Coloriasto

19.2.25

Algarobilla
(CHAPTER XIII. Tannins.)

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.

Algarobilla consists of the pods of the Caesalpinia brevifolia of Chili. The tannin appears to consist principally of ellagitannin, and this lies in resinous particles loosely attached to the somewhat open framework of the fibre. It is one of the strongest tannin matters known, and contains on the average 45 per cent. In character it resembles divi-divi, its extract being somewhat liable to fermentation. It is very suitable for tanning and also for dyeing purposes.

18.2.25

Dividivi
(CHAPTER XIII. Tannins.)

Kaikki kuvat (kemialliset kaavat) puuttuvat // None of the illustrations (of chemical formulas) included.

Divi-divi consists of the dried pods of the Caesalpinia coriaria (Willd.), a tree 20-30 feet high, found in the West Indies and Central America. The pods are about 3 inches long and j inch broad, are very thin, and frequently resemble in shape the letter S. From 40-45 per cent, of tannin is present, which consists of ellagitannin and probably also a gallotannin. Extracts of this material have a somewhat unfortunate tendency to ferment, with simultaneous development of a deep-red colouring matter; but this can be prevented to some extent by the use of antiseptics. Divi-divi is largely imported for the preparation of leather, and is also employed for black dyeing, but its use is far more limited in this latter respect than myrobalans.

17.2.25

Valonia
(CHAPTER XIII. Tannins.)

Kaikki kuvat (kemialliset kaavat) puuttuvat // None of the illustrations (of chemical formulas) included.

Valonia (Valonée, Fr.; Valonea, Ackcrdoppen, Orientalische Knoppern (Ger.)), an important tanning material, is the acorn cup of certain species of oak, usually Quercus aegilops (Linn.), and probably Q. macrolepsis, Q. graeca, Q. ungeri, and Q. coccifera (Linn). The former is most prolific in the highlands of Morea, Roumelia, Greek Archipelago, Asia Minor, and Palestine, whereas the Q. macrolepsis forms great forests in Greece. These acorn cups have a diameter up to about 1½ inches, and in good condition possess a bright colour.

The fruit ripens in Asia Minor about July or August, and the trees are then shaken, and the material left on the ground to dry; this is subsequently collected into heaps, and allowed to ferment for some weeks, until the acorn contracts and falls from the cup. The acorn, which contains but little tannin, is employed for feeding purposes.

In Greece distinct qualities of valonia are known, the best (chamada) collected about April before the fruit is ripe, a second (rhabdisto) in September or October, and a third little-used inferior variety (charcala).

Smyrna valonia may contain 40 per cent., Greek 19-30, and Candia valonia 41 percent, of tannin matter, which consists of a mixture of a gallotannin and an ellagitannin. Valonia is, indeed, an excellent source for the preparation of ellagic acid, because it so readily yields a product easy to purify. Extract of valonia frequently undergoes fermentation with deposition of ellagic acid, and to avoid this the employment of antiseptics is to be recommended.

Valonia is especially suited for the manufacture of sole leather, and together with gambier and other materials for dressing leather, but is little employed for dyeing purposes (cf. Procter, "Principles of Leather Manufacture," 259).

Myrobalans
(CHAPTER XIII. Tannins.)

Kaikki kuvat (kemialliset kaavat) puuttuvat // None of the illustrations (of chemical formulas) included.

This very important tanning material consists of the fruit or nuts of the Terminalia chebula, a tree of from 40-50 feet high, which grows in China and the East Indies. These nuts, which resemble a somewhat shrivelled plum, contain from 30-40 per cent, of tannin, the unripe fruit containing the largest amount. They should be bright in colour, and not soft, and require to be kept in a dry place, otherwise they absorb moisture and are then difficult to grind. The tannin present consists of a gallotannin, which is at least in part chebulinic acid, together with a fairly large amount of ellagitannin, and this, owing to fermentation and other causes, is decomposed to some extent during lixiviation into ellagic acid. Myrobalans from the dyer's point of view is one of the most serviceable tannin materials at the present time. Enormous quantities of its extract, especially as purified or decolorised extracts, are manufactured, and these are employed for cotton dyeing, in the black dyeing of silk, and in tanning.

Sumach
(CHAPTER XIII. Tannins.)

Kaikki kuvat (kemialliset kaavat) puuttuvat // None of the illustrations (of chemical formulas) included.

True sumach consists of the dried and usually powdered leaves of the genus Rhus (order Terebinthacæ], and is useful for tanning the finer kinds of leather, and also in dyeing and calico printing on account of the tannin matter present in it.

Sicilian sumach, the variety most esteemed in this country and throughout Europe, consists of the leaves of the Rhus coriaria (Linn.), a shrubby bush cultivated to a large extent in Sicily, where the sumach industry is of considerable importance. When the plant is about to flower the younger twigs are removed, dried in the sun, and subsequently beaten to remove the leaves and flower panicles. The sumach is imported sometimes in leaves, but more often in the form of powder, and should contain about 25 per cent, of tannin, although as much as from 27-32 per cent, may occasionally be found.

According to Löwe (Zeitsch. anal. Chem., 12, 128), the tannin matter present, C₁₄H₁₀O₉, is ordinary gallotannin; indeed it is well known that when an aqueous extract of the sumach is boiled with dilute sulphuric acid, considerable quantities of gallic acid are produced. On the other hand, Strauss and Geschwender (Zeitsch. angew. Chem., 1906, 1121), who isolated the tannin according to Löwe's instructions, detected the presence of a methoxy group, and suggest the formula C₃₂H₂₉O₁₁.OMe.

Sicilian sumach contains also a trace of an ellagitannin and myricetin, C₁₆H₁₀O₈, to the extent of about 0,1173 per cent. (Perkin and Allen, Chem. Soc. Trans., 1896, 69, 1299), the latter colouring matter having been previously mistaken by Löwe (Zeitsch. anal. Chem., 1874, 12, 127) for quercetin.

Considerable quantities of sand and sometimes particles of magnetic iron ore, which cause black stains, are often to be found in sumach (Procter, "Principles of Leather Manufacture," 1903, 271); (compare also Trotman, J. Soc. Chem. Ind., 1904); and it is frequently highly adulterated in the ground condition with the leaves and twigs of various plants. Of these, the Pistacia lentiscus (Linn.) ("schinia" or "skens"), Coriaria myrtifolia (Linn.), (French sumach or "stinco"), Tamarix africana (Poir.) (brusca), Tamarix gallica (Linn.), Ailanthus glandulosa (Desf.), Ficus carica (Linn.), Vitis vinifera (Linn.), other species of the Rhus family and also the ground branches ("gambuzza," "gammuzza") of the Rhus coriaria itself, are known to be employed. These sumach adulterants also contain tannin matters, but for tanning and dyeing purposes are as a rule much inferior to sumach itself.

The Pistacia lentiscus (Linn.), (mastic tree), a small tree about 20 feet high with evergreen leaves, grows abundantly in Cyprus. The leaves of this plant constitute the most important sumach adulterant, and about 10,000 tons are said to be exported from Tunis to Sicily annually and re-exported thence (as sumach?). According to Procter (loc. cit.) the leaves contain 12-19 per cent, of a catechol tannin. A good plump leather can be obtained from this material, but of a faintly reddish tint, the result being intermediate in character between those which are given by oak bark and sumach. Its presence in sumach is to be deprecated, and in many cases leads to injurious results. A considerable quantity, however, is consumed at Lyons in France as an assistant dyeing material for silk stuffs.

According to Perkin and Wood (Chem. Soc. Trans., 1898, 73, 374), these leaves contain a tannin closely allied to, if not identical with, ordinary gallotannic acid, as when an aqueous extract is boiled with dilute sulphuric acid a considerable quantity of gallic acid is produced. A second tannin or tannin glucoside is also present which, although possessing the general characteristics of the socalled "catechol" tannins in that it yields a red phlobaphene, and as noted by Procter, a reddish-coloured leather, gives, by fusion with alkali, gallic acid and phloroglucinol.

In addition to these tannins, myricetin (probably as glucoside) C₁₅H₁₀O₈ is also present to the extent of about 0,15 per cent.

Tamarix africana (Poir.) is a small shrub or tree characterised by its twiggy branches and minute scale-like leaves. The small twigs are collected in Tunis and imported into Sicily for the adulteration of sumach (Procter).

According to Perkin and Wood (loc. cit.} the leaves contain a tannin probably identical with gallotannin, in addition to a small quantity of an ellagitannin. A trace of yellow colouring matter is also present and consists of a quercetin methylether, C₁₆H₁₂O₇.

The Tamarix gallica (Linn.) closely resembles in appearance the Tamarix africana and flourishes in Cyprus, where the latter is not found. According to Procter, it contains 8,4 per cent, of tannin matter.

Ailanthus glandulosa (Desf.) is a tree of large size and handsome appearance, native of India and China, but common on the Continent. The leaves contain 11,2 per cent, of tannin matter (Procter), and this resembles gallotannin, although a trace of an ellagitannin is also present (Perkin and Wood). Curiously enough, although so high a percentage of tannin is present, leather is scarcely tanned by an extract of these leaves, but is merely stained a dull dirty colour. This material is therefore of little use for tanning purposes, and as an adulterant of sumach exerts a deleterious influence. A small quantity of quercetin can be isolated from the leaves.

The leaves of the Ficus carica (Linn.) (common fig tree) contain 1,6 per cent, of tannin (Procter) and a trace of a yellow colouring matter (Perkin and Wood). Skin is untanned by an extract of these leaves, but acquires, during the process, a dirty olive tint.

Gambuzza consists of the small stalks branching from the main root of the Rhus coriaria (Linn.), which are ground to powder and mixed with sumach. The material contains some quantity of a tannin resembling gallotannin, together with a trace of myricetin.

Attempts to detect the presence of these adulterants in sumach by chemical methods have not given satisfactory results, but should a prolonged boiling of the extract with dilute sulphuric acid cause the gradual precipitation of phlobaphene, the presence of the leaves of the Pistacia lentiscus may be suspected.

More satisfactory results can be obtained by microscopical examination, and an elaborate work on this subject has been carried out by Andreasch ("Sicilianischer Sumach und seiner Verfalschung," Wien, 1898); the book, however, is unsuitable for abstraction. A useful method, now generally adopted by leather trades chemists, has been devised by Lamb and Harrison (J. Soc, Dyers, 1899, 14, 60), and is based upon the behaviour of the leaf mixture with warm nitric acid. Under such treatment, the more delicate leaf structure of sumach itself is completely destroyed, whereas the strong cuticles of Pistachia lentiscus, Coriaria myrtifolia, Tamarix africana, and Ailanthus glandulosa, are unaffected and can then be readily recognised (compare also Lamb, ibul. t 1904, 20, 265). Again, the leaves of the R. coriaria are very easily distinguished from those of other plants employed for their adulteration, in that both upper and lower cuticles are covered with a dense growth of hairs (Priestman, J. Soc. Chem. Ind., 1905, 24, 231).

Venetian sumach or Turkish sumach consists of the leaves of the Rhus cotinus (Linn.), a small tree, the wood of which constitutes the yellow dyestuff known as "Young Fustic". The material contains about 17 per cent, of tannin, which resembles ordinary gallotannin, together with a trace of an ellagitannin. The presence of myricetin, C₁₅H₄O₂(OH)₆, in these leaves is interesting, in view of the fact that in the wood itself, fisetin, C₁₅H₆O₂(OH)₄, is present (Perkin, Chem. Soc. Trans., 1898, 73, 1016).

American sumach. The leaves of numerous varieties of Rhus are employed in the United States for tanning and dyeing purposes, and of these the R. glabra (Linn.) very largely takes the place of Sicilian sumach. It contains about 25 per cent, of a tannin closely resembling gallotannin, but produces a leather of very much darker colour than the Sicilian product.

Of the other varieties, R. typhina (Linn.) or "Virginian sumach" (10-18 per cent.), R. cotinoides (Nutt.) (21 per cent.), R. semialata (Murr.) (5 per cent), R. aromatica (Ait.) (13 per cent.) (Procter), R. metopium (Linn.) (about 8-2 per cent, of tannin, probably gallotannin, together with traces of both quercetin and myricetin), (Perkin, Chem. Soc. Trans., 1900, 77, 427), R. copallina (Linn.), R. pumila (Michx.), and R, toxicodendron (Linn.), are to be found in the States. Among these, R. glabra and R. copallina are considered to be worthy of extended cultivation.

In India, numerous species of the genus Rhus are known to exist (Watt's Dictionary, "Economic Products of India "), and again in Algeria the R. pentaphylla (Desf.) is used by the Arabs for tanning goat-skins. Finally, the Anaphrenium argenteum (E. Mey), (R. thunbergii). (Cape of Good Hope), 28 per cent, of tannin (bark), probably of the catechol class (Procter), and the Rhodosphaera rhodanthema (Engl.) (Rhus rhodanthema) (New South Wales), 9-5 per cent, of tannin (leaves), resembling gallotannin, are worthy of mention. The latter plant, also known as the "yellow cedar," closely resembles the R. cotinus, and it is interesting to note that although the wood of this tree contains fisetin, C₁₅H₄O₂OH)₄ the colouring matter of the leaves is quercetin, C₁₅H₃O₂(OH)₆ (Perkin Chem. Soc. Trans., 1898, 73, 1017).

French sumach is derived from the Coriaria myrtifolia (Linn.), a low deciduous shrub, native of Southern Europe, and has been referred to above as an adulterant of Silician sumach. In addition to tannin (15,6 per cent., Procter), which consists probably of ordinary gallotannin together with a little ellagitannin, it contains the poisonous glucoside coriamyrtin (Riban, Chem. Zeit, 1867, 663) and a trace of quercetin (Perkin, Chem. Soc. Trans., 1900, 77, 428). According to Procter, the colour of leather tanned by these leaves is very satisfactory and practically equal to that produced by genuine sumach (R. coriaria). It is also employed in black dyeing.

Cape sumach consists of the leaves of the Colpoon compressum (Berg.) (Osyris compressa, Fusanus compressus, Thesium colpoon), and is much used in South Africa under the name of "Pruimbast". The bush is found in the mountains, where it grows to the height of about 6 feet, and only the younger leaves are gathered. These leaves contain about 23 per cent, of tannin (Procter), which has been isolated as a hygroscopic transparent glassy mass and is probably a phlobatannin glucoside. With boiling dilute acid, a reddishbrown phlobaphene gradually separates, and on fusion with alkali protocatechuic acid is produced (Perkin, Chem. Soc. Trans., 1897, 71, 1135). In addition to tannin there is also present a considerable quantity of the quercetin glucoside Rutin (Osyritrin) (Chem. Soc. Trans., 1910, 98, 1776). According to Procter, this material forms a useful substitute for Sicilian sumach.

In lieu of the Colpoon compressum, a tanning agent known as "broach leaves" (botanical origin lacking) appears to be considerably employed in South Africa. It contains about 19,9 per cent, of tannin of the so-called "catechol" variety, together with traces of both quercetin and myricetin (Chem. Soc. Trans., 1898, 73, 384).

Russian sumach consists of the leaves of the Arctostaphylos uvaursi (Spreng.) (Bearberry), and is said to contain about 14 per cent, of tannin, which, according to Perkin (Chem. Soc. Trans., 1900, 77, 424), consists of a gallotannin together with traces of an ellagitannin. Minute amounts of both quercetin and myricetin have been isolated from this material.

Considerable quantities of "sumach extract" are now manufactured for dyeing and tanning purposes from genuine Sicilian sumach, and this is usually found on the market as a brown treacly liquid of about 52° Tw. So-called decolorised extracts are also prepared to compete with ordinary tannic acid, and for this purpose the addition of blood albumen to the dilute extract at about 48°, then raising the temperature to 70°, and subsequently filter-pressing, gives the most satisfactory results. Sulphurous acid again is employed to brighten the colour of extracts, and acts partly as a weak acid in decomposing the inorganic salts of the tannin or colouring matter and partly as a reducing agent. In this case it is usual to pass sulphur dioxide through the liquor before concentration (Procter).

15.2.25

Chestnut Extract.
(CHAPTER XIII. Tannins.)

Kaikki kuvat (kemialliset kaavat) puuttuvat // None of the illustrations (of chemical formulas) included.

The wood of the Spanish chestnut, Castanea vesca, though it contains only 3-6 per cent, of tannin, is the source of the much-valued chestnut extract. The bark contains more tannin than the wood (17 per cent), but is not much used. The tree, which grows to from 60 to 80 feet in height, is abundant in Italy, the South of France, and Corsica, where it forms immense forests, and it is also very common in America.

Trimble ("The Tannins"), who very carefully examined the tannin, obtained analytical data and reactions which indicated that it was identical, or nearly so, with gallotannin, but it is probable that this wood also contains traces of a catechol tannin, for a certain quantity of a red colouring matter is also present, which resembles in character a phlobaphene. Some writers have suggested that chestnut tannin is a methyl ether of ordinary gallotannin, but there is apparently no definite evidence in support of this theory.

Chestnut is employed almost entirely in the form of extract, the strength of which varies, but usually contains from 26 to 32 per cent, of tannin. The extract is frequently decolorised, and some times mixed with quebracho extract and other materials. Chestnut tannin is the tannin which is most largely employed for the dyeing of silk. Castanea vesca appears to be frequently confused with the horse-chestnut, Æsculus hippocastanum. The tannin derived from this latter is, however, of little or no practical value.

Gall Nuts.
(CHAPTER XIII. Tannins.)

Kaikki kuvat (kemialliset kaavat) puuttuvat // None of the illustrations (of chemical formulas) included.

These are morbid excrescences produced by the puncture of an insect called Cynips gallæ-tinctoria upon the leaves and young twigs of certain kinds of oak trees, more especially those of Quercus infectoria (Oliver), Q. lusitanica (Lam.), growing in the East Indies, Persia, the Levant. If the fully developed nut be broken open it will be seen to contain a central cavity, in which the larva of the insect will be found. As a rule the galls are collected before the larvae are fully developed, and therefore before they have perforated the galls and escaped as mature insects. In this condition they contain the most tannic acid and are known as blue, black, or true nuts. The less valuable or perforated variety are larger and paler in colour and are known as white or false nuts.

Aleppo galls are one of the best varieties on the market, and should contain from 50-60 per cent, of tannic acid. This same oak, the Quercus infectoria, also bears a large gall known as the Apple of Sodom, due to a different insect, which contains from 24-34 per cent, of tannin and has been used for tanning purposes.

Other varieties are Smyrna galls, Austrian, and Hungarian galls, and of these the former are considered best. English oaks yield several species of galls and oak apples, which, however, are not of much value.

Chinese galls are produced by the action of an insect termed the Aphis chinensis on a species of sumach, Rhus semilata. These are hollow and possess very thin walls, but are much larger and more irregular in shape than the ordinary Aleppo variety, moreover, when freshly gathered, they are covered with a very fine down. They are much esteemed owing to their richness in tannin matter, of which frequently as much as 70 per cent, is present. On this account they are largely employed for the manufacture of tannic acid.

Gall-nut extract is employed for mordanting purposes when very delicate shades are required. In addition to tannic acid, all varieties of gall-nuts appear to contain minute traces of ellagitannic acid.

Group III. Catechol or Phloratannins.
(CHAPTER XIII. Tannins.)

Kaikki kuvat (kemialliset kaavat) puuttuvat // None of the illustrations (of chemical formulas) included.

The catechol tannins are characterised among other distinctive features by the important fact that when digested with boiling dilute mineral acids a red precipitate, known as "anhydride" or "phlobaphene," is produced. The designation "catechol" arises from the fact that the majority of these substances give a green coloration with ferric chloride, and protocatechuic acid or catechol as one of their decomposition products. Such a classification is, however, misleading, in that by comparison with gallotannin, one is led to infer that these substances are similarly constituted and derived from diprotocatechuic acid only (OH)₂C₆H₃.CO.O.C₆H₃(OH)COOH

When hydrolysed, however, diprotocatechuic acid gives two molecules of protocatechuic acid without production of phlobaphene (Emil Fischer, private communication). Moreover, certain phlobaphene- yielding compounds are now known to exist in which a catechol nucleus is absent. Thus the cyanomaclurin of Jakwood (q.v.) is an instance in point, for although it contains only phloroglucinol and resorcinol nuclei, it readily yields a red anhydride of this character (cf. also Mimosa tannin, Pistachia tannin, and Maletto tannin). Evidently therefore the phlobaphene reaction is either due to the special structure of the tannin itself, or to the presence of a second phenolic grouping other than catechol in the molecule. This structure may perhaps vary in certain cases, but until this is clear it appears to be preferable to include all phlobatannins under one group rather than to complicate the subject by the introduction of subdivisions.

Bottinger and Etti (loc. cit.} have suggested a benzophenone structure for certain tannins, and this applied to the phlobatannins has, at first sight, the merit that maclurin (see Old Fustic) with dilute hydrochloric acid gives rufimoric acid (Wagner, Jahres., 1851, 420), a red amorphous phlobaphene-like mass. Kinoin (see Kino), usually regarded as the methyl ether of a pentahydroxybenzophenone, is readily transformed into kino-red, whereas aromadendrin (see Kino), apparently also a benzophenone derivative, resembles, according to Maiden and Smith, catechin in many of its properties. It has not yet been ascertained if the carboxylic acids of certain hydroxybenzophenones are tannins, but on the other hand it is to be remembered that compounds possessing such a structure will be strong mordant dye-stuffs, a property which is generally absent from tannins of the so-called catechol type. As already indicated, many phlobatannins are known which contain two distinct nuclei, more frequently phloroglucinol in addition to catechol, a fact which in these cases accounts for their reactivity with diazobenzene. More difficult to understand, however, is the precipitation of all phlobatannins in aqueous solution with bromine; this could be accounted for in those instances in which phloroglucinol or resorcinol nuclei are present in the compound, but curiously enough no difference in this respect appears to have been observed in cases where evidence as to the existence of these latter groupings has been of a negative character. Though by incautious alkali fusion, the existing phloroglucinol nucleus may escape detection and probably in many instances has done so, this evidence is such that it is hard to presume that in all cases bromine precipitation arises from the presence in these compounds of a specific phenolic grouping.

It is most probable that the phlobaphene reaction is, in many cases, to be assigned to the well-known reactivity of the phloroglucinol group present in these substances, and this has also been suggested by Emil Fischer. Thus coloured compounds can be obtained by the interaction of phloroglucinol with many aldehydes, of which the red phloroglucinol vanillein is an example, and moreover the phloroglucides of Counder (Ber., 1895, 28, 26), prepared by passing hydrogen chloride into mixtures of phloroglucinol and various sugars in aqueous solution, are coloured, the d-galactose phloroglucide possessing a red tint. Again, by boiling dextrose and phloroglucinol with dilute hydrochloric acid, a brownish-red precipitate of a phlobaphene-like character can be easily produced (private communication). Interesting in this respect also is the "phlorotanninred" of Schiff (Annalen, 245, 40), which he obtained by heating his so-called diphloroglucinol carboxylic acid to 160-175°.

That phlobatannins like gallotannin owe their tanning property, as a rule, to the depside grouping appears likely, and it has indeed been observed by Trimble ("The Tannins," ii., 91) that tannins from various species of oak, on long digestion with boiling 2 per cent. hydrochloric acid, give not only phlobaphene but some quantity of protocatechuic acid.

In the light of the researches of E. Fischer and Freudenberg (loc. cit.), it is also to be anticipated that many of these compounds will be eventually found to possess a sugar nucleus.

Closely connected with the phlobatannin group are the welldefined crystalline substances catechin, aca-catechin and cyanomaclurin, to the first of which the constitution has been assigned by v. Kostanecki and Lampe.

Catechin, although not a tannin, reacts with pine wood and hydrochloric acid, gives with bromine water the insoluble bromcatechuretin and readily yields substances of a phlobaphene character. Though the evidence is not precise it is stated by Loewe and also by Etti that catechin can be readily transformed into catechutannic acid, a substance existing side by side with it in the plant and possessing properties typical of the phlobatannin group. It has indeed been surmised that the catechol tannins of certain plants may owe their origin to the prior existence of substances of the catechin type, and that in addition to the compounds previously enumerated, the so-called quebracho resin and guarana catechin are intermediate products of tannin formation. Again, according to Procter (private communication), a colourless catechin-like substance is to be found in mangrove cutch.

With the exception of coca-tannic acid, no crystalline catechol tannins have been described, but all are amorphous and very similar in appearance to natural gallotannin, although on keeping, especially in the moist condition, they are apt to develop a red tint. According to Perkin, most phlobatannins can be prepared in a nearly colourless condition by extracting a solution of the crude tannin to which sodium bicarbonate has been added with ethyl acetate (cf. Gallotannin).

Numerous phlobatannins have been isolated, and a description of them is given below, although it is extremely doubtful whether the majority of these are individuals. In many cases, indeed, the general reactions of these compounds are the same, and considering the extreme difficulty in the effective purification of amorphous preparations of this character, specific differences which have been observed will no doubt, on further investigation, largely disappear. Trimble (loc. cit., ii., 132), who submitted certain phlobatannins to careful purification, found that there was a fair approximation in their percentage compositions, and it is thus natural to presume that the known members of this group are not so numerous as was formerly considered to be the case.

Anachueta wood and bark contain a tannin which gives a green coloration with iron salts.

Aspertannic acid, C₁₄H₁₆O₈, was obtained by Schwarz from Woodruff (Asperula odorata, Linn.), and gives a green colour with ferric salts. It does not yield precipitates with albumen, gelatin, and tartar emetic solutions.

Atherospermatannin, from the bark of Atherosperma moschatum (Labill.), gives a green colour with ferric salts. A lead salt, C₁₀H₁₄PbO₃, has been described by Zeyer (Jahrb. Min., 1861, 769).

Barbitamao tannic acid, from the bark of the Stryphnodendron barbatimum (Mart), (Wilbuszwitcz, Ber. Ref., 1886, 19, 349), is an amorphous red powder, which yields a phlobaphene, and on fusion with alkali, protocatechuic acid and phloroglucinol.

Beech tannin, from the bark of the red beech, contains a tannin of the composition C₂₀H₂₂O₉ (Etti, Monatsh., 10, 650).

Caffetannic acid, C₁₅H₁₈O₉, occurs in coffee berries in the form of calcium and magnesium salts (Rochleder, Annalen, 59, 300); in cainia root, Chiococca brachiata (Ruiz, et Pav.), (Rochleder and Hlasiwetz, ibid., 1848, 66, 35); Nux vomica (Sander, Arch. Pharm., 1897, 235, 133); St. Ignatius beans (ibid.}, and Paraguay tea, Ilex paraguensis (A. St. Hill.), (Rochleder, Annalen, 1847, 66, 39), (cf. also Graham, Stenhouse and Campbell, J. pr. Chem., 1856, 69, 815; Arata, Ber., 1881, 14, 2251; Kunz-Krause, Arch. Pharm., 1893, 231, 613; Ber., 1897, 30, 1617).

It is an amorphous powder readily soluble in water, gives a darkgreen coloration with ferric chloride, and when boiled with potassium hydroxide solution gives caffeic acid and a sugar (Hlasiwetz, Annalen, 1866, 142, 220), which is glucose (Sander, loc. cit.). On dry distillation catechol is formed, and by fusion with alkali protocatechuic acid is obtained. The ammoniacal solution becomes green on exposure to air with formation of viridic acid, a substance which is also present in coffee berries in the form of its calcium salt (Rochleder, Annalen, 63, 197). According to Griebel (Inaugural Diss. Munich, 1903) caffetannic acid is C₁₈H₂₄O₁₀, and a penta-acetyl derivative corresponding to this formula is described.

Callutannic acid, C₁₄H₁₄O₉, contained in heather, Calluna vulgaris (Salisb.), is an amber-coloured substance. It gives a green coloration with ferric chloride, and when heated with dilute mineral acids yields an amorphous anhydro derivative, C₁₄H₁₀O₇ (Rochleder, Annalen, 84, 354; Sitz. Ber., 9, 286).

Canaigre tannin is present in the tuberous roots of the Rumex hymenosepalus (Torr.), an important American tanning material. It has been submitted to an elaborate examination by Trimble ("The Tannins," 1894, ii., 115), who describes it as a yellowish-white powder readily soluble in water, which gives with lead acetate solution a yellow precipitate, and with ferric chloride a green precipitate. It reacts with bromine water to give a yellow deposit, and on heating to 160-190° is decomposed with production of catechol. Boiling 2 per cent, hydrochloric acid yields an insoluble red phlobaphene together with some protocatechuic acid. Sugar is not formed in this decomposition. Analysis gave C = 58,10; H = 5,33, figures which approximate to, though they are somewhat lower than, those given by the best-known phlobatannins.

Catechutannic acid (see Catechu).

Cherry bark tannin, C₂₁H₂₀O₁₀+½H₂O, is present in the bark of the Prunus cerasus (Linn.), (Rochleder, Sitz. Ber., 59, 819), and gives a green coloration with ferric chloride. It is not a glucoside, but when digested with boiling dilute acids gives a red phlobaphene, C₂₁H₁₆O₈+¾H₂0.

Cocatannic acid, C₁₇H₂₂O₁₀+2H₂O (?), present in the leaves of the Erythroxylon coca (Linn.), (Niemann, Jahrb. Min., 1860, 386), is sparingly soluble in cold water and gives a green coloration with ferric salts. According to Warden (Pharm. J., 18, 985) it can be obtained in microscopic crystals of a sulphur yellow colour.

Colatannin or Kolatannin, C₁₆H₂₀O₈, a light red amorphous powder, exists in the Kola nut, Cola acuminata (Schott and Endl.), in combination probably with caffeine and theobromine (Knox and Prescott, Amer. Chem. J., 1897, 19, 63). With ferric acetate it gives a green coloration, and on fusion with alkali protocatechuic acid is obtained. Penta-acetylkolatannin, C₁₆H₁₅O₈(C₂H₃O)₅, colourless powder, tribromkolatannin, C₁₆H₁₇O₈Br₃, red-brown powder, penta-acetyl-tribromkolatannin, C₁₆H₁₂O₈Br₃(C₂H₃O)₅, pentabrom, and hexabromkolatannin have been described. On heating, kolatannin yields various anhydrides, (C₁₆H₁₉O₇)₂O at 107-110°, (C₁₆H₁₇O₆)₂O at 135-140°, and C₁₆H₁₆O₆ at 155-156°.

Cortepinitannic acid, C₃₂H₂₄O₁₇, occurs together with pinicortannic acid in the bark of the Scotch fir, Pinus sylvestris (Linn.), It consists of a bright red powder, the aqueous solution of which gives an intense green coloration with ferric chloride (Kawalier, Sitz. Ber., 11, 363).

Euphrasia tannin is present according to Enz (Vierteljahrsch. Pharm. J., 8, 175) in the green parts of Euphrasia officinalis (Linn.), and gives a green colour reaction with ferric salts. Its lead salt has the composition C₃₂H₂₀Pb₃O₂₀.

Fragarianin, v. Strawberry tannin.

Filitannic acid, C₄₁H₃₆NO₁₈ (?), exists in fern-root (Aspidium filix mas, Swartz.), (Malin, Annalen, 143, 276), and forms a red-brown powder which, in aqueous solution, gives an olive-green coloration with ferric chloride. By boiling with dilute sulphuric acid it gives filix red, C₂₀H₁₈O₁₂, an amorphous compound, and this when fused with alkali gives phloroglucinol and protocatechuic acid (cf. also Reich, Arch. Pharm., 1900, 238, 648).

Fraxitannic acid, C₂₆H₃₂OH (?) occurs in the leaves of the as tree, Fraxinus excelsior (Linn.). It consists of a brownish-yellow deliquescent powder, and when heated at 100° loses water and forms an almost insoluble anhydride, C₂₆H₃₀O₁₃ (Gintl and Reinitzer, Monatsh., 3, 745). Aqueous ferric chloride produces a dark green coloration, and when oxidised with permanganate this tannin yields quinone. Acetylfraxitannic acid, C₂₆H₂₈O₁₄(C₂H₃O)₄, benzoylfraxitannic acid, C₂₆H₂₈O₁₄(C₇H₅O)₄, tribromfraxitannic acid, c₂₆H₂₉Br₃O₁₄, and tetra-acetyltribromfraxitannic acid, C₂₆H₂₅Br₃O₁₄(C₂H₃O)₄, have been described.

Galitannic acid, C₁₄H₁₆O₁₀, H₂O, exists in the bark of the Galium verum (Linn.), (Schwarz, Annalen, 83, 57). It gives a yellow precipitate with basic lead acetate and a green colour reaction with ferric chloride.

Guarana tannin is present in the Paullinia cupana (H. B. and K.), the seeds of which, known as "guarana," contain theine and are extensively used in South America for medicinal purposes. According to Nierenstein (Die Gerbstoffe, 20) the tannin consists of small colourless crystals, melting-point 199-201°, and yields an acetyl derivative, melting-point 134-136°. It is laevo-rotatory, having [a]_D²⁰=72,4° in water, and [[a]_D²⁰=-39,1° in alcohol.

Hemlock tannin, C₂₀H₁₈O₁₀ (?) is present in Hemlock bark, Tsuga [Abies] canadensis (Carr.), the extract of which is prepared on a very large scale in North America for tanning purposes. According to Bottinger (Ber., 1884, 17, 1125), it is probably related to the quercitannic acid of the oak, and on heating with sulphuric or hydrochloric acid gives the anhydride hemlock red, C₄₀H₃₀O₁₇. With hydrochloric acid at 180, hemlock red evolves methyl chloride; when heated with acetic anhydride the acetyl compound is produced, whereas bromine gives a mixture of the compounds C₄₀H₂₀Br₁₀O₁₁ and C₄₀H₁₆Br₁₄O_i7. Bromine added to the diluted tannin extract precipitates tetrabromhemlock tannin, C₂₀H₁₄Br₄O₁₀, a yellow powder which yields the penta-acetyl derivative C₂₀H₉Br₄O₁₀(C₂H₃₀)₆, and by the further action of bromine the bromine compound C₂₀H₁₂Br₆O₁₀ (cf. Trimble, Amer. J. Pharm., 1897, 69, 354, 406; J, Soc. Chem. >Ind., 1898, 17, 558) is produced.

Hop tannin, C₂₂H₂₆O₉, present in hops (Humulus lupulus, Linn.), (Etti, Annalen, 180, 223; Monatsh., 10, 651), has all the properties of a catechol tannin. It gives with ferric chloride a dark green coloration and with boiling dilute mineral acid the phlobaphene "hop red," C₃₈H₂₆O₁₅. Hop red forms a cinnamon-coloured powder and on fusion with potash yields phloroglucinol and protocatechuic acid.

Horse chestnut tannin, C₂₆H₂₄O₁₂, is present in nearly all parts of the Aesculus hippocastanum (Linn.) and in the root bark of the apple tree (Rochleder, Sitz. Ber., 53, [ii.], 478; 54, [ii.j, 609). It consists of a nearly colourless powder, the solution of which gives a green coloration with ferric chloride, and when boiled with dilute mineral acid the phlobaphene C₂₆H₂₂O₁₁ or C₂₆H₂₀O₁₀. On fusion with alkali, phloroglucinol and protocatechuic acid are produced. V. also phyllascitannin and CHESTNUT EXTRACT.

Ipecacuanhic acid, C₁₄H₁₈O₇, was obtained by Willigt (Annalen, 76, 345) from the roots of Psychotria ipecacuanha (Stokes), and consists of a reddish-brown, bitter hygroscopic substance. Its solution gives a green coloration with ferric chloride.

Japonic acid (see CATECHU).

Kino (see KINO).

Larch tannin. The bark of the larch, Larix europaea (DC.), contains considerable quantities of a tannin which was examined by Stenhouse (Phil. Mag., 23, 336). It forms an olive-green precipitate with ferric salts, and when boiled with dilute sulphuric acid a red phlobaphene is produced.

Maletto tannin occurs in the bark of Eucalyptus occidentalis (Endl.) and other species of eucalyptus. According to Strauss and Geschwender (Zeitsch. angew. Chem., 1906, 19, 1121) it possesses the formula (C₄₃H₅₀O₂₀)₂, and appears to be identical with quebracho tannin. Dekker (Arch. Neerland., 1909, ii., 14, 50) prefers the formula (C₁₉H₂₀O₉)_?, and describes the acetyl derivative C₃₈H₂₈O₁₆Ac₁₀ and benzoyl derivative C₁₉H₂₅O₁₂Bz₅. Heated with zinc-dust and sodium hydroxide solution the tannin gives small quantities of gallic acid and phloroglucinol, whereas dry distillation yields pyrogallol and traces of other phenols. Boiling dilute sulphuric acid forms "maletto red," C₅₇H₅₀O₂₂ from which the acetyl derivative can be obtained.

Mangrove tannin, C₂₄H₂₆O₁₂. This important tannin is derived from the Rhizophora mangle (Linn.), R. mucronata (Lam.), Ceriops candolleana (Arn.), C. roxburghiana (Arn.), and other allied species. It is described as an amorphous red powder, which on fusion with alkali gives protocatechuic acid, and with boiling dilute sulphuric acid the phlobaphene C₄₈H₄₆O₂₁. The monacetyl derivative C₂₄H₂₅O₂₃(C₂H₃O) melts at 205° (Nierenstein, Die Gerbstoffe). This tannin closely resembles in its properties catechutannic acid (see CATECHU), and indeed mangrove cutch and catechu may be employed in many cases for the same purpose. Possibly these two substances are identical, and Procter (private communication) has isolated from mangrove cutch a small quantity of a colourless crystalline substance resembling catechin. Perkin who examined an ethyl acetate extract of the fresh bark of the C. candolleana, prepared in Borneo, was unable to detect the presence of a catechin, but obtained the tannin as a pale yellow powder, which gave a green coloration with ferric chloride solution, and resembled catechutannic acid in many respects.

Mimosa tannin is derived from various species of Mimoseae, such as the Acacia arabica (Willd.) of Egypt, the so-called "Wattles" of Australia, and numerous others. The tannin present is interesting in that though it possesses the reactions of a phlobatannin, such as phlobaphene production, precipitation by bromine water and solubility of its lead compound in acetic acid, etc., it gives a bluish-violet coloration with ferric chloride. Ammonium sulphide gives a precipitate with a mimosa solution, when after removal of the excess by boiling, a few drops of sulphuric acid are added, followed by a small quantity of salt. All other phlobatannins, except Maletto and Pistaschia tannins, give no precipitate by this method (Stiansy, private communication).

Moritannic acid, see Maclurin.

Oxypinitannic acid, v. Pinitannic acid.

Oak bark tannin or Quercitannic acid is found in the bark of the oak, and is not to be confused with the tannin of oak-wood from which it is distinct, and which is described below under the name of quercinic acid. In 1792 George Swayne communicated to the Society of Arts his results on the use of oak leaves in tanning (Trimble, "The Tannins," ii., 51), and the subject was again discussed by Berzelius in his Lehrbuch, 1827, and by Liebig (Handbuch der Chemie, 1843). Assumed at that time to be identical with the tannin of nut-galls (gallotannin), it was first shown by Stenhouse (Phil. Mag., 22, 425) to differ from this substance. According to Grabowski (Sitz. Ber., 56, [ii.], 388), Oser (ibid., 72, [ii.], 178), Johanson (Arch. Pharm., [in.], 9, 210), Bd'ttinger (Ber., 14, 1598), quercitannic acid is in reality a glucoside, but Etti (Ber., 14, 1826; Monatsh., 4, 512) found that the pure substance did not yield a trace of sugar. Various formulæ have been assigned to this substance, viz. Eckert (Jahres., 1864, 608), C₂₈H₂₀O₂₀; Oser (Jahres., 1875, 600), C₂₀H₂₀O₁₁; Löwe (Zeitsch. anal. Chem., 20, 210), C₂₈H₃₀O₁₅; Bottinger (Ber., 1883, 16, 2712), C₁₅H₁₂O₉, 2H₂O, and Etti (Monatsh., 10, 650), C₁₇H₁₆O₉, C₁₈H₁₈O₉, and C₂₀H₂₀O₉.

Quercitannic acid is described by Etti as a reddish-white powder almost insoluble in water. One of its most characteristic properties is the readiness with which it forms reddish-brown anhydrides when heated by itself or with dilute acids. Thus at 130-140° the first anhydride, C₃₄H₃₀O₁₇, is produced, and this by heating with dilute sulphuric or hydrochloric acid gives the second anhydride, C₃₄H₂₈O₁₆, Boiling dilute sulphuric acid again converts the original tannin into the third anhydride, 2C₁₇H₁₆O₉ 3H₂O = C₃₄H₂₆O₁₅.

These compounds are insoluble in water, but colour a solution of ferric chloride blue (?). Löwe (loc. cit.) again examined an anhydride, C₂₈H₂₄O₁₂, and an "oak red," C₂₈H₂₂O₁₁. Bottinger, who prepared the oak red from the tannin by means of dilute sulphuric acid, adopted the formula (C₁₄H₁₀O₆)₂, H₂O. Etti (Ber., 1884, 17, 1823) isolated from the bark of the Quercus pubescens (Willd.) a tannin C₂₀H₂₀O₉, practically identical with the substance C₁₇H₁₆O₉ previously found in the Q. robur (Linn.), but giving a green solution with ferric chloride, and not a blue, as formerly stated.

When heated with dilute sulphuric acid at 130-140°, quercitannic acid gives in addition to anyhydride 1,5 per cent, of gallic acid, and with strong hydrochloric acid at 150-180° the formation of oak red was accompanied by the evolution of methyl chloride (Etti, Monatsh., 1, 274). On dry distillation it yields dimethylcatechol and catechol, and on fusion with alkali protocatechuic acid, catechol, and phloroglucinol.

By the action of bromine on the aqueous bark extract, Bottinger (Ber., 1883, 1 6, 2710) obtained dibromquercitannic acid, C₁₉H₁₄Br₂O₁₀, as a yellow precipitate, and from this penta-acetyldibromquercitannic acid was prepared. Heated with hydrochloric acid at 180°, methyl chloride was evolved, and by means of hydroxylamine hydrochloride the compound C₁₉H₁₅Br₂NO₁₀ was produced. Reduced with sodium amalgam hydroquercinic acid, C₁₅H₁₈O₇ or C₁₅H₁₆O₆, and hydroquergalic acid, C₁₄H₁₄O₆, are formed (Annalen, 263, 121). When suspended in chloroform and further brominated it yields tetrabromdehydroquerdtannic acid. Quercitannic acid was thus C₁₉H₁₆O₁₀, contained five hydroxyls, and the group COCH₈.

According to Etti (Monatsh., 10, 647) a further investigation of the tannins C₁₇H₁₆O₉ and C₂₀H₂₀O₉ has proved that they are not glucosides, but derivatives of a ketone acid, C₆H₂(OH)₃.CO.C₆H(OH)₃COOH

Trimble ("The Tannins," 1894, ii., 77) carried out an elaborate investigation of the tannins present in the barks of the Quercus alba (Linn.), Q. coccinea (Wangenh.), Q. discolor (Ait.), Q.falcata (Michx.), Q. palustris (Du Roi), Q. prinus (Linn.), Q. bicolor (Willd.), Q. obtusiloba (Michx.), Q.phellos (Linn.), Q. rubra (Linn.), Q. robur (Linn.), Q. semicarpifolia (Sm.), employing acetone for the purpose of extraction. The tannins in most cases had a pale yellow colour, gave with ferric chloride a green coloration, and practically identical results with all the usual tannin reagents.

Heated with glycerol at 160, the tannins of Q. tinctoria, Q. palustris, Q. falcata, and Q. phellos yielded catechol, whereas with fused alkali the eight samples examined gave protocatechuic acid. Though all these compounds produce a violet colour on pine wood moistened with hydrochloric acid, phloroglucinol or other phenol was not detected among their decomposition products. Heated with 2 per cent, hydrochloric acid for two and a half hours, a phlobaphene separated, whereas the solution contained protocatechuic acid. Analyses of nine of these tannin preparations showed but little variation, the average being C = 59,79; H = 5,08, and approximately correspond with those of Etti (loc. cit.) for the tannin C₂₀H₂₀O₉ from Q. pubescens, and of Kraemer (Amer. J. Pharm., 1890, 236) for the tannin of Q. alba.

Oak wood tannin, Quercin, Quercic acid, Quercinic acid, C₁₅H₁₂O₉, 2H₂O, consists of a light brownish-yellow substance, and is distinguished from the quercitannic acid of oak bark in that its aqueous solution gives a blue, not green, coloration with ferric chloride, and does not yield a precipitate with bromine water (Böttinger, Ber., 1887,20,761).

With the object of isolating the tannin in a pure condition, Bottinger acetylated a purified extract of the wood, and decomposed the acetyl compound C₁₅H₇(C₂H₃O)₅O₉ by heating it with water at 135°. By the action of sodium amalgam on the acetyl derivative, Böttinger (Annalen, 263, no) obtained hydroquercic acid, C₁₅H₁₈O₇ or C₁₅H₁₆O₆, querlactone, C₅H₆O₂, and an acid which is probably trihydroxybutyric acid.

Hydroquercic acid is a grey-brown, bitter hygroscopic powder, which forms the acetyl derivative C₁₅H₁₄(C₂H₃O)₂O₆, the barium salt (Cl5H15Ofl ) 2Ba, and the lead salt (C₁₅H₁₅O₆)₂Pb. Querlactone, on the other hand, forms the salt (C₅H₇O₃)₂Pb. As above noted, hydroquercinic acid could also be obtained in a similar manner from qiiercitannic acid.

Etti (Monatsh., 10, 647) isolated from the wood of the Slavonian oak a tannin C₁₆H₁₄O₉ which appeared to be a ketonic compound. This is present in the wood in the form of a readily soluble salt (probably magnesium salt). Crystallised from alcohol, it forms brownish-red microscopic warty spherical masses, insoluble in water, and having the properties of a monobasic acid (cf. Fuchs, Monatsh., 9, 1132). With phenylhydrazine it gives the compound C₂₂H₂₀N₂O₈, forms a brown amorphous oxime C₁₆H₁₅NO₉, and with dilute sulphuric acid at 120-130° yields gallic acid in addition to a red anhydride. Heated alone at 130-135°, or in a sealed tube with water at 100°, anhydrides are also produced, which on boiling with hydriodic acid evolve methyl iodide. On long digestion of the tannin C₁₆H₁₄O₉ with hydrochloric acid at 100°, a methoxyl group is split off, with production of a yellow coloured acid C₁₅H₁₂O₉ in which a methoxyl group is still present. This tannin is therefore probably the dimethyl ether of the ketonic acid formulated above.

On boiling the tannin with dilute sulphuric acid, the anhydride C₃₂H₂₄O₁₆ is produced, whilst on heating in a closed tube the anhydrides C₃₂H₂₀O₁₄ and C₃₂H₁₈O₁₃ were obtained.

The varied results of many of these workers with the oak tannins appear to be due, as suggested by Trimble, to the fact that in many cases they employed oak tannin extracts of doubtful authenticity. Thus it is possible that Etti, who in his earlier work describes the tannin as producing a blue coloration with ferric chloride, was in reality examining oak-wood and not oak-bark preparations, and again the peculiar insoluble property of certain of his tannins, also commented on by Trimble, suggests that in these cases he investigated an anhydride rather than the tannin itself. That oak barks contain a phlobatannin possessing a catechol nucleus appears to be certain from the investigations of Trimble, and it seems probable that in the wood either a pyrogallol tannin or a phlobatannin containing a pyrogallol group is present. Though, as stated above by Etti in the case of the Slavonian oak, this yields phlobaphenes, Stiasny (private communication) considers that such is not usually the property of oak-wood extracts. An interesting point, moreover, apparently not stated in the literature, though well known to tanners, is that oakwood extracts give some ellagic acid, and on this account impart to leather the "bloom" so characteristic of this substance.

Oenotannin, C₁₉H₁₆O₁₀ (?), was obtained by Gautier from red wine (Bull. Soc. chim., 1877, 27, 496), who describes it as a colourless substance readily soluble in water. It gives a green coloration with ferric chloride solution, by fusion with alkali protocatechuic acid and phloroglucin, and when exposed to moist air becomes converted into an insoluble red phlobaphene-like substance. According to Heise (Ber. Ref., 22, 823), oenotannin contains gallotannin and is a mixture of three compounds.

Pistachia tannin is present in the leaves of the Pistacia lentiscus (Linn.) in addition to some quantity of a gallotannin (Perkin and Wood, Chem. Soc. Trans., 1898, 73, 378), and consists of a pale brown brittle mass which with iron alum solution gives a blue-black coloration. With boiling dilute sulphuric acid a phlobaphene quickly separates, and when fused with alkali, gallic acid and phloroglucinol are produced.

Phyllœscitannin is the name given by Rochleder to a tannin present in the small leaflets of the horse chestnut, as long as they remain enclosed in the buds (Zeitsch. fur Chem., 1867, 84). It is described as an amorphous red-brown substance of the formula C₂₆H₂₄O₁₃, H₂O, having a strongly astringent taste.

Pinicortannic acid and cortepinitannic acid occur in the bark of the Scotch fir, Pinus sylvestris (Linn.), and can be separated owing to the fact that in aqueous solution the former only is precipitated by means of lead acetate. Pinicortannic acid forms a reddish-brown powder of the composition (C₁₆H₁₈O₁₁)₂, H₂O, which after drying is sparingly soluble in water. It gives a green coloration with ferric chloride, and when boiled with dilute acids gives the phlobaphene C₄₈H₅₀O₂₁ (Kawalier, Sitz. Ber., 11, 361).

Pinitannic acid and oxypinitannic acid occur in the needles of the Scotch fir, Pinus sylvestris (Kawalier), and are distinguished from one another by the fact that the former only is precipitated by lead acetate solution. Pinitannic acid, according to Rochleder (Sitz. Ber., 2 9> 6p), also present in the Thuja occidentalis, is a reddish-yellow substance which gives a red-brown coloration with ferric chloride and when boiled with dilute acids a sparingly soluble red product (phlobaphene).

Oxypinitannic acid, on the other hand, yields a green solution with ferric chloride (Kawalier).

Quebracho tannin or Quebrachitannic acid, see QUEBRACHO COLORADO.

Quinotannic acid or Cinchonatannic acid obtained from cinchona bark is a light yellow very hygroscopic substance, a solution of which gives a green precipitate with ferric salts. On digestion with boiling dilute sulphuric acid, it is converted into a sugar and cinchona red C₂₈H₂₂O₁₄ (Rembold, Annalen, 143, 270), and from the latter by fusion with alkali, protocatechuic and acetic acids are produced (Hlasiwetz, ibid., 143, 307). According to Schwarz (Sitz. Ber., 7, 250), quinotannic acid has the composition C₁₄H₁₆O₉, whereas cinchona red is to be represented as C₁₂H₁₄O₉.

>Quinovatannic acid, contained in the bark of the Cinchona nova, in many respects resembles quinotannic acid (Hlasiwetz, Annalen, 79, 129). With ferric chloride it gives a dark green coloration, and with boiling dilute acids quinova red C₁₂H₁₂O₅ is produced. On fusion with potash it yields protocatechuic acid.

Rhamnotannic acid (so-called), present in buckthorn berries, is in reality not a tannin matter.

Rhatany tannin, C₂₀H₂₀O₉, from the bark of rhatany root. Krameria triandra (Ruis and Pav.), (Willstein, Jahres., 1854, 656) is described by Raabe (ibid.) 1880, 1060) as a light yellow powder, readily soluble in water. Its solution gives with ferric chloride a green coloration. When heated with dilute acids it yields rhatany red, C₂₂H₂₂O₁₁, and a sugar (Grabowski, Annalen, 143, 274), whereas according to Raabe (loc. cit.) no sugar is thus produced and the red substance possesses the composition C₂₀H₁₈O₈. By dry distillation rhatany red yields catechol, and protocatechuic acid and phloroglucinol when fused with alkali.

Rheotannic acid or Rhubarb tannic acid, C₂₆H₂₆O₁₄, derived from rhubarb, forms a yellowish-brown readily soluble powder, the solution of which gives with ferric chloride a black-green precipitate. With boiling dilute acids it gives rheic acid (rheumic acid), C₂₀H₁₆O₉ and a fermentable sugar (Kubly, Zeitsch. fur Chem., 1868, 308), although according to Tschirch and Neuberger (Schweiz, Wochenschr. Chem. Pharm., 1902, 282) in this manner rheum-red, C₄₀H₃₂O₁₈, cinnamic acid, gallic acid, and sugar are produced. According to Gilson (Chem. Zentr., 1903, i., 722, 882), two glucosides are present, glucogallin, C₁₃H₁₆O₁₀, giving gallic acid and dextrose, and tetrarin, C₃₂H₃₂O₁₀, from which rheosurin, C₁₀H₁₂O₂, cinnamic acid, and gallic acid can be produced. According to Krembs (Inaugural Diss., 1903, Berne), a catechin is also present in rhubarb.

Rhodotannic acid, 4C₁₄H₁₂O₇, 3H₂O, found in the leaves of Rhododendron ferrugineum (Linn.), is an amber-coloured substance which gives a green coloration with ferric chloride solution. Heated with dilute mineral acids, a reddish -yellow precipitate of Rhodoxanthin, C₁₄H₁₄O₈, is produced (Schwarz, Sitz. Ber., 9, 298).

Rubinic acid, v. CATECHU.

Rubitannic acid, 2C₁₄H₂₂O₁₂ + H₂O, was obtained by Willigt (Annalen, 82, 340) from the leaves of Rubia tinctorium (Linn.). It gives a green colour reaction with ferric chloride.

Sequiatannic acid, C₂₁H₂₀O₁₀, was isolated from the cones of Sequoia gigantea (Torr), (California), by Heyl (Pharm. Zentr., 1901, 42, 379) as a reddish-brown powder, soluble in water and yielding the salts MgC₂₁H₁₈O₁₀ and CaC₂₁H₁₈O₁₀. Boiled with dilute sulphuric acid, a phlobaphene, gallic acid, and a sugar are produced. The hexa-acetyl, C₂₁H₁₄O₆(C2H₃O)₆, hexabenzoyl, C₂₁H₁₄O₁₀(C₇H₅O)₆, and bromine, C₂₁H₁₅O₁₀Br₅, derivatives of this tannin are amorphous.

Sorbitannic acid, from the juice of the ripe berries of the mountain ash, Sorbus ancuparia (Linn.), forms a thick syrupy mass, which gives a green coloration with ferric chloride solution. It yields catechol on dry distillation, and protocatechuic acid and phloroglucinol when fused with alkali (Vincent and Delachanal, Bull. Soc. chim., [ii.], 47, 492).

Spruce-bark tannin, C₂₁H₂₀O₁₀ (?) gives, according to Bottinger, an unstable bromo derivative C₂₁H₁₄Br₆O₁₀. This reacts with hydroxylamine hydrochloride, and with hydrochloric acid at 180-190° evolves methyl chloride. The penta-acetylpentabromo derivative C₂₁H₁₀Br₅O₁₀(C₂H₂O)₅ was also prepared. With boiling dilute hydrochloric acid the tannin yields spruce red which gives the acetyl derivative C₄₂H₂₇(C₂H₃O)₇O₁₇ and when suspended in chloroform and treated with bromine the compound C₄₂H₂₄Br₁₀O₁₇ (Bottinger, Ber., 17, 1127).

Strawberry-root contains a tannin fragarianin (Phipson, Jahres., 1878, 891), the solution of which gives a green colour with ferric chloride. Boiling dilute hydrochloric acid forms glucose and a red substance fragiarin. On dry distillation the tannin gives traces of catechol, and when fused with alkali protocatechuic acid is produced.

Tannecortepinic acid, C₂₈H₂₆O₁₂, according to Rochleder and Kawalier (Sitz. Ber., 29, 23), can be isolated from the bark of young Scotch firs collected in the spring time. Ferric chloride gives a green coloration and boiling dilute acid a phlobaphene in addition to a little sugar.

Tannopinic acid, C₂₈H₃₀O₁₃ (?) is sometimes present in the needles of the Scotch fir gathered in the spring (Rochleder and Kawalier). In the winter time, oxypinitannic acid (loc. cit.) appears to take its place.

Tea tannin is probably identical with the quercitannic acid of oak bark (Stenhouse, Phil. Mag., 23, 332; Rochleder, Annalen, 63, 205; and Hlasiwetz and Malin, J. pr. Chem., [i.], 101, 109).

Tormentilla tannin, C₂₆H₂₂O₁₁, from the root of Potentilla tormentilla (Neck.), is an amorphous reddish powder, which colours ferric chloride solution blue-green. Boiled with dilute acids it produces tormentil-red without appreciable formation of sugar, and this appears to have the same composition as the tannin itself. With fused alkali phloroglucinol and protocatechuic acid are obtained. The root also contains a substance which yields ellagic acid when boiled with potash solution (Rembold, Annalen, 145, 5).

Viridic acid, C₁₄H₂₀O₁₁ (?), which exists in coffee beans as a calcium salt (Rochleder, Annalen, 63, 197), is obtained by the air oxidation of an ammoniacal solution of caffetannic acid, and forms a brown amorphous mass, the alkaline solutions of which are green. The salts, Ba₂C₁₄H₁₆O₁₁, PbC₁₄H₁₂O₈, and PbCuH₁₄O₉, have been described (cf. also Vlaanderen and Mulder, Jahres., 1858, 261).

Willow bark tannin. The bark of Salix triandra (Linn.) contains a glucoside tannin which gives a green colour reaction with ferric chloride, and when boiled with dilute sulphuric acid a brownred precipitate (Stenhouse, Proc. Roy. Soc., u, 403; Johanson, Arch. Pharm., [iii.], 13, 103).

Tannins are frequently accompanied in the plant by yellow colouring matters, and it has been pointed out by Perkin that a relationship is usually to be observed between these compounds in respect of the phenolic nuclei present in each. Thus catechu contains catechutannic acid and quercetin, both of which contain phloroglucinol and catechol groups, whereas both the cyanomaclurin and morin of Jakwood (Artocarpus integrifolia, Linn.) yield phloroglucinol and β- resorcylic acid. Again, in sumach (R. coriaria, Linn.), Pistacia lentiscus, Linn, (leaves) and Hæmatoxylon campeachianum, Linn, (leaves), a gallotannin and myricetin exist, both of which are pyrogallol derivatives.

Other similar instances of this relationship could be cited; and where a divergence of this rule at first sight seems evident, this is frequently more apparent than real, as in the case of Young Fustic (R. cotinus, Linn.), which is known to contain a gallotannin and fisetin (catechol and phloroglucinol). From the reactions of the wood extract, however, catechol tannin must also be present.

Group II. Diphenylmethylolid or Ellagitannins.
(CHAPTER XIII. Tannins.)

Kaikki kuvat (kemialliset kaavat) puuttuvat // None of the illustrations (of chemical formulas) included.

This tannin group, which has for its mother substance diphenylmethylolid (2), is at present only represented by one well-authenticated member known as ellagitannin, unless indeed pomegranate tannin (see below) is a distinct compound. Whereas Perkin and Nierenstein (Chem. Soc. Trans., 1905, 87, 1428) considered that ellagitannin was probably formed by the condensation of two molecules of digallic acid and possessed the formula (1) or was a glucoside of this substance, according to Nierenstein, ellagitannin (q.v.) (cf. also Ber., 1912, 45, 365) is a glucoside of luteoic acid (3).

Numerous natural tannin matters yield ellagic acid, but these have been little investigated, and it is likely that glucosides giving ellagic acid by hydrolysis but differing from ellagitannin itself in the character ot their sugar nuclei will be later discovered.

Though various hydroxydiphenylmethylolid compounds have been synthetically prepared, e.g. metellagic acid, C₁₅H₅O₄(OH), catellagic acid, C₁₄H₄O₄(OH)₂, flavellagic acid, C₁₄HO₄(OH)₅, and coeruleoellagic (cyanellagic) acid, C₁₄O₄(OH)6, (loc. cit.), ellagic acid is the only member of this group which has been isolated from natural sources.

Pomegranate tannin, C₂₀H₁₆O₁₃, is an amorphous greenish-yellow substance contained in the root bark of Punica granatum (Linn.). Boiling dilute sulphuric acid hydrolyses it with formation of a sugar and ellagic acid (Rembold, Annalen, 143, 385).

I. Depside Group.
(CHAPTER XIII. Tannins.)
(Osa artikkelista)

Kaikki kuvat (kemialliset kaavat) puuttuvat // None of the illustrations (of chemical formulas) included.

Tannin, Tannic acid, Gallotannin, Gallotannic acid is found to the largest extent in galls which arise from the puncture of insects of the genus Cynips on the leaves and buds of various species of oak, more especially the Quercus lusitanica (Lam.), and on a species of sumach, Rhus semilata (Murr.). Aleppo galls, derived from the young shoot of the oak and which are the best.variety of oak gall on the market, contain 50-60 per cent, of gallotannic acid, whereas Chinese galls from sumach yield as much as 70 per cent. In smaller amount it occurs also in numerous plants, and is probably the main tannin of various sumachs, of valonia, divi-divi, and algarobilla.

The methods employed for the isolation of gallotannic acid from galls are more or less modifications of that of Pelouze, which consists in extracting finely powdered galls with commercial ether. The extract separates into two layers, the upper consisting of an ethereal solution of gallic acid, wax, and resinous substances, whereas the lower represents a concentrated solution of gallotannic acid which, on evaporation, remains as a porous mass. In place of the ether a mixture of 75 per cent, commercial ether and 25 per cent, alcohol can be employed. The method of Leconnet (Annalen, 1836, 18, 179) consists in stirring powdered galls into ether until a thin cream results, pressing the mixture and repeating the operation until tannin is no longer removed. According to Domine (J. Pharm. Chim., 1844, [3], 5, 231), it is advantageous to allow the powdered galls before extraction to remain for some time in a moist atmosphere, and Pelouze's process witji this modification was adopted by the British and United States pharmacopœias.

For the manufacture on the large scale, Chinese or Japanese galls are preferably employed owing to their richness in tannin. The finely powdered material is stirred with sufficient water at 50-60° to form a concentrated aqueous extract, and after filtration the clear liquid is agitated with one-fourth of its volume of ether until an emulsion results. After standing for several days, the upper ethereal liquid which has separated is removed, and the lower layer, which contains all the tannin matter, is run into a still and the ether which is present recovered. After cooling the syrupy liquid is. spread out on sheets of tin, and heated by means of a steam coil, when the gallotannic rapidly puffs up and dries.

Thus prepared the commercial tannin contains some quantity of gallic acid, plant wax, glucose and other impurities; to remove these the material may be washed with ether, or the aqueous solution shaken with ether, or the aqueous liquid fractionally precipitated with common salt, the precipitate dissolved in ethyl acetate, and the tannin recovered by evaporation under reduced pressure. Trimble ("The Tannins," 85) treats a 5 per cent, solution of the tannin with 10 per cent, lead acetate, drop by drop, until the precipitate, at first yellow coloured, ceases; to be granular and is colourless. This is collected, the filtrate agitated two or three times with ethyl acetate and the extract evaporated. The colourless product, which still contains gallic acid, is redissolved in water, and the solution, after agitation with ether, evaporated under reduced pressure. Finally, the residue dissolved in ether by the aid of a little water is again brought rapidly to dryness under reduced pressure, and thus obtained is colourless and gives no reactions for gallic acid or glucose. With the object of preparing a homogeneous product, Walden (Ber., 1898, 30, 3154) nas employed dialysis, and also the precipitation of a solution of the purest commercial tannin in ethyl acetate with benzene. Rosenheim and Schidrowitz (Chem. Soc. Trans., 1898, 73, 882) point out the extreme difficulty of removing the last traces of gallic acid from the tannin and employ a mixture of ether and acetone for this purpose. Paniker and Stiasny (Chem. Soc. Trans., 1911, 99, 1821) state, however, that the process of Rosenheim and Schidrowitz only partially removes gallic acid. The method suggested by Perkin, which consists in neutralising a solution of the tannin with sodium bicarbonate, extracting the mixture with ethyl acetate, and subsequently precipitating the substance from the extract with benzene, gives a product free from even traces of gallic acid (see also Iljin, Ber., 1909, 42, 1731). Fischer and Freudenberg (Ber., 1912, 45, 919) find the addition of dilute sodium hydroxide to the tannin solution until faintly alkaline before extraction with ethyl acetate is similarly beneficial.

Gallotannic acid as found in commerce consists of an amorphous powder possessing a faint yellow colour, although when exhaustively purified it is colourless. It is readily soluble in water and alcohol, more sparingly in ethyl acetate, insoluble in pure ether, chloroform, or benzene. With solutions of ferric salts, gallotannic acid gives a bluishblack coloration or precipitate, according to the concentration, whereas ferrous salts give with strong solutions only a white precipitate which gradually turns blue in the air. Many metallic salts give precipitates with the tannin, those of lead and tin being colourless, whereas the copper and silver compounds possess a brown tint. Cold alkaline solutions absorb oxygen from the air and darken in colour with production of the so-called metagallic acid. When boiled hydrolysis occurs, gallic acid being formed. Gallotannic acid precipitates most alkaloids and gives precipitates with albumen and gelatin, the latter, according to Trunkel (Zeitsch. Biochem., 1910, 26, 458), in quantitative amount.

Analyses of gallotannin by Berzelius, Pelouze, Mulder, Bijlert, Strecker, Gautier, Trimble, Dekker, and Walden have been in fairly close agreement, varying from about C = 51.5 to C = 52.3; H = 3.7 to H = 4.1 per cent. Iljin (Ber., 1909, 42, 1735) has suggested higher numbers (C = 54.13; H = 3.22), the correctness of which he has again emphasised (J. pr. Chem., 1910, [ii.], 82, 422; cf. Nierenstein, ibid.) 1909, 42, 3552). Mulder (J. pr. Chem., 1849, 48, 90) was the first to assign to gallotannin the formula C₁₄H₁₀O₉, and this was subsequently adopted by Schiff (Ber., 4, 231), and until recently generally accepted as correct. Very numerous salts of gallotannic acid have been described which are in fair agreement with this view, of which ammonium tannate, NH₄C₁₄H₉O₉, potassium tannate, KC₁₄H₉O₉, sodium tannate, NaC₁₄H₉O₉, and barium tannate, Ba(Cl4H8O9) 2 (Buchner, Annalen, 53, 361) may be given as examples. For a complete list reference should be made to Beilstein (1896, ii., 1926).

Gallotannic acid isolated from plants is said to contain free glucose which is difficult to eliminate. Strecker (Annalen, 90, 340) indicated the possible existence of a glucoside of the formula C₂₇H₂₂O₁₇, although Schiff (ibid., 170), while agreeing that unaltered tannin is probably a glucoside of digallic acid, preferred the formula
C₃₄H₂₈O₂₂ ( = C₆H₁₂O₆ + 2C₁₄H₁₀O₉ - 2H₂0)
This corresponds to a yield of 23. per cent, of glucose; whereas in natural tannin about 22 per cent, is said to have been detected.

When gallotannic acid is heated at from 160-215°, water, carbon dioxide, and pyrogallol are evolved, and a dark coloured non-volatile substance known as metagallic acid is produced. According to Trimble (loc. cit.) the best yields of pyrogallol are obtained by raising the temperature slowly to 215° and then keeping it between 190-210° for half an hour. Digested with boiling dilute mineral acids, gallic acid is produced, and Wetherill, who employed for this purpose 50 grams of tannic acid and 500 c.c. of sulphuric acid (1 vol. acid + 4 of water), obtained a yield of 87.4 per cent, of gallic acid. Knop (Annalen, 170, 44) states he obtained 95 per cent., and Stenhouse the theoretical amount, whereas Trimble (loc. cit.) considers that when pure gallotannic acid is heated with a 2 per cent, solution of absolute hydrochloric acid, gallic acid only is produced.

[---]

A number of other synthetic products of this nature have also been described by Fischer (cf. Ber., 1914, 47, 2485).

For further references to gallotannin see Harnack (Arch. Pharm., 1896, 234, 537), colour reaction; Ljubavin (J. Russ. Phys. Chem. Soc., 1901, 33, 680), tannin and tartar emetic; Thibault (Bull. Soc. Chim., 1903, (iii.), 29, 745), tannin and bismuth; Vigneron (J. Pharm. Chim., 1906, (vi.), 23, 469), iodotannin; Farbewerke vorm. Meister, Lucius, and Brüning (D.R.P. 173729), mixed anhydrides of tannic and cinnamic acids; Biginelli (Gazzetta, 1907, 37, ii., 205; ibid., 1903, 38, i, 559), tannates of quinine; Hildebrandt (D.R.P. 188318), tannin and formaldehyde; Francis and Nierenstein (Collegium, 1911, 335), action of benzoyl chloride and potassium cyanide on benzoyl-hydroxybenzoic acids and on acetylated hydroxybenzoylhydroxybenzoic acids; Nierenstein (Die Gerbstoffe).

Chestnut tannin.

Chestnut tannin has been examined by Nass (Inaugural Diss., 1884, Dorpat, Russia) and by Trimble ("The Tannins," ii., 119). According to the latter author it is probably identical with ordinary gallotannin (see Chestnut extract).

Chebulinic add or Eutannin.

This tannin was isolated by Fridolin from myrobalans, Terminalia chebula (Retz.), which also contain an ellagitannin. It crystallises in rhombic prisms, is sparingly soluble in cold water, gives with ferric chloride a blue-black precipitate, and by heating with water is converted into gallic acid and a new tannin.

Thorns (Chem. Zentr., 1906, i., 1829; Apotheker Zeit., 1906, 21, 354) has found that commercial eutannin is identical with chebulic acid, C₂₈H₂₂O₁₉. It consists of small colourless needles, containing water of crystallisation, reacts acid to litmus paper, decomposes at 234°, and has [a]_D initially +61,7°, gradually rising to +66,9°. An ennea-acetyl and methyl derivative are described, the latter giving trimethylgallic acid by the action of sodium hydroxide solution. With water at 100-150° the tannin yields gallic acid and eutannin hydrate, C₂₈H₂₄O₂₀, a colourless powder decomposing at 200-210°. When eutannin is dissolved in cold sodium hydroxide, the solution acidified with acetic acid, and then treated with lead acetate solution, the resulting precipitate, after decomposition with sulphuretted hydrogen, gives gallic acid, and a tannin C₁₄H₁₆O₁₂ or C₁₄H₁₄O₁₁, which consists of a yellow powder, having [a]_D+26° at 15°, and giving a blue coloration with ferric chloride. To chebulinic acid the following constitution is assigned: [KUVA PUUTTUU] According, however, to Fischer and Freudenberg (Ber., 1912, 45, 915), when hydrolysed chebulinic acid gives also dextrose.

Hamamelitannin.

This compound, one of the few gallotannins as yet isolated in a crystalline condition, occurs in the bark of the Hamamelis virginiana (Linn.), a tree, 10 to 12 feet high, common in North America. The bark, previously extracted with light petroleum to remove plant wax, is exhausted with ether-alcohol (5:1), the solution evaporated, the residue dissolved in a little alcohol and treated with ether to precipitate certain impurities. Evaporation of the ethereal liquid gives a product, a hot aqueous solution of which, after treatment with alumina and animal charcoal, deposits, on cooling, the substance in the form of small colourless needles. From a dilute aqueous solution, hamamelitannin, C₁₄H₁₄O₉, crystallises with 5H₂O, but deposited from strong solutions the crystals contain 2½H₂O. The air-dried substance melts at 115-117°, although when dried at 100 the melting-point is 203°. Hydrolysed with boiling dilute sulphuric acid, gallic acid only was produced, and the presence of a sugar could not be detected. Hamamelitannin is dextro-rotatory, [a]_D=+35,43°. According to Fischer and Freudenberg (Ber., 1912, 46, 2712), however, this tannin probably contains a sugar nucleus.

Benzoylhamamelitannin, C₁₄H₉O₉(C₇H₅O)₅, is a yellow powder which melts at about 125-132° (Gruthner, Arch. Pharm., 236, 303).

-

Oak wood tannin is probably a member of this group, but is described under the heading of Phlobatannins.

Sumach tannin (see Sumach).

Diprotocatechuic acid is prepared by coupling monocarbomethoxyprotocatechuic acid with dicarbomethoxyprotocatechuyl chloride in alkaline solution and subsequently hydrolysing the product. It possesses the constitution (OH)₂C₆H₃.CO.O.C₆H₃(OH)COOH, and consists of fine needles which begin to sinter at 230° and melt with decomposition at 237-239°. It is much more sparingly soluble in water than protocatechuic acid, gives with ferric chloride a bluishgreen coloration, and possesses tanning properties (Fischer and Freudenberg, Annalen, 1911, 384, 2, 238).

Di-β-resorcylic acid, prepared from β-resorcylic acid by the same general method, (OH)₂C₆H₃.CO.O.C₆H₃(OH)COOH, forms small microscopic needles, melting at 215° (corr.), sparingly soluble in water. It gives with ferric chloride a violet-red coloration and behaves as a tannin.

Digentisic acid, (OH)₂C₆H₃.CO.O.C₆H₃(OH)COOH, crystallises in fine needles, melting-point 208-209° (corr.), sparingly soluble in water. The aqueous solution precipitates gelatin and gives a blue coloration with ferric chloride.

Fischer and Hoesch (loc. cit., 224) and Fischer and Lepsius (loc. cit., 224) have prepared numerous other acids of this type, but it is not stated as yet if these are to be regarded as tannins. Lecanoric acid, ramalic acid, evernic acid, and no doubt other lichen acids (loc. cit.) which structurally belong to this group may, it is possible, also possess tanning property.

CHAPTER XIII. Tannins. (osa)

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 ").

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