Catechin is a white crystalline substance, contained to the extent of some 30 per cent. in cube gambier, and in smaller proportion in block gambier and cutch, and very probably in all tanning materials yielding catechol-tannins. It melts at 4221/2° F. (217° C.) and yields a sublimate of catechol on further heating. It is readily soluble in alcohol and in boiling water, but requires 1133 parts of cold water for its solution. Hence it separates on cooling from a hot solution of gambier, and may be purified by redissolving in hot water and treatment with animal charcoal, and subsequent crystallisation. It may also be extracted from its aqueous solution by agitation with ether. It possesses no acid properties, though some writers have incorrectly called it catechuic acid. The aqueous solution gives precipitates with lead acetate and mercuric chloride, and reduces ammonia-nitrate of silver; but, unlike tannins, it does not precipitate gelatin, alkaloids, or tartar emetic. It is oxidised by permanganate in presence of free acid, and hence, when in solution, is estimated by the Löwenthal method as "not tannin." It dissolves in concentrated sulphuric acid with a deep purple coloration. By fusion with caustic alkalies it yields protocatechuic acid and phloroglucol together with hydrogen. Its constitution is very uncertain, but it is probable that the phloroglucol stands in a somewhat similar relation to the protocatechuic acid that glycerin does to the fatty acids in natural fats; and that its decomposition by fused alkalies is a process much akin to saponification. This constitution is represented by the following formula:—
| C19H18O8 or C7H8O2 | O.C6H3(OH)2 O.C6H3(OH)2 |
When acted on by heat and dilute acids the following anhydrides are produced—the formulæ given must be taken as to some extent provisional.
| Catechin 2C19H18O8 | = | C38H36O16 | Not acid; does not precipitate gelatin. | |
| Catechutannic acid | C38H34O15 |  | Acid, precipitate gelatin. | |
| Dianydride | C38H32O14 | |||
| Trianydride | C38H30O13 | | Insoluble in water. | |
| Catechuretin | C38H30O12 | Soluble in alcohol and alkalies. |
The white deposit which occurs on pit sides and in the interior of leather where gambier is largely used, and which is sometimes called "the whites," consists of catechin. It may be decomposed by warm sulphuric acid. This deposit is favoured by the use of hot gambier liquors. It is probable that by exposure to the air and by boiling, catechin is gradually converted into catechutannic acid during the tanning process, and hence the practical tanning value of cube gambier is probably greater than analysis indicates. Hunt has, however, shown (Jour. Soc. Chem. Ind., iv. 266) that, as estimated by the Löwenthal process, the tannin is to some extent lessened by boiling.
Kinoin, C14H12O6 (Etti, Berl. Ber., xi. 1879), obtained from green or malabar kino, a product very similar to cutch, by boiling with dilute hydrochloric acid and extraction by agitation with ether, is very similar in its properties to catechin. It does not itself precipitate gelatin, but like catechin, yields a series of anhydrides or reds, which do so. On dry distillation it yields catechol and common phenol; and when heated with hydrochloric acid at 248°-266° F. (120°-130° C.) methyl chloride, catechol and gallic acid. Hence its constitution is probably that of methyl-catechol gallate.
Quebracho-catechin was found by P. N. Arata (Chem. Soc. Jour., xl. 1152) in the wood of quebracho colorado ([p. 40]), but in too small quantity for detailed investigation. It probably bears a similar relation to quebrachitannic acid that ordinary catechin does to catechutannic acid. It is insoluble in cold and only slightly soluble in hot water, but very soluble in alcohol and ether. Its solution is clouded by normal lead acetate, and gives rose-coloured precipitates with basic lead acetate and mercurous nitrate, and blackish with ferric acetate; it reduces silver-nitrate and gold chloride, and is coloured yellow by nitric acid, red by sulphuric acid, yellowish by sodium hypochlorite, and green by Fehling's solution. It does not precipitate gelatin, or alkaloids.
Catechutannic acid has been pretty fully described under catechin, of which it is the first anhydride ([p. 80]). It is possibly identical with mimo-tannic acid, the tannin of cutch and mimosa bark, which is chemically very similar, but greatly differs in its practical effect in tanning. For some further reactions of cutch and gambier infusions see [p. 113]. It gives a greyish-green precipitate with ferric salts, and (distinction from gallotannic acid) precipitates cupric sulphate but not tartar emetic.
Quebrachitannic acid is got from the wood of the quebracho colorado, Quebrachia lorentzii (formerly Loxopterygium) which must not be confounded with the bark of the Aspidospermum quebrachia, which is valuable, not for its tannin, but for an alkaloid, aspidospermin, which is used for medical purposes. It has been pretty thoroughly investigated by P. N. Arata (Chem. Soc. Jour., xxxiv. 986 and xl. 1152). It seems, however, a little doubtful to the writer whether the substance investigated by Arata was not the anhydride of the tannin, rather than the tannin itself, as it presents many points of analogy to catechu-red and was less soluble in water than quebracho tannin appears in practice to be.
According to Arata, quebrachitannic acid is a pale red amorphous mass, having an astringent taste and yielding a light cinnamon coloured powder. It is insoluble in carbon-bisulphide, turpentine oil, and benzene. Its aqueous solution gives a white precipitate with both normal and basic lead acetate, which when heated, acquires first a rose, and then a chocolate colour; with ferric chloride a green liquid is produced, which changes after a time to red and becomes black on addition of sodium acetate. It forms white precipitates with gelatin, albumen, and alkaloids. By dry distillation it yields catechol. By fusion with potash or the action of sulphuric acid, phloroglucol and protocatechuic acid, while nitric acid converts it into oxalic and picric acids. While it shows great similarity in its reactions to catechutannic acid, it differs materially in percentage composition, containing only 52·5 as compared with 62·0 per cent. of carbon.