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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. Paternò [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.
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 C_98 H_82 N_14 O_96) and proposed the formula, C_58 H_40 O_33,
the constitution of which would be--
R_1 R_1
| |
}C--O--O--C{
| | |
R_2 | R_2
O
R_1 | R_1
| | |
}C--O--O--C{
| |
R_2 R_2
where R1= CO C_6 H_2 (OH)_3
and R2= C_6 H_2 (OH)_2
Nierenstein [Footnote: _Ber._, 1905, 38, 3841; 1907, 40, 917; 1908, 41,
77 and 3015; 1909, 42, 1122 and 3552; _Chem. Ztg._, 1907, 31, 72; 1909,
34, 15.] considers tannin to be a mixture of digallic acid and
leucotannin, the latter possessing the formula--
^-------CH.OH--O----^ OH
| | | |
HO V OH HOOC V OH
OH
The optical activity of tannin is expressed in this formula and its
probability is corroborated by Nierenstein, who was able to resolve the
acetylated tannin by fractional precipitation into pentacetyl tannin
(M.P. 203°-208° C.) and pentacetyl leucotannin (M.P. 166° C.). By
oxidation, the former is converted into ellagic acid, and on hydrolysis
with dilute sulphuric acid readily yielded gallic acid. Hydrolysis of
the pentacetyl leucotannin, however, yielded gallic aldehyde, and
oxidation yielded purpurotannin (a naphthalene derivative) in addition
to ellagic acid.
Nierenstein [Footnote: _Ber._, 1910, 43, 628.] also succeeded in
converting tannin into carboethoxytannin, the latter on saponification
yielding crystalline, inactive digallic acid. On acetylating pentacetyl
leucotannin with acetyl chloride a hexacetyl derivative (M.P. 159° C.)
is obtained, the strychnine salt of which is resolved into both of the
active components. This proves the presence of digallic acid and
leucotannin in tannin lev. pur. Schering investigated by Nierenstein.
The latter author [Footnote: Liebig's _Ann._, 1912, 386, 318; 388, 223.]
later considered tannin to be polydigalloylleucodigallic acid anhydride
and the simplest tannin to be a digalloylleucodigallic acid
anhydride. This view, however, would not stand subsequent criticisms,
being in disagreement with the earlier observations of molecular weight
and acidic properties of tannin. Manning [Footnote: _Ibid._, 1912, 34,
918.] believed to have isolated a pentethylester of the pentagalloyl
glucoside from tannin, but this was shown to be the ethyl ester of
gallic acid.
Feist [Footnote: _Ber._, 1912, 45, 1493.] had arrived at the conclusion
that tannin was a glucose compound, and maintained that tannin from
Turkish galls was a compound of glucogallic acid combined as an ester
with 2 molecules gallic acid. But Fischer and Strauss [Footnote:
_Ibid._, 1912, 45, 3773.] synthetically prepared a glucoside of gallic
acid exhibiting differences from Feist's preparation which were so great
that the latter no longer could be considered a single glucoside of
gallic acid.
Fischer and Freudenberg [Footnote: _Ibid._, 1912, 45, 2717; 1913, 46,
1127.] subsequently elaborated a method of purifying tannin, and on
investigating the purified substance, arrived at the conclusion that
no other hydroxybenzoic acid than gallic acid was present in
tannin. On repeating Strecker's hydrolysis they obtained 7-8 per cent,
sugar, and hence concluded that 1 molecule of glucose was combined
with about 10 molecules of gallic acid. Owing to the difficulty of
isolating the intermediary hydrolysis products, and the subsequent
impossibility of drawing any conclusions as to the constitution of
tannin, the latter investigators decided to adopt the methods offered
by synthesis. Their basic idea was the absence of carboxylic groups in
tannin, and that hence the total gallic acid must be present in ester
form. These conditions are fulfilled if one views tannin as being an
ester compound of 1 molecule of glucose and 5 molecules of digallic
acid, of similar construction as, for example, pentacetyl
glucose. Fischer and Freudenberg succeeded in preparing the former by
shaking a mixture of finely powdered glucose, chloroform, and
quinoline with an excess of tricarbomethoxygalloyl chloride for
twenty-four hours and precipitating the resulting product with methyl
alcohol; suitably purified, a light amorphous colourless substance was
obtained which proved to be penta-(tricarbomethoxygalloyl)
glucose. Careful saponification with excess alkali in acetone-aqueous
solution at room temperature yielded a tannin very closely resembling
tannin, identified as pentagalloyl glucose. It is doubtful, however,
whether this substance is homogeneous, and it is probably a mixture of
two stereoisomers.
Fischer and Freudenberg, therefore, further concluded that tannin is
mainly an ester compound of glucose and 5 molecules _m_-digallic
acid. Elucidation on this point offered itself advantageously in
Herzwig's methylotannin, [Footnote: _Ber._, 1905, 38, 989.] which is
obtained by the interaction of diazomethane and tannin. The first step
was then to prepare pentamethyl-_m_-digallic acid
CH_3.O_______ ______COOH
CH_3.O{_______}--CO.O--{______}
CH_3.O CH_3.O O.CH_3
from trimethylgalloyl chloride and the _m-p_-dimethyl ether of gallic
acid; the chloride of this substance, coupled with [Greek: a]- and
[Greek: b]-glucose, yields--
_CH.OR
| |
| CH.OR H_______O.CH_3
| | R=CO{_______}O.CH_3
O{ CH.OR H O
| | H_____O.CH_3
| CH CO{_____}O.CH_3
| | H O.CH_3
|_CH.OR
CH_2.OR
[Illustration: Penta-(pentamethyl-_m_-digalloyl)-glucose.]
The [Greek: a]- and [Greek: b]-derivatives thus obtained differ in their
behaviour towards polarised light, and are, again, probably mixtures of
two stereoisomers, _i.e._, mixtures of derivatives of [Greek: a]- and
[Greek: b]-glucose. Compared to methylotannin, these preparations
exhibit very close resemblance to the former, from which it may be
concluded that they are closely related to this substance, and probably
possess the same or a very similar structure; the result of the above
experiments has, therefore, brought us at least in close proximity to
the structure of tannin. It must, however, be borne in mind that the
analysis and hydrolysis of tannin does not afford an explanation of the
question as to whether tannin is a compound of glucose and 10, 9, or 11
molecules of gallic acid; it is also possible, though not probable, that
tannin would contain a polysaccharide instead of glucose
itself. Similarly to sugar, the true glucosides can be coupled with
hydroxybenzoic acids, which is proved by the preparation of
tetra-galloyl-[Greek: a]-methyl glucoside; this substance, also,
exhibits tannoid character.
2. DIGALLIC ACID
Whereas, until recently, tannin had been considered to be gallic acid
anhydride, or digallic acid, closer investigations have revealed that
neither is tannin digallic acid nor is the synthetically prepared
digallic acid identical with tannin. Schiff [Footnote: _Ber._, 1871,
231 and 967.] prepared digallic acid by the interaction of phosphorus
oxychloride and gallic acid, and believed the product obtained to be
identical with tannin; to this latter he first ascribed an ether formula
(I.), later an ester formula (II.)--
(OH)_2 (OH)_2
¦¦ ¦¦
C_6H_2---0---C_6H_2
¦ ¦
COOH COOH
(I.)
(OH)_2
¦¦
C_6H_2(OH)_3--C--O.C_6H_2
¦¦ ¦
O COOH
(II.)
Froda [Footnote: _Gasz. chim._, 1878, 9.] held that Schiff's
condensation product contained phosphorus or arsenic acid and ascribed
its tanning properties to the latter; according to this investigator,
digallic acid, when completely freed from arsenic acid, does not react
with gelatine or quinine. Biginelli [Footnote: _Ibid._, 1909, 39,
ii. 268 and 283.] did not consider the action of arsenic acid that of a
catalyst, but held that it entered into reaction; according to his
investigations products containing arsenic (C_7H_7O_8As and
C_14H_11O_12As) are obtained when gallic acid is heated with arsenic
acid.
In his preparation of digallic acid, Iljin [Footnote:
_Jour. f. prakt. Chem._, 1911, 82, 451.] could only obtain gallic acid,
and the ethyl ether of gallic acid showing no characteristics of the
tannins; when, however, he heated gallic acid with arsenic pentoxide, he
obtained bodies exhibiting the reactions given by tannins.
Bottinger [Foonote: _Ber._, 1884, 1503.] made the first attempt at
synthesising tannin; he heated gallic acid or its ethyl ester with
glyoxylic acid or pyroracemic acid, and obtained a substance of the
composition C_14H_10O_9.2H_2O, which certainly showed some of the
characteristics exhibited by tannin, but which by no means was identical
with the latter. Bottinger's preparation is probably identical with
[Greek: b]-digallic acid, one of two dibasic isomers having the
composition--
C_6H_2(OH)_2COOH
|
C_6H(OH)_3COOH
the other possible isomer having the composition
C_6H(OH)_3COOH
CO |
C_6H_2(OH)_3
Fischer [Footnote: _Ber_., 1908, 41, 2875.] obtained a digallic acid
(M.P. 275°-280° C) by coupling tricarbomethoxygalloyl chloride with
dicarbomethoxygallic acid.
Nierenstein [Footnote: _Ibid_., 1910, 43, 628.] obtained, from the
carbethoxy compound of tannin, a crystalline, optically active digallic
acid, M.P. 268°-270° C. The pentacetate of this substance, obtained by
reduction and acetylisation, yielded hexacetylleucotannin. A
pentamethyldigallic acid methyl ester of the composition
((O.CH_3)_3)C_6H_2----COO-----C_6H_2((OCH_3)_2)COO.CH_3
was obtained by Mauthner [Footnote: _Jour. f. prakt. Chem_., 1911, 84,
140.] from the chloride of trimethylgallic acid and the methyl ester of
the acid from the glucoside of syringin; on saponification with caustic
potash the former compound yielded trimethylgallic acid and syringic
acid.
Fischer [Footnote: _Ber_., 1913, 46, 1116.] synthesised the so-called
_m_-digallic acid by coupling tricarbomethoxygalloyl chloride with
carbonylgallic acid and subsequent splitting off of CO_2. The
_m_-digallic acid appears as rather thick, colourless, microscopic
needles containing about 16 per cent. water of crystallisation,
M.P. 271° C. They are slightly soluble in cold, soluble in hot water,
and very soluble in methyl and ethyl alcohols. Their aqueous solution
gives dark blue coloration with ferric chloride, and precipitates
gelatine and quinine.
Fischer and his students [Footnote 5: _Ibid_., 1912, 45, 915, 2709;
1913, 46, 1116.] prepared quite a number of digallic acid derivatives,
amongst which are the following:--
Pentamethyl-_m_-digallic acid methyl ester, C_20H_22O_9.
Pentacetyl-_m_-digallic acid, C_24H_20O_14.
Pentamethyl-_m_-digallic acid, C_19H_20O_9.
Pentamethyl-_m_-digalloyl chloride, C_19H_19O_8Cl.
Pentamethyl-_p_-digallic acid, C_19H_20O_9.
Pentamethyl-_p_-digallic acid methyl ester, C_20H_22O_9.
Hydrolysis of digallic acid yields gallic acid; oxidation, on the other
hand, ellagic acid and luteic acid (Luteo Säure), which can be separated
by shaking with pyridine. The reduction of digallic acid yields, by
different methods, the same reduction compound, [Footnote: Nierenstein,
Abderhalden's "Handb. d. biochem. Arbeitsm.," vi. 154.] viz., the
racemic leucodigallic acid, which differs from digallic acid by being
devoid of any tannoid properties; the latter distinction may be ascribed
to the transformation of the tannophor group--CO.O--, to the
tannoid-inactive group CH(OH)--O--.
The successful resolving of racemic leucodigallic acid into both of its
optically 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 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
| | | |
HO V ---O.CO--- V OH
OH OH
By heating ellagic acid for three-quarters of an hour at 185° C. with
concentrated sulphuric acid, ceruleo-ellagic acid (dioxyellagic acid),
C_14H_6O_10, is formed as yellowish needles, M.P. 360° C., which are but
little soluble in the usual solvents. The acid is slightly soluble in
strong caustic soda solution, the colour of the solution, on diluting,
changing to green and blue.
LUTEIC ACID (Luteo Saure, pentoxybiphenylmethylolide carboxylic
acid),C_14H_8O_9, occurs, in addition to ellagic acid, in myrabolams--
[Footnote: _Ber_., 1909, 42, 353.]
----CO.O----
^ ---------- ^ OH
| | | |
HO V OH HOOC V OH
OH OH
It is obtained by extracting myrabolams for one hour and a half, under
reflux condenser, with pyridine, filtering and adding twice the volume
of water to the filtrate and boiling till complete solution is
obtained. After about thirty hours a reddish powder deposits, from which
ellagic acid may be extracted with pyridine; the mother-liquor on being
concentrated yields luteic acid. It is also obtained by oxidising tannin
with hydrogen peroxide, the other oxidation product being ellagic acid,
and the two may then be separated as indicated above. Luteic acid forms
reddish needles which are decomposed, with evolution of gas, at
338°-341° C. Heated with 10 per cent. caustic soda solution it yields
ellagic acid. In pyridine solution the carboxyl group maybe eliminated
by hydrogen iodide, whereby pentoxybiphenylmethylolide is formed as long
silky needles, which do not melt below 300° C. The same substance may
also be obtained when ellagic acid is boiled with concentrated caustic
potash solution. When luteic acid is treated with diazomethane, it
yields the methyl ester of pentamethoxybiphenylmethylolidcarboxylic
acid.
4. DEPSIDES
The most common decomposition products of the natural tannoids are
hydroxybenzoic acids, notably gallic and proto-catechuic acids;
furthermore, other aromatic and aliphatic hydroxy compounds frequently
occur. So far, however, attempts at explaining the constitution of the
complex decomposition products obtained by hydrolysing high molecular
tannoids have not been successful. On the other hand, the constitution
of the simpler natural tannoids is known to a greater or less extent; of
these, lecanoric acid (Lecanorsäure) is the best known, being an ester
anhydride of orsellic acid (a dihydroxytoluylic acid). It combines with
erythrite, forming another tannoid, erythrine. The fact that
hydroxybenzoic acids are constantly encountered together with the
products obtained on hydrolysis of the tannins, seems to point toward
the conclusion that anhydrides of hydroxybenzoic acids are frequent
constituents of the natural tannoid molecules.
The assumption that, for instance, in tannin at least part of the gallic
acid radicals are combined with one another is highly probable, and is
supported by the formation of tri- and dimethylgallic acid from
methylotannin, [Footnote: Herzig, _Monatshefte f. Chemie_, 1909, 30,
343.] and by the formation of ellagic acid when tannin is oxidised.
[Footnote: Nierenstein, _Ber_., 1908, 41, 3015.] Further proof is
brought forward by the existence of the pentacetyl-tannin, [Footnote:
Schiff, _Ann. d. Chem_., 1873, 170, 73.] and by the results of
hydrolysis which has yielded up to 104 per cent. anhydrous gallic acid
fiom tannin [Footnote: Sisley, _Bull. Soc. Chim_. 1909, 5, 727.]
Of the three classes of isomeric anhydrides which can be formed from
hydroxybenzoic acids, the chemistry of the natural tannins is only
concerned with the class comprising the ester anhydrides. If the
carboxyl of the first molecule combines with a hydroxyl of the second
molecule (ester formation), then a substance possessing character
similar to that of a hydroxybenzoic acid is formed, which is capable of
combining up with a further molecule in the same way. It is natural to
assume that this ester form is much more prevalent in Nature than a
combination of two carboxyls by the elimination of water. From the point
of view of the chemistry of the tannins, therefore, the starting-point
would naturally be that of synthesising the ester anhydrides of
hydroxybenzoic acids. Amongst the small number of synthetically prepared
ester anhydrides of hydroxybenzoic acids, a few occur exhibiting the
properties of the natural tannoids.
In order to simplify the terminology of these substances, Fischer
[Footnote: Liebig's _Ann_., 1910, 372, 35.] proposed the name
"Depsides" from [Greek: depheiv] = to tan. In analogy with peptides
and saccharides, the names di-, tri-, and polydepsides of hydroxybenzoic
acids would be suitable for these substances.
The principles underlying the synthesis of depsides are the
following:--If the chlorides of carbomethoxy (or carbethoxy)
hydroxybenzoic acids are coupled with the sodium salts of hydroxybenzoic
acids, esters are formed, _e.g._,
CH_3CO O.O.C_6H_4.CO.Cl + NaO.C_6H_4.COO.Na = NaCl
+ CH_3.COO.O.C_6H_4.CO.O.C_6H_4.COO.Na
On gently saponifying the esters, these are converted into the
corresponding hydroxy derivatives--
OH.C_6H_4.CO.O.C_6H_4.COOH
According to Fischer and Freudenberg, [Footnote: Liebig's _Ann._, 1909,
372, 32.] this method possesses the following advantages:--
1. The synthesis takes place at low temperatures, so that any
intramolecular rearrangements are improbable.
2. The composition of the substances is controlled by the intermediary
compounds, the carboalkyloxy derivatives.
3. The synthesis permits of more definite evidence as regards the
structure of the resulting compounds.
4. The substances obtained are easily purified.
Depsides produced in this manner are by no means new, and were obtained
by Klepl by simply heating _p_-hydroxy-benzoic acid (_cf._ Introduction,
p. 4). This simple procedure, however, is not applicable to most other
hydroxybenzoic acids which are decomposed at the high temperature
necessary to induce reaction. Lowe and Schiff (_loc. cit._) have
obtained products similar to tannins, the latter investigator by
removing the elements of water from gallic acid, protocatechuic acid,
salicylic acid, _m_-hydroxybenzoic acid, cresotinic acid, phloretinic
acid, and pyrogallolcarboxylic acid. These depsides, however, are
amorphous substances, and it is hence difficult to substantiate their
homogeneity.
Carbomethoxylation of Hydroxybenzoic Acids
Amongst other compounds chlorphydroxybenzoic acid is used in the
preparation of the materials employed in the synthesis of depsides; the
free phenolic group, however, exerts a disturbing influence when
aromatic acids are acted upon by phosphorus chloride, and another group,
which can subsequently be easily removed, must therefore be introduced
to cover the disturbing influence referred to. For this purpose, Fischer
[Footnote: _Ber_., 1908, 41, 2860.] chose the carbomethoxy group, and
this investigator succeeded, by the action of chlorocarbonic alkyl
ester and alkali upon hydroxybenzoic acid in cold aqueous solution, in
obtaining substances with the properties required. [Footnote: _Ber._,
1908, 41, 2875.] In such substances (_e.g._, salicylic acid) where the
hydroxyl occupies the ortho-position to the carboxyl, complete
carbomethoxylation does not take place, whereas the _m_- or _p_-
positions offer no hindrance. In the case of the _o_-position, however,
the action of chlorocarbonic alkyl ester is successfully assisted by the
presence of dimethylaniline in an inert solvent, _e.g._,
benzene.[Footnote: U.S. Pat, 1,639,174, 12, xii., 1899.] The difficulty
encountered by the _o_-position is eliminated when the carboxyl is not
directly linked to the benzene nucleus, _e.g._, _o_-cumaric acid. Many
hydroxybenzoic acids require an excess of chlorocarbonic methyl ester,
which then also, to some extent, attacks the carboxyl group; but on
dissolving the product in acetone and treating it with bicarbonate the
carboxyl group as such is again restored without splitting off the
carbomethoxy group.[Footnote: _Ber._, 1913, 46, 2400.] In this way all
hydroxybenzoic acids may be carbomethoxylated. [Footnote: _Ibid._,
1908, 41, 2877, 2881, 2882; 1909, 42, 226, 218, 223, 225; Liebig's
_Ann._, 1912, 391, 357, 366; _Ber._, 1913, 46, 1145, 2390, 2400.] The
carbomethoxy group is easily removed by excess of aqueous alkali in the
cold, and is also partially removed when insufficient alkali is present;
the latter fact is of importance in the synthesis of didepsides.
Chlorides of Carbomethoxyhydroxybenzoic Acids
The chlorides of these compounds are obtained when phosphorus
pentachloride is allowed to act upon the acids, and are as a rule
crystalline. For the purpose of synthesis they may be employed as
follows:
1. They readily form esters with alcohols, which on subsequent
saponification with alkali are converted into the esters of the free
hydroxybenzoic acids.
2. The chlorides interact energetically with esters of amino-acids, and
may be coupled with amino-acids in aqueous alkaline solution. On
subsequently removing the carbo-methoxy group derivatives of
hydroxybenzoic acids are obtained, _e.g._,
CH_3.CO_2.O.C_6H_4.CO.Cl + 2NH_2CH_2.CO.C_2H_5
= NH_2.CH_2.CO_2.C_2H_5 + HCl + CH_3.CO_2.O.C_6H_4 CO.NH.CH_2CO_2C_2H_5.
CH_3.CO_2.O.C_6H_4.CO.NH.CH_2.CO_2.C_2H_5 + 3NaOH
= Na_2CO_3 + C_2H_5OH + CH_3OH + HO.C_6H_4.CO.NH.CH_2.COONa.
3. In the presence of AlCl_3 the chlorides easily combine with benzene,
and on removing the carbomethoxy group unsymmetrical hydroxy derivatives
of benzophenone are formed:--
CH_3.CO_2.O.C_6H_4.CO.Cl + C_6H_6 = CH_3.CO_2.O.C_6H_4.CO.C_6H_5 + HCl
CH_3.CO_2.O.C_6H_4.CO.C_6H_5 + 3NaOH
= NaO.C_6H_4.CO.C_6H_5 + Na_3CO_3 + CH_3OH + H_2O
4. The chlorides may be coupled with free hydroxybenzoic acids, and on
removing the carbomethoxy group didepsides are obtained. Repetition of
these operations yields tri- and tetradepsides.
Preparation of Didepsides
A simple application of these syntheses is offered by _p_-hydroxybenzoic
acid. When the chloride of its carbomethoxy derivative is allowed to
interact with _p_-hydroxybenzoic acid in aqueous alkaline solution, in
the cold, the alkali salt of carbomethoxy-_p_-hydroxybenzoic acid is
formed:--[Footnote 1: _Ber._, 1909, 42, 216.]
CH_3.CO_2.O.C_6H_4.CO.Cl + NaO.C_6H_4.COONa
= CH_3.CO_2.O.C_6H_4.CO_2.C_6H_4.CO_2.Na + NaCl.
Being sparingly soluble, the salt in this case is readily deposited as
crystals, but is readily converted into the free acid by hydrochloric
acid. In most other cases, however, the alkali salts are easily soluble
and the aqueous solution is then directly acidified with a mineral
acid. The chlorides, being for the most part solids, the mode of
procedure is as follows:--the hydroxybenzoic acid required for coupling
is dissolved in normal or double-normal alkali (the volume calculated
per molecule acid), a little acetone added, and the mixture well cooled;
a further molecule of 2N caustic soda and the chloride (I molecule)
dissolved in dry acetone are added in small portions, whilst stirring,
to the mixture. In spite of the low temperature the coupling proceeds
quickly and the sparingly soluble product can in most cases be
precipitated from the solution by acidifying and diluting with water. In
case of more easily soluble coupling products the acetone is driven off
under reduced pressure or the liquid acidified and diluted, and the
substance extracted with ether. Instead of alkali, dimethylaniline may
be employed, with the exclusion of water as a solvent for the purpose of
coupling.
Another suitable method of obtaining _o_-didepsides is that of treating
_o_-hydroxybenzoic acids with phosphorus trichloride and dimethylaniline
(_e.g_., synthesis of disalicylic acid, Boehringer & Sons).[Footnote:
Ger. Pat., 211,403.]
The carbomethoxy derivatives of the depsides are as a rule crystalline
substances of distinct acidic character, and decompose alkaline
carbonates.
The elimination of the carbomethoxy group may be brought about by dilute
alkaline solutions in the cold, or by aqueous ammonia. If the depside
formed is so stable as to resist the action of alkali for several hours,
the use of the latter is very convenient for the purpose required. The
substance is dissolved directly in sufficient normal alkali to
neutralise the carboxyl group and a further 2 molecules of caustic soda
for each carbomethoxy group to be eliminated are added. The temperature
should be about 20° C., when the reaction as a rule is completed after
one-half to three-quarters of an hour. It is usual, however, to use an
aqueous ammonia solution in considerable excess, whereby the temperature
should again be about 20° C., and the solution of ammonia normal or half
normal.
The didepsides so far investigated are crystalline bodies, sparingly
soluble in cold water; they--as a rule--decompose when fused, possess
acid reaction, and are dissolved by bicarbonates. On account of the
presence of a free phenolic group they give a coloration with ferric
chloride; if the phenolic group occupies the _o_-position to carboxyl,
the coloration with ferric chloride is red or bluish-violet Excess of
dilute alkali resolved all didepsides into their components at ordinary
temperatures. The didepsides of gallic, proto-catechuic, gentisinic, and
[Greek: b]-resorcylic acids precipitate gelatine and quinine acetate,
and in this respect approach the natural tannins.
The following summary gives an account of depsides which have been
prepared synthetically or which occur naturally:--[Footnote 1: _Ber._,
1908, 41, 2888; 1909, 42, 217; 1913, 45, 2718; 1913, 46, 1130, 2396,
1141, 1143; Liebig's _Ann._, 384, 230, 233, 238; 391, 356, 362.]
Di-_p_-hydroxybenzoic acid.
Di-_m_-hydroxybenzoic acid.
Disalicylic acid.
Diprotocatechuic acid.
Digentisinic acid.
Di-[Greek: b]-resorcylic acid.
_p_-Diorsellic acid.
_o_-Diorsellic acid.
_m_-Digallic acid.
Disyringic acid.
Di-_o_-cumaric acid.
Diferulic acid.
Di-[Greek: b]-hydroxynaphthoic acid.
_p_-Hydroxybenzoyl-_m_-hydroxybenzoic acid.
_m_-Hydroxybenzoyl-_p_-hydroxybenzoic acid.
Salicyl-_p_-hydroxybenzoic acid,
Vanilloyl-_p_-hydroxybenzoic acid.
Feruloyl-_p_-hydroxybenzoic acid.
[Greek: a]-Hydroxynaphthoyl-_p_-hydroxybenzoic acid.
Orsellinoyl-_p_-hydroxybenzoic acid.
Protocatechuyl-_p_-hydroxybenzoic acid.
Galloyl-_p_-hydroxybenzoic acid.
Pyrogallolcarboy _p_-hydroxybenzoic acid.
Syringoyl-_p_-hydroxybenzoic acid.
_p_-Hydroxybenzoyl-syringic acid.
Pentamethyl-_m_-digallic acid.
Pentamethyl-_p_-digallic acid.
Vanilloyl vanillin.
Preparation of Tridepsides
Monohydroxybenzoic acids allow theoretically of tri-depsides of the type
HO.C_6H_4COO.C_6H_4.COO.C_6H_4.COOH only; if, on the other hand, di- or
trihydroxybenzoic acids are dealt with, two formulae are possible,
viz.:--
HO.C_6H_4.COO
} C_6H_3.COOH
HO.C_6H_4.COO
Of the former type, two compounds are known, _i.e._,
di-_p_-hydroxybenzoyl-_p_-hydroxybenzoic
acid and vanilloyl-_p_-hydroxybenzoyl-_p_-hydroxybenzoic acid--
HO
} C_6H_3.COO.C_6H_4.COO.C_6H_4.COOH
CH_3O
The first named of these two compounds was obtained by Klepl, in
addition to the didepside, by heating _p_-hydroxybenzoic acid. Fischer
and Freudenberg obtained a beautifully crystalline form in the following
way: carbethoxyhydroxy-benzoyl chloride was coupled with
_p_-hydroxybenzoyl-_p_-hydroxybenzoic acid in alkaline solution, the
compound dissolved in a mixture of pyridine and acetone, and ammonia
added for the purpose of removing the carbethoxy group. The tridepside
was then obtained as long needles by re-dissolving in acetone.
Both tridepsides melt well above 200° C., are practically insoluble in
water, and are but sparingly soluble in practically all organic
solvents. In alcoholic solution they give colour reaction with ferric
chloride similar to those given by _p_-hydroxybenzoic acids.
Preparation of Tetradepsides
[Footnote: Fischer and Freudenberg, Liebig's _Ann._, 1910, 372, 32.]
Here, again, two forms are known, _e.g._,
tri-_p_-hydroxybenzoyl-_p_-hydroxybenzoic acid--
HO.C_5H_4.COO.C_6H_4.COO.C_6H_4COO.C_6H_4 COOH
and vanilloyl-di-_p_-hydroxybenzoyl-_p_-hydroxybenzoic acid--
HO
} C_6H_3.COO.C_6H_4.COO.C_6H_4.COO.C_6H_4.COOH
CH_3O
The former has been prepared from
carbethoxyhydroxy-benzoyl-_p_-hydroxybenzoyl chloride and
_p_-hydroxybenzoyl-_p_-hydroxybenzoic acid in alkaline solution; the
second tetradepside was prepared from
carbomethoxyvanilloyl-_p_-hydroxybenzoyl chloride and
_p_-hydroxybenzoyl-_p_-hydroxy-benzoic acid.
The preparation of these compounds is rendered difficult by the slight
solubility of the substances and their slight affinities for entering
into reaction. Both tetradepsides were obtained in crystalline form, and
are but very little soluble in most organic solvents. They decompose on
being fused.
Tannoid Substances of the Tannin Type
The preparation of pentagalloyl glucose has proved this compound to be
nearly identical with tannin obtained from galls (_tannin_); a few other
natural tannins belong to this type which Fischer terms acyl compounds
of sugar with hydroxybenzoic acids. The method of preparation employed
in the synthesis of pentagalloyl glucose may be easily applied to other
hydroxybenzoic acids, _e.g._ penta[_p_-hydroxybenzoyl] glucose
[Footnote: Fischer and Freudenberg, _Ber._, 1912, 45, 933.] was
prepared in this way. Similar characteristics are exhibited by
pentasalicylo glucose. Mention must also be made of the corresponding
derivative of pyruvic acid and the compound with pyrogallolcarboxylic
acid, penta-[pyrogallolcarboyl]glucose. [Footnote: Fischer and
Rapoport, _Ber._, 1913, 46, 2397.] The latter is isomeric with
pentagalloyl glucose and possesses similar properties; there is,
however, a vast difference in the solubility of the two. Whereas the
galloyl compound is easily soluble in cold water, its isomer is hardly
soluble in hot, and completely insoluble in cold water. Considering the
very similar structure of these two tannins, such differences appear
surprising, but an analogy may be readily found in the existence of
colloidal solutions of tannin and the (nearly) identical pentagalloyl
glucose. These properties clearly show how dependent is the colloidal
state on small differences in the structure of two substances. On the
other hand, the formation of hydrosols is of the greatest importance
relatively to the part played by these substances in Nature as well as
relating to their chemical characteristics; thus it is extremely
difficult to make a solution of penta-[pyrogallolcarboyl]-glucose, at
the same time ascertaining its astringent taste and its property of
precipitating gelatine.
The experience gained by the methyl glucosides makes it exceedingly
probable that the simpler polyhydric alcohols also are suitable
substances to employ in these syntheses; as a matter of fact, glycerol
has been condensed with gallic acid. [Footnote: Fischer and Freudenberg,
_Ber., 1912, 45, 935.]
One of the chief characteristics of synthetic tannins is their high
molecular weight; for instance, the molecular weight of
penta-[tricarbomethoxygalloyl]-glucose is 1,810, that of
penta-[pentamethyl-_m_-digalloyl]-glucose 2,051. Employing gallic acid
derivatives, especially the tribenzoyl compounds, coupled with glucose,
_e.g._, mannite, yielded a neutral ester of molecular weight 2,967.
The determination of the elementary composition of compounds of high
molecular weight is greatly facilitated by employing their halogen
derivatives; so, for instance, is _p_ iodophenyl maltosazone very
suitable. Coupling the latter with tribenzoylgalloyl chloride yielded
hepta-[tribenzoyl-galloyl]-_p_-iodophenyl maltosazone, the structure of
which is represented by--
CH:N_2H.C_6H_4I
|
C:N_2H.C_6H_4I
|
CH.O.R R = CO.C_6H_2(O.CO.C_6H_6)_2
|
CH.O.R
|
CH.O.R R R R R
| O O O O
| | | | |
CH_2.O.CH.CH.CH.CH.CH.CH_2
| |
---O---
The molecular weight of this substance is 4,021, and probably represents
the highest molecular organic body obtained in any chemical synthesis.
From a physiological standpoint the recognition of tannins as esters of
glucose and hydroxybenzoic acids, possessing characteristics similar to
those of tannin, is of great importance. Especially interesting appears
the fact of plants utilising sugars for the esterification of acids,
just as glycerol or monohydric alcohols may be employed for the same
purpose. Free acids, as a rule, are only tolerated in certain parts of
the organism, the latter usually striving to neutralise acidic groups
which may be brought about by salt formation; formation of amino
compounds (proteins) or esterification (fats); and, lastly,
esterformation by means of sugars.
Why Nature should always build up substances of very complex
constitution can only be explained by biochemical investigations, but it
may, at any rate, be assumed that by this means any substance poisonous
to the living organism is rendered inactive. The function of the tannins
present in plants may thus be explained; if, for instance, phenols are
formed by the oxidation of corresponding sugars, [Footnote: Mielke,
"Ueber die Stellung der Gerbstoffe im Stoffwechsel der Pflanzen"
(Hamburg, 1893).] the poisonous character of the former would be
lessened by the introduction of the carbonic acid esters and subsequent
coupling of the substances (depside formation). The depsides thus formed
would serve as vehicle of the sugars and transport the migrating
tannins, [Footnote: Kraus, "Grundlinien zu einer Physiologie der
Gerbstoffe" (1889).] and, after subsequent deposition of the sugars,
would then be eliminated from the plant organism, either by oxidation
into ellagic acid and phlobaphenes or by condensation with the formation
of cork.
Diagrammatically, the following would represent the physiology of the
tannins:--[Footnote: Nierenstein, "Chemie der Gerbstoffe" (Stuttgart,
1910).]
Sugar-->Phenol-->Hydroxybenzoic Acid-->Depside-->
|Phlobaphene
-->Migrating Depside-->Glucoside-->Free Depside-->-{Ellagic Acid
|Cork.
[Illustration: Chart Showing the Decomposition of Products of Tannin.]