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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.C6H4.SO2.O.C6H4HSO3. 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.NH2 R.NH- +O.C.H. = CH2 + H2O R.NH2 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. Patern [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:—
HOOH 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.). Knig 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, C6H5-COO-C6H5.
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—
R1 R1 }C O O C{ R2 R2 O R1 R1 }C O O C{ R2 R2
where R1= CO C6 H2 (OH)3 and R2= C6 H2 (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
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 C6H2—-0—-C6H2 COOH COOH (I.)
(OH)2 C6H2(OH)3—C—O.C6H2 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 (C7H7O8As and C14H11O12As) 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 C14H10O9.2H2O, 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, C20H22O9. Pentacetyl-m-digallic acid, C24H20O14. Pentamethyl-m-digallic acid, C19H20O9. Pentamethyl-m-digalloyl chloride, C19H19O8Cl. Pentamethyl-p-digallic acid, C19H20O9. Pentamethyl-p-digallic acid methyl ester, C20H22O9.
Hydrolysis of digallic acid yields gallic acid; oxidation, on the other hand, ellagic acid and luteic acid (Luteo Sure), 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
-CO.O - ^ ^ OH HO V OH COOH V OH OH OH
-CO.O - ^ ^ OH HO V O.CO V OH OH OH
COOH COOH ^ ^ HO V -O - V OH OH OH
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. 329-336C. 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 (Lecanorsure) 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.,
CH3CO O.O.C6H4.CO.Cl + NaO.C6H4.COO.Na = NaCl + CH3.COO.O.C6H4.CO.O.C6H4.COO.Na
On gently saponifying the esters, these are converted into the corresponding hydroxy derivatives—
OH.C6H4.CO.O.C6H4.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:—
CH3.CO2.O.C6H4.CO.Cl + C6H6 = CH3.CO2.O.C6H4.CO.C6H5 + HCl CH3.CO2.O.C6H4.CO.C6H5 + 3NaOH = NaO.C6H4.CO.C6H5 + Na3CO3 + CH3OH + H2O
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.]
CH3.CO2.O.C6H4.CO.Cl + NaO.C6H4.COONa = CH3.CO2.O.C6H4.CO2.C6H4.CO2.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.C6H4COO.C6H4.COO.C6H4.COOH only; if, on the other hand, di- or trihydroxybenzoic acids are dealt with, two formulae are possible, viz.:—
HO.C6H4.COO } C6H3.COOH HO.C6H4.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.C5H4.COO.C6H4.COO.C6H4COO.C6H4 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.
SECTION II
SYNTHESIS OF TANNING MATTERS
1. AROMATIC SULPHONIC ACIDS
In organic chemistry distinction is made between sulphonic acids of the aliphatic and the aromatic series, the characteristic group of these acids being the so-called _sulphonic acid group_, HSO_3.
When sulphides or mercaptans in glacial acetic acid solution are heated with permanganate, the resulting sulphonic acid compounds exhibit great similarity to compounds containing free carboxyl groups. The sulphonic acid group may also be directly introduced either by concentrated, or by fuming sulphuric acid, or by elimination of halogen by the action of sodium or silver sulphite on the halogen derivatives of the aliphatic compounds. Saturated hydrocarbons do not react with sulphur trioxide, but unsaturated hydrocarbons are readily attacked by SO_3. Similarly, halogenated compounds and alcohols react with concentrated or fuming sulphuric acid forming sulphonic and hydrosulphonic acids respectively. The aromatic compounds form, as a rule, sulphonic acids with much greater facility. Benzene, for instance, is easily converted into the _m_-disulphonic acid by gently heating with fuming sulphuric acid; stronger heating converts the _m_- into the _p_-disulphonic acid, and at 190 C. the trisulphonic acid is formed. Toluene treated with fuming sulphuric acid first yields _o_- and _p_-sulphonic acids, finally _o_- and _p_-disulphonic acids, ethylbenzene at the boiling point _p_-ethylbenzene-sulphonic acid. Of the three isomeric xylenes _o_- and _m_-xylene dissolve in concentrated, _p_-xylene in fuming sulphuric acid only.
The action of sulphuric acid on naphthalene is stronger even than on benzene. Equal parts of naphthalene and sulphuric acid heated to 100 C. yield 80 per cent. [Greek: a] and 20 per cent. [Greek: b]-monosulphonic acid. At 160-170C. 25 per cent [Greek: a]- and 75 per cent. [Greek: b]-sulphonic acid is formed, and at higher temperatures [Greek: b]-monosulphonic acid only. If, on the other hand, 8 parts of naphthalene are heated with 3 parts of concentrated sulphuric acid to 180 C., two different naphthyldisulphonic acids are obtained.
Complete solution of the substance in sulphuric acid is, generally speaking, a criterion of complete sulphonation. A completely sulphonated compound should remain clear on dilution with water, or, in case precipitation occurs, the precipitate should be completely soluble in alkali or ammonia. It is necessary to submit the product to this test, since many organic substances are soluble in concentrated sulphuric acid without undergoing any alteration in composition.
Phosphoruspentoxide or potassium sulphate considerably increase the sulphonating property exhibited by fuming sulphuric acid.
The separation of the sulphonic acids from sulphuric acid is effected by salting out the former with common salt, or by removing the sulphuric acid with calcium, barium, or lead salts, provided that the sulphonic acid salts of these metals are soluble in water.
The sulphonic acid, in its chemically pure state, is best obtained from its crystalline barium salts, which are decomposed with the equivalent of sulphuric acid; another way is to decompose the calcium salts of the sulphonic acids with oxalic acid. The sulphonic acids are frequently hygroscopic and are easily soluble in water; the majority of their barium and lead salts are also soluble in water. The sulphonic acids are insoluble in ether. The halogens do not easily react with sulphonic acids, but when they do they usually replace the sulphonic acid group. In order to prepare the halogen substitution products, therefore, use is made of sulphonic chlorides. The latter are obtained by the action of chlorosulphonic acid on aromatic hydrocarbons; a simpler method, however, is to treat the dry alkali sulphonates with phosphorus pentachloride—
C6H5SO3Na + PCl5 = C6H5SO2.Cl + NaCl + POCl3
Derivatives of sulphonic chlorides are sulphonamides, which are easily prepared from the former by grinding with ammonium carbonate—
C_6H_5SO_2.Cl + (NH_4)_2CO_3 = C_6H_5.SO_2.NH_2 + NH_4Cl + CO_2 + H_2O
Sulphonic chlorides react with alkaline sulphides to form thiosulphonic acids—
C_6H_5SO_2.Cl + K_2S = C_6H_5SO_2.SK + KCl
Sulphonic chlorides, dissolved in ether, yield sulphinic acids on reduction with zinc dust or metallic sodium—
C_6H_5SO_2.Cl + H_2 = C_6H_5SO_2.H + HCl
In the sulphonic acid compounds it is assumed that the sulphur is hexavalent, and it is hence possible to consider the sulphones to be esters of sulphinic acid.
O R—SO —H
The sulphones are mostly solid bodies, which soften prior to melting when heated. They are very stable towards chemical reagents; for instance, saponification of a mono-sulphone very rarely yields sulphinic acid.
If a hydroxyl is substituted for a hydrogen atom in the aromatic hydrocarbons, the action of sulphuric acid is greatly facilitated; thus, by merely mixing phenol with sulphuric acid, the sulphonic acid is at once formed, whereby, in the cold, o-phenolsulphonic acid prevails which on heating for some time to 100-110 C. is completely converted into p-phenolsulphonic acid. In the absence of free sulphuric acid the conversion of o- into p-phenolsulphonic acid is brought about by heating the aqueous solution. Phenol-2,4-disulphonic acid is prepared from o- or p-phenolsulphonic acid, whereas phenol-2,4,6-trisulphonic acid is prepared directly from phenol by heating with concentrated sulphuric acid in presence of phosphorus pentoxide. Phenolsulphonic acids are also obtained by fusing benzenedisulphonic acid with alkali.
Cresol is not so easily sulphonated as is phenol; o-cresol when heated eight to ten hours at 90 C. with one and one-half times its weight of concentrated sulphuric acid, yields o-cresol-p-sulphonic acid.
The phenolsulphonic acids are strong, rather stable acids; their alcoholic hydroxyl-hydrogen atom may, similarly to that of the phenols, be substituted by a metal or an alkyl radical.
From [Greek: a]- and [Greek: b]-naphthol a number of sulphonic acids may easily be prepared; viz., mono-, di-, and trisulphonic acids. Nearly all these acids are important as basic materials in the dyestuff industry, especially 2,6-[Greek: b]-naphtholmonosulphonic acid (S-acid), 2,3,6-[Greek: b]-naphtholdisulphonic acid (R-acid) and 2,6,8-[Greek: b]-naphtholdisulphonic acid (G-acid).
2. Condensation of Phenols
Phenolsulphonic acids exhibit pronounced tendencies to condensation, for which purpose A. v. Baeyer (1872) employed aldehydes. The reaction is rather violent, and yields, in addition to well-defined crystalline substances, amorphous bodies resembling rosins. In addition to formaldehyde, paraformaldehyde, trioxymethylene, methylal, hexamethylene-tetramine, and other substances containing a reactive methylene group, as well as acetaldehyde, benzaldehyde and other aldehydes may be employed to induce reaction.
A number of these condensation products are derivatives of diphenylamine or hydroxybenzyl alcohols. When the latter are heated, either by themselves or in presence of acids, anhydrides and polymerisation products are formed producing hard, brittle, fusible substances, insoluble in water but fairly soluble in organic solvents. The same substances are formed when phenols are condensed with formaldehyde, especially in the presence of acid contact substances and excess of phenol by sufficiently long heating at certain temperatures. The substances referred to are termed "Novolak": similar to these are the so-called "Resols," insoluble and non-fusible substances, very resistant to chemical and physical action. Another member of the series is the so-called "Bakelite" or "Resitol," which does not fuse but softens when heated and swells in organic solvents. The ultimate product of this class of substances is "Resit" which is obtained when concentrated hydrochloric acid is allowed to act upon a mixture of phenol and formaldehyde; the temperature rises spontaneously, and a hard, porous, insoluble mass of great resistance is obtained. By heating resols, resitols are formed which, on further heating, are finally converted into resits. [Footnote: Ber., 1892, 25, 3213.]
Of all these products, bakelite (resitol) has found the greatest industrial application; in its purest form, this substance is a nearly colourless or light yellow body of sp. gr. 1.25 and, being a poor conductor of heat and electricity, constitutes an excellent insulating material; it is exceedingly resistant towards most chemical reagents even in concentrated forms of the latter. Its pronounced refractivity, and the ease with which it may be worked, makes bakelite a favourite substitute for amber (Ger. Pat, 286, 568). Similarly, the resols which can be easily moulded are used either as such or mixed with sand, pulverised cork, asbestos or wood, and the moulded substances then converted into the more highly resistant bakelite by heating.
The constitution of these bodies no doubt depends largely on their method of preparation; Baekeland [Footnote: Chem. Ztg., 1913, 73, 733.] considers resit a polymerised hydroxybenzylmethylene glycol anhydride; Raschig, a diphenylmethane derivative (e.g., dihydroxydiphenylmethane alcohol); Wohl [Footnote: Ber., 1912, 45, 2046.] considers them polymerisation products of methylene derivatives of tautomeric phenol.
CH==CH H_2C:C{ }CO CH==CH [Note: Lower Right CH has double bond to CO]
This group possesses the characteristic property of being capable of converting animal hide into leather when suitably dissolved. The author has dissolved a number of these water-insoluble condensation products in alkali and alcohol and was able to demonstrate their tanning effects on pelt; bakelite is easily soluble in alkali; a faintly alkaline solution partially precipitates gelatine, and completely so when the alkali is neutralised. This latter solution gives a dirty brown precipitate with iron salts.
These condensation products gained extraordinary importance for the tanning trade when Stiasny [Footnote: Ger. Pat, 262,558; Austr. Pat, 58,405.] succeeded in preparing them in water-soluble form when they are enabled to directly exert their tannoid properties. This may be done by acting upon two molecules of concentrated phenolsulphonic acid with one molecule of formaldehyde, the temperature thereby not exceeding 35C. By condensation, however, considerable heat is liberated, and hence the rise in temperature can only be limited by adding the diluted formaldehyde drop by drop, whilst stirring and cooling, to the phenolsulphonic acid. The original letters patent is worded as follows: 10 kilos each of crude phenol and sulphuric acid (66 B.) are heated with stirring for two hours at 105-106C., cooled to about 35C., and 463 kilos 30 per cent. formaldehyde added during three hours, the temperature thereby not exceeding 35C.; the stirring is continued for a couple of hours after the final addition of formaldehyde. This yields about 24 kilos of the crude condensation product. On a commercial scale, however, cresol (cresylic acid) is substituted for phenol. There are three isomers of cresol, viz., o-, m-, and p-cresol, and it was naturally of interest to investigate whether one or the other of the isomers exerted any particular influence on the properties of the final product. It was found, however, that condensation products from the three isomers were distinguishable from one another neither in physical nor in tannoid properties. It is hence possible to employ crude cresol, which contains varying quantities of the o-, m-, and p-compounds, in the manufacture of these tanning matters. [Footnote: Gen Pat, 291,457.]
The tar obtained from the Rochling coal-gas generator contains considerable quantities of phenols (B.P.=200-250C.), and the author has protected the use of these for the production of synthetic tannins by Ger. Pat, 262,558. A deep brown viscous mass is obtained which, when partly neutralised, yields similar results to those given by the product above referred to.
It may be anticipated that by analogy from the chemical reactions taking place in the condensation of phenols on the one hand and cresolsulphonic acid on the other, that all other homologues of phenol, its polyvalent derivatives, substitution products and acids, would yield similar condensation products.
The particular position occupied by the aromatic hydroxy compounds in the chemistry of substance possessing tannoid character is not only evidenced by the natural classification of the tannins, tannin derivatives, and decomposition products so far isolated and investigated, but also by other chemical behaviour shown by these substances. Meunier and Seyewetz [Footnote:Collegium, 1908, 315, 195.], for example, were able to show that phenol, p-aminophenol, chlorophenol, trinitrophenol, catechol, resorcinol, hydroquinone, monochlorohydroquinone, orcinol, pyrogallol, and gallotannic acid precipitate gelatine from its aqueous solution, that is, to a certain extent possess tanning properties.
The author has extended this series somewhat and obtained the following results:—
Relative Behaviour Towards Substances Gelatine. Hide Powder. Pelt. Tribromophenol Slight ppte. Tans Surface tannage [Footnote: In alcoholic solution] o-Nitrophenol No ppte. " " Br-o-Nitrophenol Slight ppte. " " Tribromopyrogallic Ppte. " " acid Bromophloroglucinol " " No tannage Galloflavine Slight ppte. " " Bromosalicylic acid " " " Bromo-[Greek: b] " " Tans -naphthol [Footnote: In alcoholic solution] Rosolic acid " " " [Footnote: In alcoholic solution] Gallic acid No ppte. No tannage No tannage
By the condensation of their sulphonic acids, it may be demonstrated experimentally how the tannoid properties of nearly every member of the series are intensified. Investigattion in this direction, however, has not been systematically undertaken, for which reason the author determined to examine this subject; but the enormous number of samples required, obtainable only with great difficulty during the war, made it impossible to conclude completely the researches in this field. What little has so far been done relatively to this subject should, when collected, indicate the way to be pursued in this wide field of investigation. What follows will hence comprise the conversion of a few of the most important members of this series of substances into their methylene-condensation products with a brief discussion of the qualitative and tannoid reactions of the latter.
The didepside of phenolsulphonic acid is obtained by condensing carbomethoxyphenolsulphonic chloride with sodium phenolsulphonate in the presence of the calculated amount of caustic soda. A product of the composition
CH_3.0.COO.C_6H_4SO_2.0.C_6H_4.SO_3Na
is first obtained, which on saponification with soda yields the pure didepside—
HO.C6H4.SO2.C6H4.SO3.Na
By acidifying the concentrated solution the didepside is obtained as a white crystalline substance; a solution of which precipitates gelatine without, however, exhibiting any tanning effect upon animal hide. If, on the other hand, the above ester is converted into the chloride
CH_3O.COO.C_4H_4SO_2.O.C_6H_4.SO_2Cl
by treatment with PCl_5, and the chloride thus obtained further condensed with sodium phenolsulphonate, saponified, and the solution acidified, the pure tridepside
HO.C_6H_4.SO_2.O.C_6H_4.SO_2.O.C_6H_4.SO_3Na
is precipitated as white crystalline needles which not only precipitate gelatine, but are capable of converting animal hide into leather.[Footnote: Chem. Ztg., 1919, 43, 318.]
Of the class of hydroxy-cymenes thymol,
C_6H_3.CH_3.C_3H_7OH,
was converted into the water-soluble sulphonic acid by warming with concentrated sulphuric acid at 50 C., the sulphonic acid being subsequently easily condensed with formaldehyde by slightly heating the mixture. The condensation product thus obtained is a viscous brown mass which is easily soluble in water, precipitates gelatine completely, gives a bluish-black coloration with iron salts, and gives a precipitate with aniline hydrochloride. To investigate its tannoid properties, the mixture was brought to the acidity 1 gm = 10 c.c. N/10 NaOH and a piece of bated calf skin was then introduced into a solution measuring about 2 B. After eighteen hours the pelt was nearly tanned through, and a further twenty-four hours completed the tanning process, after which a light fat-liquor was given. The dried leather was brownish-grey in colour, possessed soft and full feel and good tensile strength.
On account of their importance, the three dihydroxybenzenes were examined with a view to test their suitability for conversion into tannoid substances.
o-Dihydroxybenzene, catechol, yields a sulphonic acid easily soluble in water, which on the careful addition of formaldehyde assumes a blue colour. The compound thus obtained may be heated to 100 C., without depositing insolubles. A further addition of formaldehyde, however, results in the formation of a considerable quantity of insolubles whilst the liquid assumes a brown coloration. If, on the other hand, the sulphonic acid is diluted with twice its volume of water, formaldehyde added and the mixture heated on the water bath, the liquid immediately turns brown, the formaldehyde is completely fixed, and a condensation product soluble in water results. The latter gives a brownish-black coloration with ferric chloride, completely precipitates gelatine, but gives no opalescence with aniline hydrochloride. Tanning experiments with the partly neutralised (1 gm.= 10 c.c. N/10 NaOH) substance resulted in both grain and flesh being tanned with a black colour, whereas the interior of the pelt was pickled (white colour). After a further forty-eight hours, however, the black colour penetrated the pelt, and tannage was complete. The washed and lightly fat-liquored leather was soft, of full feel and good tensile strength, and was greyish coloured throughout.
With regard to the black colour possessed by leathers tanned with synthetic tannins the following should be noted. When sulphonating and especially when condensing substances, black dyestuffs or very finely divided carbon in the colloidal state are often formed. Such a substance does not deposit the black particles, even when filtered through kaolin, and hence convert pelt into leather possessing black colour on the surface. The hide in this case acts as a perfect filtration medium, whereby the surface layers retaining the coloured particles assume their colour; thus only the pure tanning matter enters into the interior, which then, according to the composition of the former, imparts a colour varying from white to light brown to the inner layers.
m-Dihydroxybenzene, resorcinol, is also easily sulphonated by concentrated sulphuric acid, the brownish-coloured sulphonic acid being easily soluble in water. If the sulphonic acid is diluted with three times its volume of water, cooled down, a few drops of formaldehyde added and the mixture heated on the water bath to completely fix the formaldehyde, and this process repeated till no more formaldehyde is taken up, a brown water-soluble condensation product results, the aqueous solution of which precipitates gelatine completely, aniline hydrochloride only partly and which gives a deep blue colour with ferric chloride.
A piece of calf skin immersed in a solution of the partly neutralised (as above) product was tanned through in twenty-four hours; when lightly fat-liquored, the resulting leather possessed a yellowish-green colour and good tensile strength, and was soft and full.
p-Dihydroxybenzene, hydroquinone, was converted into the water-soluble sulphonic acid by heating it with concentrated sulphuric acid at 100 C.; the sulphonic acid, mixed with formaldehyde at ordinary temperature, immediately solidifies to a white mass, which is soluble in water and which had completely fixed the formaldehyde. If, however, this mass is heated for some time to 100C, it assumes a light brown coloration and its solubility in water is diminished. A slight excess of formaldehyde and the application of heat result in dark violet insoluble condensation products. The aqueous solution precipitates gelatine, gives a deep blue colour with ferric chloride, but gives no precipitate with aniline hydrochloride; on the other hand, addition of potassium nitrite produces the yellow colour characteristic of hydroquinone.
The product effects a slower tannage (seven days) than the former product, when a brown, soft, but rather empty leather of good tensile strength is obtained.
Of the trihydroxybenzenes pyrogallol and phloroglucinol only were included in these investigations.
When pyrogallol is sulphonated with concentrated sulphuric acid a violet-coloured sulphonic acid, soluble in water, is obtained, which, when treated with formaldehyde first in the cold and then when heated, yields a solid deep red-coloured mass, which precipitates gelatine but not aniline hydrochloride, and gives a blackish-brown colour with ferric chloride. The partly neutralised substance in aqueous solution tans pelt in twenty-four hours with black colour on the surface only, the intermediary layer being pickled (white colour) only, but the black-coloured tanning matter ultimately penetrates the pelt, which tanned through in seven days. The resultant leather is coloured black throughout, is full, soft, and possesses good tensile strength. |
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