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Vegetable Tanning by Arthur W

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Vegetable Tanning by Arthur W Sept., 1922 THE JOURNAL OF INDUSTRIAL AND ENGINEERING CHEMISTRY 829 75-Jablonski and Einbeck, Ledntech. Rundschau, 13 (1921), 41. 82-Thomas, J. Am. Leather Chem. Assoc., 15 (1920), 221. 76-Schultz. J. Am. Leather Chem. Assoc., 15 (19201, 283. 83-Matos, Ibid., 16 (1920), 553. 77-Hart, Ibid., 16 (l920), 404. 84--Welwart, Chem. Ztg., 44 (1920), 719. 78--Hart, Ibid., 16 (1921), 159. SSHoffman, Bn., 63 (1920), 224. 79-Bunicke. Ibid., 16 (l921), 7. 86-Froboese, Z. Nahr.-Genussm., 41 (1921), 113. SO-Pickering, J. SOC.Leather Trades’ Chem., 89 (1920), 305. 87-Bogue, Chem. Met. Eng., 2s (1920), 6, 61, 105, 154, 197 81-Kern, J. Ind. Eng. Chem., 12 (1920), 785. Vegetable Tanning By Arthur W. Thomas CHEMICALLABORATORIES, COLUMBIA UNIVERSITY, New YORK,N. Y. HE NATURE of tannin has beenunder study by organic Nierenstein is also studying the constitution of the catechu chemists for many years, but the outstanding feature tannins. He has recently found3 that Paullinia tannin T of organic chemical investigation was the synthesis of from Paullinia cupunu seeds is a crystalline normal glucoside gallotannic acid (the tannin of oak galls) by Emil Fischer in consisting of 1 molecule of dextrose and 2 molecules of 1918,l which rendered all previous work practically obsolete. gambier-catechin-carboxylic acid, forming a depside. The study of the hydrolytic products of purified nut- K. Freudenberg, formerly associated with Fischer, is con- galls tannin showed that 1 molecule of glucose was combined tinuing that great master’s work on the tannins. He has with a number of molecules of gallic acid, and that gallo- shown4 that Hamameli tannin, which has been isolated in the tannic acid was-actually a pentadigalloyl glucose, with crystalline condition, yields on hydrolysis with tannase the formula gallic acid and an unidentified aldo-hexose in the proportions required for a digalloylhexose. \ HH 1 HH Freudenberg’s investigations6 of chebulinic acid, which oc- HC-C-C-C-C-CH curs in myrabolans, showed that it is a crystalline material OOOHOO RRR RR sparingly soluble in cold water, which cannot be hydrolyzed by tannase, and that it is probably a compound of a digalloyl- where R .= digalloyl radical. glucose and a new phenolic acid, the latter forming a glu- By condensing 5 molecules of tricarbomethoxygalloyl coside with the glucose. He showed6 chlorogenic acid, the chloride with 1 molecule of glucose in the presence of quino- tannoid constituent of coffee, to be a simple depside of caffic line (the aarbomethoxy groups protecting the hydroxy groups and quinic acids. Gambier-catechin has been shown7 to of the gallic acid from the action of phosphorus penta- have an oxydiphenylpropane nucleus, to be related to the chloride) and the carbomethoxy groups being subsequently flavone dyes and anthocyanidins, thus disproving an older removed by cold alkali, pentadigalloyl glucose, similar in view that it was a derivative of ethyldiphenylmethane. properties to gallotannic acid, was obtained. The tannins of the woods of the Spanish chestnut and native Fischer also prepared a series of acyl glucoses, many of German oak have recently been found by this same chemist* which have tanning properties. The monacyl glucoses do not to contain quercitrin, glucose, and ellagic acid. precipitate gelatin, but trigalloylglucose shows pronounced The leaves of Acer ginnala yield a crystalline tannin, tanning properties. Trigalloylglycerol and hexagalloylman- acertannin, C~oHzo0~3,and an amorphous mixture consisting nitol precipitate gelatin in aqueous solution. of ellagic acid, quercitrin, an amorphous tannin, mainly Fischer prepared penta-m-digalloyl-6-glucose, which was galloyl-aceritols with small amounts of a flavonol glucoside, proved to be an isomer of Chinese tannin, and in the first and a substance probably a phlobo-catechol tannin according paper mentioned he proved that the “glucogallin” isolated by to Perkin and U~eda.~Acertannin hydrolyzes to 2 molecules Gilson from Chinese rhubarb is 1-galloyl-p-glucose. of gallic acid and a dextrorotatory sugar, aceritol. Aceritol Nierenstein2 raises a number of objections to Fischer’s behaves as a polyhydric alcohol and is probably an anhydro- formula, mainly on the ground that whereas Fischer’s penta- hexitol derived from mannitol or sorbitol. digalloylglucose when methylated with diazomethane yields A new classification of natural tannins, more discriminating glucose on hydrolysis, methylogallotannin gives under iden- than the older pyrogallol-catechol categories, has been pro- tical conditions tetramethylglucose, showing that in gallo- posed by Perkin and Everest :Io tannin 4 hydroxyl groups are not replaced by digalloyl 1-Depsides (old gallotannins). radicals. Nierenstein proposes a modified long chain formula 2-Diphenyl methoid (old ellagitannins). based on the supposition that gallotannin is probably a 3-Phlobatannins (old catechol tannins). glucoside of the following polydigalloylleuco-digallic acid Freudenbergll has recently offered classification much anhydride: a more distinctive than the above: (0H)sCsHa.CO.[O.CoHa(OH)s.COl% .O.Ce.Hz(OH).CO.O.CaH2(0H)n.CH(OH) .O.C~HI(OH)Z a J. Chem. SOC., 121 (1922), 23. I I 4 Ber., 52 (ISIS), 177; 53 (1920), 953. 0- ________.---I-co 5 Ibid., 52 (1919), 1238. 6 Ibid., 63 (19201, 232. or of its free acid. Five reasons in support of this formula 7 Ibid., 63 (1920). 1416. are offered by Nierenstein, one of which is that it is in accord- 8 Ibid., 54 (1921), 1695; Naturwisscnschaften, 8 (1920), 005. ance with the formation of tetramethylglucose from methylo- 9 J. Chem. SOC., 121 (1922), 66. gallotannin. 10 “The Natural Organic Coloring Matters,” Longmans, Green & Co , New York, 1918. 1 Bn., 61 (1918), 1760; 52 (1919). 829. 11 “Die Chemie der Nattirlichen Gerbstoffe,” Julius Springer, Berlin, 2 J. SOG.Chem. Ind., 41 (?922), 29T. 1920. 830 THE JOURNAL OF INDUSTRIAL AND ENGINEERING CHEMISTRY Vol. 14, No. 9 1-Hydrolyzable tannins in which the benzene nuclei are two explanations of astringency to one and the same thing, united to larger complexes through oxygen atoms. 2-Condensed tannins, in which the nuclei are held together although it must be noted that the charge is not the sole through carbon linkages. factor in fixation by pelt, since undoubtedly one mechanism of the non-tans in reducing astringency lies in their high diffusive Where both kinds of compounds are present in the molecule, power and loose fixation in the pelt, thus preventing “case e. g., in ellagic acid, the classification is decided according to hardening” by the tannins and retarding and evening the the genetic connection with other tannins. fixation of the same. Group 1 embraces (a) esters of phenolcarboxylic acids with The significance of Hf ion to the charge of tannin particles each other or with other oxyacids (depsides), (b) esters of was indicated in the paper by Thomas and Foster, and a phenolcarboxylic acids with polyatomic alcohols and sugars continuation of the study of this question is in progress. (tannin class), and (c) glucosides. The most important Considerable data on the influence of electrolytes in pre- criterion for Group 1 is hydrolysis to simple components by cipitation of tannins are also given in this paper, throwing enzymes, particularly tannase or emulsin. light on their colloidal properties. Tannins of Group 2 are not decomposed to simple compo- nents by enzymes. They are generally, but not always, FIXATIONOF TANNINS BY PELT precipitable by bromine, and when treated with oxidizing agents or strong acids condense to high molecular amorphous A study of the time and concentration factors in the fixation tannins or “reds.” They are divided into two classes accord- of tannins by pelt is being pursued by Thomas and Kelly, a ing to whether or not phloroglucin is present. With the preliminary paper on quebracho and gambier having appeared exception of some simpler ketones, oxybenzophenones and recent1y.l’ This paper shows a striking difference between oxyphenylatyrylketones, the catechines belong to the phloro- the action of astringent quebracho and mild gambier. The concentration curves for gambier rise regularly and slowly like glucin, class, e. g. ,the very important quebracho, and probably oak tannin are in this class. the well-known “adsorption curve,” while those for quebracho Freudenberg has made valuable use of the enzyme tannase rise very steeply to a maximum at 30 to 40 g. total solids per secreted by Aspergillus niger. A recent paper by him and liter of solution and then drop off abruptly. They explain Vollbrecht discusses the isolation and determination of the this action of quebracho by the fact that owing to the lack activity of this enzyme.12 For a complete summary of the of non-tans in this astringent material the surfaces of the hide present knowledge of the organic chemistry of natural tannins substance particles are so heavily tanned that they are Freudenberg’s very valuable bookll should be consulted. rendered impermeable to tannin and hence the interiors are unaffected, thus accounting for the smaller amount of tannin ELECTROLYSISOF TANNIKS fixed by the hide in the strong solution. Wilson and Kern1*have shown that prolonged washing with Extensive studies of the conduct of tannins upon electrol- water neither hydrolyzes vegetable leather nor removes any of ysis have been made by Grasser,l3 but the results are greatly the tannin therefrom. They contend that the definition of confused due to oxidation, reduction, and decomposition. tannin should be restricted to those substances which ir- He found that mangrove and catechin are sodium salts, and reversibly combine with pelt. They made the valuable dis- that in some instances a separation of two qualitatively covery that when completely detannized vegetable liquors are different tannins can be effected by electrophoresis. evaporated in the presence of air, tannin is generated owing T;ITilson,14 in extension of his application of the Procter- to oxidation and polymerization of organic non-tans present.
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