T H E JOURNAL OF INDUSTRIAL A N D ENGINEERING CHEMISTRY
Sept., 1922
75-Jablonski and Einbeck, Ledntech. Rundschau, 13 (1921), 41. 76-Schultz. J . Am. Leather Chem. Assoc., 15 (19201, 283. 77-Hart, Ibid., 16 (l920), 404. 78--Hart, Ibid., 16 (1921), 159. 79-Bunicke. Ibid., 16 (l921), 7. SO-Pickering, J . SOC.Leather Trades’ Chem., 89 (1920), 305. 81-Kern, J . I n d . Eng. Chem., 12 (1920), 785.
829
82-Thomas, J . A m . Leather Chem. Assoc., 15 (1920), 221. 83-Matos, Ibid., 16 (1920), 553. 84--Welwart, Chem. Ztg., 44 (1920), 719. S S H o f f m a n , B n . , 63 (1920), 224. 86-Froboese, Z . Nahr.-Genussm., 4 1 (1921), 113. 87-Bogue, Chem. Met. Eng., 2s (1920), 6, 61, 105, 154, 197
Vegetable Tanning By Arthur W. Thomas CHEMICAL LABORATORIES, COLUMBIA UNIVERSITY, New YORK,N. Y.
HE NATURE of tannin has beenunder study by organic chemists for many years, but the outstanding feature of organic chemical investigation was the synthesis of gallotannic acid (the tannin of oak galls) by Emil Fischer in 1918,l which rendered all previous work practically obsolete. The study of the hydrolytic products of purified nutgalls tannin showed that 1 molecule of glucose was combined with a number of molecules of gallic acid, and that gallotannic acid was-actually a pentadigalloyl glucose, with the formula
T
\ H H 1 HC-C-C-C-C-CH O O O H R R R
H H O O R R
where R .= digalloyl radical. By condensing 5 molecules of tricarbomethoxygalloyl chloride with 1 molecule of glucose in the presence of quinoline (the aarbomethoxy groups protecting the hydroxy groups of the gallic acid from the action of phosphorus pentachloride) and the carbomethoxy groups being subsequently removed by cold alkali, pentadigalloyl glucose, similar in properties t o gallotannic acid, was obtained. Fischer also prepared a series of acyl glucoses, many of which have tanning properties. The monacyl glucoses do not precipitate gelatin, but trigalloylglucose shows pronounced tanning properties. Trigalloylglycerol and hexagalloylmannitol precipitate gelatin in aqueous solution. Fischer prepared penta-m-digalloyl-6-glucose, which was proved to be an isomer of Chinese tannin, and in the first paper mentioned he proved that the “glucogallin” isolated by Gilson from Chinese rhubarb is 1-galloyl-p-glucose. Nierenstein2 raises a number of objections to Fischer’s formula, mainly on the ground that whereas Fischer’s pentadigalloylglucose when methylated with diazomethane yields glucose on hydrolysis, methylogallotannin gives under identical conditions tetramethylglucose, showing that in gallotannin 4 hydroxyl groups are not replaced by digalloyl radicals. Nierenstein proposes a modified long chain formula based on the supposition that gallotannin is probably a glucoside of the following polydigalloylleuco-digallic acid anhydride: (0H)sCsHa.CO.[O.CoHa(OH)s.COl% .O.Ce.Hz(OH).CO.O.CaH2(0H)n.CH(OH) .O.C~HI(OH)Z
I
I
0-________.---I-co
or of its free acid. Five reasons in support of this formula are offered by Nierenstein, one of which is that it is in accordance with the formation of tetramethylglucose from methylogallotannin. 1 2
B n . , 6 1 (1918), 1760; 52 (1919). 829. J . SOG. Chem. Ind., 41 (?922), 29T.
Nierenstein is also studying the constitution of the catechu tannins. He has recently found3 that Paullinia tannin from Paullinia cupunu seeds is a crystalline normal glucoside consisting of 1 molecule of dextrose and 2 molecules of gambier-catechin-carboxylic acid, forming a depside. K. Freudenberg, formerly associated with Fischer, is continuing that great master’s work on the tannins. He has shown4that Hamameli tannin, which has been isolated in the crystalline condition, yields on hydrolysis with tannase gallic acid and an unidentified aldo-hexose in the proportions required for a digalloylhexose. Freudenberg’s investigations6 of chebulinic acid, which occurs in myrabolans, showed that it is a crystalline material sparingly soluble in cold water, which cannot be hydrolyzed by tannase, and that it is probably a compound of a digalloylglucose and a new phenolic acid, the latter forming a glucoside with the glucose. He showed6 chlorogenic acid, the tannoid constituent of coffee, to be a simple depside of caffic and quinic acids. Gambier-catechin has been shown7 to have an oxydiphenylpropane nucleus, to be related to the flavone dyes and anthocyanidins, thus disproving an older view that it was a derivative of ethyldiphenylmethane. The tannins of the woods of the Spanish chestnut and native German oak have recently been found by this same chemist* to contain quercitrin, glucose, and ellagic acid. The leaves of Acer ginnala yield a crystalline tannin, and an amorphous mixture consisting acertannin, C~oHzo0~3, of ellagic acid, quercitrin, an amorphous tannin, mainly galloyl-aceritols with small amounts of a flavonol glucoside, and a substance probably a phlobo-catechol tannin according t o Perkin and U ~ e d a . Acertannin ~ hydrolyzes to 2 molecules of gallic acid and a dextrorotatory sugar, aceritol. Aceritol behaves as a polyhydric alcohol and is probably an anhydrohexitol derived from mannitol or sorbitol. A new classification of natural tannins, more discriminating than the older pyrogallol-catechol categories, has been proposed by Perkin and Everest :Io 1-Depsides (old gallotannins). 2-Diphenyl methoid (old ellagitannins). 3-Phlobatannins (old catechol tannins). Freudenbergll has recently offered a classification much more distinctive than the above: a
J . Chem. SOC., 121 (1922), 23.
Ber., 52 (ISIS), 177; 53 (1920), 953. Ibid., 52 (1919), 1238. 6 Ibid., 63 (19201, 232. 7 Ibid., 63 (1920). 1416. 8 Ibid., 54 (1921), 1695; Naturwisscnschaften, 8 (1920), 005. 9 J. Chem. SOC., 121 (1922), 66. 10 “The Natural Organic Coloring Matters,” Longmans, Green & Co , New York, 1918. 11 “Die Chemie der Nattirlichen Gerbstoffe,” Julius Springer, Berlin, 4
5
1920.
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T H E JOURNAL OF I N D U S T R I A L A N D ENGINEERING CHEMISTRY
1-Hydrolyzable tannins in which the benzene nuclei are united to larger complexes through oxygen atoms. 2-Condensed tannins, in which the nuclei are held together through carbon linkages. Where both kinds of compounds are present in the molecule, e. g., in ellagic acid, t h e classification is decided according to the genetic connection with other tannins. Group 1 embraces (a) esters of phenolcarboxylic acids with each other or with other oxyacids (depsides), ( b ) esters of phenolcarboxylic acids with polyatomic alcohols and sugars (tannin class), and (c) glucosides. The most important criterion for Group 1 is hydrolysis to simple components by enzymes, particularly tannase or emulsin. Tannins of Group 2 are not decomposed to simple components by enzymes. They are generally, but not always, precipitable by bromine, and when treated with oxidizing agents or strong acids condense to high molecular amorphous tannins or “reds.” They are divided into two classes according to whether or not phloroglucin is present. With the exception of some simpler ketones, oxybenzophenones and oxyphenylatyrylketones, the catechines belong to the phloroglucin, class, e. g. ,the very important quebracho, and probably oak tannin are in this class. Freudenberg has made valuable use of the enzyme tannase secreted by Aspergillus niger. A recent paper by him and Vollbrecht discusses the isolation and determination of the activity of this enzyme.12 For a complete summary of the present knowledge of the organic chemistry of natural tannins Freudenberg’s very valuable bookll should be consulted. ELECTROLYSIS OF TANNIKS Extensive studies of the conduct of tannins upon electrolysis have been made by Grasser,l3 but the results are greatly confused due to oxidation, reduction, and decomposition. He found that mangrove and catechin are sodium salts, and that in some instances a separation of two qualitatively different tannins can be effected by electrophoresis. T;ITilson,14 in extension of his application of the ProcterWilson theory of tanning,15 discusses the reasons for the different degrees of astringency of tannins, and attributes the great difference of astringency between quebracho and gambier to the differences in the electrical charges of the particles and to the content of non-tans. He found that quebracho became mild in action when gallic acid was added to it, and suggested the mechanism for the fixation of tannins by hide substance. Thomas and Foster16 actually measured the electrical charge of eight different vegetable tanning materials by the U-tube electrophoresis method. They found all to be anodic in migration (at their natural reaction, i. e., -log CH+ = ea. 4.) and that the potential difference of the Helmholtz double layer of astringent quebracho was the highest, namely, -0.028 volt, while that of mildly acting gambier was the lowest, -0.005 volt. This confirms the theory of Wilson that astringency is a function of the electrical charge. Thomas and Foster also showed that the negative electrical charge of quebracho could be gradually reduced and finally brought to zero by addition of small amounts of hydrochloric acid, while the charge for this tanning material, as well as for four others, was raised by removal of the non-tans by dialysis: as, for example, cube gambier showed a p. d. of -0.005 volt in natural state; after dialysis for 24 hrs. it became -0.024 volt. This shows that the non-tans have a profound effect upon the p. d. of the tannin particles, and partly resolves Wilson’s 2. physiol. Chem., 116 (1921), 277. 18 Collegium, 1920, 17, 49, 137, 200, 277, 332. 14 J . A m . Leather Chem. Assoc., 16 (1920). 374. 15 J . Chem. S o c , 109 (1916), 1327. 16 THISJOURNAL, 14 (1922), 191. 12
Vol. 14, No. 9
two explanations of astringency to one and the same thing, although it must be noted that the charge is not the sole factor in fixation by pelt, since undoubtedly one mechanism of the non-tans in reducing astringency lies in their high diffusive power and loose fixation in the pelt, thus preventing “case hardening” by the tannins and retarding and evening the fixation of the same. The significance of Hf ion to the charge of tannin particles was indicated in the paper by Thomas and Foster, and a continuation of the study of this question is in progress. Considerable data on the influence of electrolytes in precipitation of tannins are also given in this paper, throwing light on their colloidal properties.
FIXATION OF TANNINS BY PELT
A study of the time and concentration factors in the fixation of tannins by pelt is being pursued by Thomas and Kelly, a preliminary paper on quebracho and gambier having appeared recent1y.l’ This paper shows a striking difference between the action of astringent quebracho and mild gambier. The concentration curves for gambier rise regularly and slowly like the well-known “adsorption curve,” while those for quebracho rise very steeply to a maximum a t 30 to 40 g. total solids per liter of solution and then drop off abruptly. They explain this action of quebracho by the fact that owing to the lack of non-tans in this astringent material the surfaces of the hide substance particles are so heavily tanned that they are rendered impermeable to tannin and hence the interiors are unaffected, thus accounting for the smaller amount of tannin fixed by the hide in the strong solution. Wilson and Kern1*have shown that prolonged washing with water neither hydrolyzes vegetable leather nor removes any of the tannin therefrom. They contend that the definition of tannin should be restricted to those substances which irreversibly combine with pelt. They made the valuable discovery that when completely detannized vegetable liquors are evaporated in the presence of air, tannin is generated owing to oxidation and polymerization of organic non-tans present. This is in accord with the findings of Meunier and Seyewitz,lg that while pure phenols produce only an extremely feeble effect upon the solubility of gelatin, their oxidation products quickly render gelatin insoluble, e. g., hydroquinone has no tanning properties whereas quinone has. Wilson and Kern showed that this action takes place right in the leather, for when hide powder was drummed in various vegetable extracts and then removed, divided into two portions, one being washed immediately, the other allowed to stand in contact with air for 30 days before washing out the non-tans, the second portion showed in all cases an increased tannin content, thus explaining the importance of the time factor in the “aging” of heavy leather. Alsop’sZ0statement, that sole leather tanned slowly not only contains more tannin, but actually consumes less tanning material than rapid tannage, is thus explained. The knowledge of this chemical change was first made apparent by the introduction and use of the new method of tannin determination invented by Wilson and Kern.21 This method is radically different from any others previously proposed and used, and as a consequence has met with vigorous opposition although it is based upon sound chemical principles. The investigators, in a subsequent study22of the conduct of quebracho and gambier liquors a t different H+-ion concenTHISJOURNAL, 14 (1922), 292. Ibid., 12 (1920), 1149. 19 Collegium, 1908, 195. 20 J . A m . Leather Chem. Assoc., 15 (1920), 464. 2 1 THISJOURNAL, 12 (1920), 465; 13 (1921), 772. 22 I b z d , 13 (1921), 1025. 17
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