The Natural Organic Colouring Matters
By
Arthur George Perkin, F.R.S., F.R.S.E., F.I.C., professor of colour chemistry and dyeing in the University of Leeds
and
Arthur Ernest Everest, D.Sc., Ph.D., F.I.C., of the Wilton Research Laboratories; Late head of the Department of Coal-tar Colour Chemistry; Technical College, Huddersfield
Longmans, Green and Co.
39 Paternoster Row, London
Fourth Avenue & 30th Street, New York
Bombay, Calcutta, and Madras
1918
Kaikki kuvat (kemialliset kaavat) puuttuvat // None of the illustrations (of chemical formulas) included.
TANNINS (Acides tanniques, Fr.; Gerbstoffe, Ger.). This term has been applied to a large class of substances which have been found in many plants and are distinguished by the following characters: they have an astringent taste; give a blue-black or green coloration with ferric salts; are precipitated by a solution of gelatin, by albumen and by alkaloids; unite with hide to form leather.
It was formerly believed that only one tannin existed, and it was assumed that the difference in properties of tannin from different sources was due to the presence of foreign matter. It is, however, now well known that tannin from different sources frequently varies both in composition and in properties, and although it is probable, owing to the difficulty of obtaining these compounds in an even approximately pure condition, that the number of distinct individuals may not be so great as is usually supposed, there can be no doubt that at least three important classes of tannins exist in nature. The earliest suggestion was to divide the tannins into two classes the iron-blueing tannins and the iron-greening tannins according to their behaviour towards salts of iron, and it was considered that whereas the former were pyrogallol compounds, that the latter were derived from catechol. This differentiation appears in the main to be correct, and the employment of ferric chloride for this purpose, or better, iron alum as a preliminary step, is of general application; but on the other hand, it is to be borne in mind that the presence of acid, alkali, or organic impurity has considerable effect upon the colour production. Thus, although the method is no doubt of service in many cases even with the plant infusion, its exact significance can only be ascertained from the coloration given by the purified tannin itself.
Stenhouse (Proc. Roy. Soc., 11, 405) believed that those tannins which give blue-black precipitates with ferric salts are mostly glucosides. Wagner (Zeitsch. anal Chem.) made a distinction between pathological and physiological tannins, and considered that the former class represented by gallotannic acid only existed in pathological formations of certain species of oak and sumach (Rhus javanica, Linn., and R. semilata, Murr.), whilst the latter class include all tannins which are produced under normal conditions of plant life. As, however, gallotannic acid has been found to exist in some plants as a physiological tannin, Wagner's classification is untenable.
Bottinger (Ber., 1884, 17, 1123) has examined the action of bromine on aqueous tannin extracts, and determined the percentage of bromine contained in the precipitated bromo products. As a result, it was shown that certain tannins may be grouped together according to the amount of bromine which they take up.
Mangrove tannin... 42.15
Hemlock bark tannin... 43.6
Quebracho tannin... 44.5
Mimosa tannin... 49.36
Chestnut oak tannin... 50.48
Terra japonica tannin... 53.2
Spruce bark tannin... 52.8
A very large amount of attention has been given to the classification or identification of aqueous extracts of tanning materials, by the coloured and other effects given by certain reagents. It remains to be decided whether in all cases these reactions in reality arise from the tannin itself.
The more important methods which may be used to classify the tannins into groups according to our present knowledge of these substances are as follows:
Coloration with ferric chloride or iron alum (see above).
Digestion with boiling dilute sulphuric acid.
In this method of procedure, three definite reactions may be observed: (a) the hydrolysis of the tannin with formation of gallic acid (gallotannic acid); (b) the precipitation of ellagic acid (ellagitannic acid); (c) the gradual production of an amorphous red-coloured precipitate known as a "phlobaphene" (catechol or phlobatannin).
Precipitation with bromine water indicates the presence of a so-called catechol or phlobatannin (Procter, "Leather Industries Handbook," 1898).
Pine wood and hydrochloric acid test.
If a deal shaving be moistened with a solution of phloroglucinol and then with strong hydrochloric acid, a deep red-violet colour due to the formation of phloroglucinol vanillein is produced. Resorcinol reacts similarly giving a blue-violet (Procter). These colorations are an indicati of the presence of a phloroglucinol or resorcinol nucleus in the / tannins. Gallotannin and ellagitannin solutions do not react in this / manner.
Diazobenzene chloride.
Solutions of certain tannins in the presence of alkali or alkaline acetates give a red-coloured precipitate of the azobenzene tannin with this reagent, a fact which indicates with some certainty the presence of a phloroglucinol or resorcinol grouping. Gallotannin and ellagitannin do not react in this manner.
Fusion with alkali.
Procter ("Leather Industries Handbook," 1898) recommends adding 20 grams of tannin to 150 c.c. of a solution of potassium hydroxide of specific gravity 1.20 and concentrating the liquid during three hours until it becomes pasty. Or 5 to 10 parts of caustic potash and a few drops of water are heated with one part of the tannin to 210-240° for twenty minutes. Gallotannin, by this method, gives gallic acid, and possibly traces of pyrogallol, whereas the so-called catechol tannins yield protocatechuic acid or other allied acid, alone, or together with phloroglucinol or resorcinol, etc.
Heating with glycerol.
The tannin (1 gram) is heated with 5 c.c. of glycerol slowly raising the temperature from 160°, and keeping it for half an hour between 200-210°. The product diluted with 20 c.c. of water is extracted with ether, the extract evaporated and the residue tested for pyrogallol or catechol. According to Trimble ("The Tannins"), paraffin wax may be employed in place of glycerol.
Formaldehyde test.
When an aqueous solution of a so-called catechol tannin is treated with formaldehyde and a little hydrochloric acid and gently warmed the tannin is completely precipitated. Pyrogallol tannins do not yield an entirely insoluble compound in this manner. This reaction, discovered by Stiansy (Der Gerber, 1905, 185), has by numerous writers been assigned to Jean and Frabot (Ann. Chim. anal., 1907, 12, 49).
Lead acetate test.
(Stiasny and Wilkinson, Collegium, 1911, 475 (2 ix.), 318). All natural tannins are completely precipitated by lead acetate solution in so far as the filtrate from the precipitate does not give the iron test. In the case of catechol or phlobatannins this precipitate is dissolved by dilute acetic acid, whereas with the gallotannins the lead compound is insoluble or but partially soluble. The test is preferably made by adding 10 c.c. of acetic acid (10 per cent.) to 5 c.c. of the tannin solution, and then adding 5 c.c. of lead acetate (10 per cent.). No precipitate is thus produced in the case of the catechol or phlobatannins.
By these methods it is easy to divide the tannins into three classes, usually distinguished as (i) gallotannins, (2) ellagitannins, and (3) catecholtannins. Since the discovery of synthetical tannins by Fischer and Freudenberg (Annalen, 1911, 384, 225), it is evident that this nomenclature, as applied to the first group, is imperfect. Thus, whereas the term "gallotannin" is in reality only applicable to compounds containing pyrogallol nuclei and in fact merely relates to digallic acid and its derivatives, it is now known that diprotocatechuic acid, diresorcylic acid, and digentisic acid, members of the same group, possess tanning property. On this account it is considered more reasonable to distinguish such tannins by the term "depside," a nomenclature which is due to Fischer and Freudenberg, although it is not suggested that such a group is absent in the other tannins.
Again, as regards the so-called "ellagitannin" group (2), new tannins belonging to this class may also be either synthesised or isolated in the future. The term "ellagitannin" is therefore here replaced by "diphenyldimethylolid," the name by which the group mother substance is known.
Exception is again to be taken to the designation "catechol" tannin (3) for reasons similar to those given above and which are discussed later in this article, and this name is also replaced by "phlobatannin," in that these compounds, apparently without exception, possess the property of yielding phlobaphenes. In respect of this latter group, it is to be noted in connection with the qualitative tests above enumerated that the formation of phlobaphene and of precipitates with bromine water and with formaldehyde sufficiently indicate the presence of this variety of tannin, because these compounds may not all react with pine wood and hydrochloric acid, or diazobenzene chloride, or give protocatechuic acid as one of their decomposition products.
There appears to be ample evidence also of the existence of special varieties of glucoside belonging to this third class, distinguished by their extremely hygroscopic nature and the fact that they are insoluble or nearly so in acetic eter. They give, however, the well-marked reactions of the phlobatannins. None of these compounds has yet been obtained in a pure condition, but when they are digested with boiling dilute mineral acids a sugar and phlobaphene appear to be mainly produced.
Methods of isolation.
All the well-known tannins are dissolved by hot water, and yield precipitates with lead acetate solution, and thus by decomposing the well-washed lead precipitate from a plant infusion in the moist condition with sulphuretted hydrogen a crude solution of the tannin is obtained. This can be concentrated in vacuo over caustic potash or sulphuric acid. In the place of lead acetate, stannous chloride was employed by Proust in 1798 (Ann. Chim. Phys., 25, 225), who is credited with being the first to prepare tannic acid in a nearly pure condition. As, however, the tannin usually exists in the plant side by side with yellow colouring matter, either in the free state or as glucoside, and other secondary substances soluble in water, a fractional precipitation with lead acetate is preferably adopted, in which case the middle portion usually yields the purest tannin (Grabowski, Sitz. Ber., 1867, 55, ii., 567; Trimble, "The Tannins," 1892, i., 85). A preliminary treatment with lead acetate in the presence of a little acetic acid is serviceable in some cases for the precipitation of coloured impurities. Probably the solvent most extensively employed since 1880 in the investigation of tannins has been ethyl acetate, in which case it has been usual to agitate a solution of the substance or extract of the plant with this solvent. A preliminary addition of salt or sodium sulphate to the liquid is beneficial.
Certain tannin glucosides, owing to their sparing solubility in ethyl acetate, cannot be satisfactorily isolated in this manner, and the method is not applicable to the case of mineral salts of the tannin, in which the preliminary production and subsequent decomposition of the lead salt is to be recommended. Numerous methods have been adopted for the purification of the tannin thus prepared, and are given under the head of the special substance with which they have been employed.
In many cases it has been found preferable to extract the tannin matter with an organic solvent rather than with water. This is, as a rule, to be advised, as the crude substance is thus more readily isolated in a concentrated form. Indeed, one of the oldest methods of separating tannins from other substances is that of Pelouze (Ann. Chim. Phys., 1834, 55, 337), who exhausted powdered gall-nuts with commercial ether. Various mixtures of alcohol, ether, and water have been recommended, and also dilute alcohol in the case of gall-nuts for preparing gallotannic acid, although these methods must not be considered of special advantage for the isolation of tannins as a whole. Probably the most efficient solvent for general purposes of investigation is acetone which was employed by Trimble for the percolation of numerous tannin matters ("The Tannins ").
Ei kommentteja :
Lähetä kommentti