--like many other important discoveries--an extreme emergency for
the purpose of showing their value. The Great War provided the
opportunity of which chemical industry was to avail itself, and to-day
we do not only see synthetic tannins placed upon the market as a
veritable triumph of chemical technology and a creditable triumph of
manufacturing chemistry; we also see their immensely practical qualities
established as a fact, and, as the author aptly remarks, no modern
tanner can to-day dissociate himself from the use of synthetic tannins
for the production of leather in the true sense of this word. There is
no branch of leather-making where synthetic tannins cannot help and
improve processes already established.
The immense number of substances patented by German manufacturing
chemists for the purpose of producing synthetic tanning materials is
almost staggering. In view of this fact it is doubly pleasing to see
that British chemists have found new ways, and are able to produce
equally good and more varied synthetic tannins than has hitherto been
deemed possible. The originator of these products and his acolytes
must at least share the credit with those who, in spite of the
limitations necessarily set by the former, have been able to find new
and better ways.
In his book Dr. Grasser gives a short review of the necessary forerunner
of any work upon synthetic tannins: the investigations and syntheses of
the natural tannins. It is certainly to be hoped that we may soon see
such works as those of Fischer's and Freudenberg's, recently published,
translated into English. For the guidance of the reader it may be noted
that a short account of the works of these authors may be found in the
_Journal of the Society of Leather Trades' Chemists_, vol. v. (May
issue); in addition to this some of the matter contained in the chapter
on synthesis of tanning matters appeared in the January 1921 issue of
the _Journal of the American Leather Chemists' Association._
In addition to these two sections, the last part of this book deals with
the practical applications of synthetic tannins, and it is hoped that
the tanner will find much valuable information in these pages. The main
outlines of the synthesis of tanning matters should prove of great value
to the chemist engaged in this branch of chemical technology.
The translator takes great pleasure in the acknowledging the valuable
assistance rendered him by Mr. Robin Bruce Croad, A.R.T.C., F.I.C., and
by Mr. Arthur Harvey.
F. G. A. ENNA
CONTENTS
Introduction: Classification of Synthetic Tannins
PART I
SECTION I
The Synthesis of Vegetable Tannins
1. Tannin
2. Digallic Acid
3. Ellagic Acid
4. Depsides
Carbomethoxylation of Hydroxybenzoic Acids
Chlorides of Carbomethoxyhydroxybenzoic Acids
Preparation of Didepsides
Preparation of Tridepsides
Preparation of Tetradepsides
Tannoid Substances of the Tannin Type
Chart showing the Decomposition of Products of Tannin
SECTION II
Synthesis of Tanning Matters
1. Aromatic Sulphonic Acids
2. Condensation of Phenols
Condensation of Hydroxybenzene
Condensation of Dihydroxybenzene
Trihydroxy benzene
Polyhydroxybenzenes
Quinone
Phenolic Ethers
Nitro Bodies
Amino Bodies
Aromatic Alcohols
Aromatic Acids
3. Condensation of Naphthalene Derivatives
4. Condensation of the Anthracene Group
5. Di- and Triphenylmethane Groups
6. Summary
Table
SECTION III
Tanning Effects of Mixtures and Natural Products
1. Mixture of Phenolsulphonic Acid and Formaldehyde
2. Mixture of Phenolsulphonic Acid and Natural Tannins
3. Tanning Effects of Different Natural Substances
SECTION IV
Methods of Examining Tanning Matters
PART II
Synthetic Tannins: Their Industrial Production and Application
A. Condensation of Free Phenolsulphonic Acid
B. Condensation of Partly Neutralised Phenolsulphonic Acid
C. Condensation of Completely Neutralised Phenolsulphonic Acid
D. Condensation of Cresolsulphonic Acid
E. Relative Behaviour of an Alkaline Solution of Bakelite and
Natural Tannins
F. Dicresylmethanedisulphonic Acid (Neradol D)
1. Neradol D Reactions
2. Electro-Chemical Behaviour of Neradol D
3. The Influence of Salts and Acid Contents
on the Tanning Effect of Neradol D
4. Phlobaphene Solubilising Action of Neradols
5. Effect of Neradol D on Pelt
6. Reactions of Neradol D with Iron and Alkalies
7. Reagents suitable for Demonstrating the
Various Stages of Neradol D Tannage
8. Combination Tannages with Neradol D
(1) Chrome Neradol D Liquors
(2) Aluminum Salts and Neradol
(3) Fat Neradol D Tannage
9. Analysis of Leather containing Neradol D
10. Properties of Leather Tanned with Neradol D
11. Neradol D, Free from Sulphuric Acid
12. Neutral Neradol
G. Different Methods of Condensation as Applied to
Phenolsulphonic Acid
1. Condensation Induced by Heat
2. Condensation with Sulphur Chloride
3. Condensation with Phosphorus Compounds
4. Condensation with Aldehydes
5. Condensation with Glycerol
REGISTER OF AUTHORS
INDEX
INTRODUCTION
CLASSIFICATION OF SYNTHETIC TANNINS
In laying down a definition of "Synthetic Tannins," it is first of all
necessary to clearly define the conception of "tannin." Primarily,
tannins may be considered those substances of vegetable origin which may
be found, as water-soluble bodies, in many plants, exhibiting certain
chemical behaviour, possessing astringent properties and being capable
of converting animal hide into leather. This latter property of the
tannins, that of converting the easily decomposable protein of animal
hide into a permanently conserved substance and imparting to this
well-defined and technically valuable properties, has become the
criterion of the practical consideration of a tannin. It appears that
different substances certainly show the chemical reactions peculiar to
the tannins, and to a certain extent also exhibit astringent character
without, however, possessing the important property peculiar to the
tannins of converting hide into leather. Such substances, in our
present-day terminology, are termed pseudo-tannins (_e.g._, the "tannin"
contained in coffee-beans). Decomposition products of the natural
tannins, to which belong, for instance, gallic acid and the
dihydroxybenzenes, exhibit the well-known reactions of the tannins
(coloration with iron salts), but they cannot be regarded as tannins
from either a technical or a physiological standpoint.
As regards their chemical constitution, 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 tannin can be detected; and the yield (of leather, based on the pelt
employed), which, from a practical standpoint, is so important, is so
very low that it is hardly possible to speak of it as a tannin in the
ordinary sense of the word. Formaldehyde must, therefore, be termed a
pseudo-tannin.
The tanning effect of formaldehyde is, according to Thuau, [Footnote:
_Collegium_, 1909, 363, 211.] increased by those salts which bring
about colloidal polymerisation of the formaldehyde, the resultant
compounds being absorbed by the hide fibre. Fahrion considers this to be
a true tannage, and is supported by Nierenstein [Footnote: _Ibid._,
1905, 157, 159.]:--
R.NH_2 R.NH-|
+O.C.H. = CH_2 + H_2O
R.NH_2 | R.NH-|
(Hide.) H (Leather.)
A peculiar combination between true tannage and pickling is to be found
in the tawing process (tannage with potash, alum, and salt), whereby,
firstly, the salt and the acid character of the alum produce a pickling
effect, and secondly, the alum at the same time is hydrolysed, and its
dissociation components partly adsorbed by the hide, thereby effecting
true tannage. This double effect is still more pronounced in the
synthetic tannins which contain colloidal bodies of pronounced tanning
intensity on the one hand, inorganic and organic salts on the other,
which then act as described above. Their real mode of action can only
be explained with the aid of experimental data. The following chapters
will deal with the different behaviour of the various groups of
synthetic tannins.
PART I
SECTION I
THE SYNTHESIS OF VEGETABLE TANNINS
1. TANNIN
The first investigations of gall-tannin date from the year 1770, at
which time, however, no exact differentiation between tannin and gallic
acid was made. The first step in this direction was made when
Scheele,[Footnote: Grell's _Chem. Ann._, 1787, 3, I.] in 1787,
discovered gallic acid in fermented gall extract, and in the same year
Kunzemuller [Footnote:_Ibid._, 1787,3,413.] separated gallic acid (or
pyrogallol) as a crystalline body from oak galls. Dize [Footnote:
_Jour. Chim. et Phys._, 1791, 399.] continued the investigations, which
were brought to a conclusion with Deyeux' work [Footnote: _Ann. Chim._,
1793, 17, I.]; both recognised that the substance isolated was not a
single substance, but was a mixture of gallic acid, a green colouring
matter, a rosin (tannin?), and extraneous matter. Proust [Footnote:
_Ibid._, 1799, 25, 225.] was the first to differentiate the crystalline
gallic acid from the amorphous, astringent substance, which latter he
named "Tannin."
Amongst the numerous subsequent investigations of tannin must be
especially noted the one by Berzelius [Footnote: Pogg,_Ann._, 1827, 10,
257.], who purified the potash salt and decomposed this with sulphuric
acid. Pelouze [Footnote: Liebig's _Ann._, 1843, 47, 358.], later on,
observed the formation of the crystalline gallic acid from tannin, when
the latter is boiled with sulphuric acid; this had already been observed
by J. Liebig.[Footnote: _Ibid._1843, 39, 100.] Both had noticed the
absence of nitrogen. In addition to the methods of preparation of tannin
then in vogue neutral solvents were mainly employed by subsequent
investigators; Pelouze [Footnote: _Jour. Prakt. Chem._, 1834, 2, 301,
and 328.] treated powdered galls with ether containing alcohol and
water, and considered the upper layer to be a solution of gallic acid
and impurities, the bottom layer to contain the pure tannin.
The EMPIRICAL FORMULA of tannin has also been the subject of much
speculation by the different investigators, the difficulty here being
that of obtaining a pure specimen of the substance free from sugars, and
which could be submitted to elementary analysis. Whereas these early
purified substances were thought to correspond to the formula of
digallic acid (galloylgallic acid), C_14H_10O_9, Fischer and Freudenberg
[Footnote: _Ber._, 1912, 915 and 2709.] were able to show, with
approximate certainty, that the constitution of tannin is that of a
pentadigalloyl glucose.
Early attempts at _hydrolysing tannin_ gave varying results, some
investigators claiming the presence, and others the absence of
sugars. Here, again, E. Fischer and Freudenberg [Footnote: _Ibid._] were
able to conclusively prove that on hydrolysing tannin with dilute acids,
7.9 per cent. glucose is dissociated, and that hence glucose forms part
of the tannin molecule. Fischer and Freudenberg also determined the
optical activity of pure tannin in water: [Greek: a]_D was found to lie
between +58° and +70°.
Graham found [Footnote: _Phil. Transact._, 1861, 183.] that the _tannin
molecule_ is of considerable size, since its diffusion velocity is 200
times less than that of common salt. Paternň [Footnote:
_Zeits. phys. Chem._, 1890, iv. 457.] was the first to determine the
molecular weight of tannin, employing Raoult's method; he found that
tannin in aqueous solution behaves like a colloid and that hence
Raoult's method is not applicable. When, on the other hand, he dissolved
tannin in acetic acid, results concordant with the formula of
C_14H_10O_9, corresponding to a molecular weight of 322, were
obtained. Sabanajew [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.
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