| C6H6 | C6H5OH | C6H4(OH)2 | C6H3(OH)3 | |||||||||||||||||||
| Benzene. | Phenol. | Pyrocatechol (or catechol), Hydroquinol, Resorcinol. | Pyrogallol, Phloroglucol. | |||||||||||||||||||
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| Benzoic acid. | Salycylic acid, Oxybenzoic acid. | Protocatechuic acid (and 5 other isomeric acids). | Gallic acid, &c. |
It will be noticed that a large proportion of the formulæ given above represent several compounds identical in composition, but frequently very distinct in their properties. The explanation of these differences lies in the different relative position of the OH and CO.OH groups round the benzene ring. Thus the following diagram represents the relative positions of the pyrocatechol series. It may be noted that each phenol yields two isomeric[E] acids. Miller (C. S. Jour., xli. 398), who has investigated these acids, remarks, "Of the 3 phenols C6H4OH2, catechol alone gives a precipitate with lead acetate, and of the 6 acids, C6N5OH2, CO.OH, none yields precipitates with lead acetate, except the 2 which are obtained from catechol."
[E] Isomeric, of similar composition but different structure and properties.
| Pyrocatechol 1-2. | Resorcinol 1-3. | Hydroquinol 1-4. |
| H──C══C──O──H | | H──C C──O──H ║ ║ H──C──C──H | H──C══C──O──H | | H──C C──O──H ║ ║ H──C──C──O──H | H──C══C──O──H | | H──C C──H ║ ║ H──O──C──C──H |
All the natural tannins with which we are acquainted, are derived from, and yield on decomposition either catechol, phloroglucol, or pyrogallol, and sometimes more than one of these. Artificial products, however, with many of the reactions of tannins have been obtained from other members of the group, and most phenols and their derived acids give either purplish or greenish black with ferric salts.
Several classifications of the tannins have been suggested. The division most obvious to the tanner is into those tannins which yield the whitish deposit in the surface of the leather, called "bloom," and those which do not. Stenhouse, some years since, divided tannins into 2 classes, one of which gives a bluish, and the other a greenish-black with ferric salts. In the main these 2 classes correspond to the 2 former, as most tannins which yield a blue-black with iron acetate also give bloom to the leather. In some cases, however, the difference of tint is due to accidental impurities, and even gallotannic acid will give a decided green with strongly acid ferric chloride. These classifications both correspond to well-defined differences of constitution, and it is obviously more scientific to arrange tannins according to the products which they yield on decomposition, and which indicate their ultimate structure, rather than on any less essential point.
If those tannins which give bloom to leather are cautiously heated to about 392° F. (200° C.), they are decomposed, and a substance is volatilised which condenses in feathery crystals, and which on examination turns out to be pyrogallol. Those tannins, on the other hand, which yield no bloom, but red deposits, produce a somewhat similar sublimate of catechol. From oak-bark and valonia, which yield both bloom and red colouring matters, both catechol and pyrogallol have been obtained. We may, therefore, divide tannins broadly into derivatives of catechol, which yield no bloom, and usually give greenish-blacks with iron acetate, and which include hemlock, mimosa, cutch, gambier, quebracho, &c; derivatives of pyrogallol, which give bluish-blacks with iron, deposit bloom in leather, and embrace galls, sumach, divi-divi, myrobalans, pomegranate rind, &c., and tannins which contain both pyrogallol and catechol, such as oak-bark and valonia, and which, as is well known, yield bloom, and give blue-blacks with iron.
If tannins are boiled with dilute sulphuric or hydrochloric acids, and allowed to ferment under the influence of pectose and other natural ferments, which are always present in vegetable tanning materials, a different series of decompositions takes place. Many tannins yield glucose, or starch sugar, as one of their products, or as that of closely associated impurities. Of this more must be said later. In addition it will be found that the catechol tannins invariably yield insoluble reddish-brown bodies which have been called phlobaphenes, and which differ from the original tannins in containing one or more molecules less water, and which, in chemical language, are anhydrides of their respective tannic acids. The pyrogallol tannins, on the other hand, yield gallic acid, or ellagic acid (the deposit forming bloom) either alone or in mixture. Oak-bark and valonia give both bloom and insoluble reds, and by digestion with acids in sealed tubes also gallic acid.
If the red anhydrides, which are produced from the catechol tannins, be fused with caustic potash, or in many cases, if they be simply boiled with concentrated potash solution, they are broken up still further, and from the fused mass, protocatechuic acid (which bears the same relation to catechol that gallic acid does to pyrogallol) may always be obtained. This is in many cases accompanied by phloroglucol, a phenol isomeric with pyrogallol, as may be seen by the table on [p. 61], but which tastes sweet like a sugar. Cutch, gambier, mimosa, quebracho, and probably many others, are phloroglucide tannins. The tannins which do not yield phloroglucol frequently give acetic acid, and other acids of the "fatty" group, along with protocatechuic acid. We may summarise this classification in the following table:—
Tannins boiled with dilute sulphuric acid yield (frequently glucose, and) Insoluble Reds, which fused with potash yield protocatechuic acid, and,──  Phloroglucol, as chestnut, gambier, kino, cutch, quebracho, rhatany, fustic, horse-chestnut, tormentil. Acetic acid, coffee, Peruvian bark, male-fern. | ||
| Reds and gallic and ellagic acids; no glucose |  | Oak-bark and valonia tannins. |
| No reds, but gallic and ellagic acids |  | Galls, myrobalans, sumach, divi-divi, pomegranate rind. These are probably mixtures of two tannins which yield |
| Gallic acid only | Digallic, or pure gallotannic acid. | |
| Ellagic acid only | Pure ellagitannic acid. | |