Synthetic Tannins, Their Synthesis, Industrial Production and Application - Georg Grasser

HO } C_6H_3.COO.C_6H_4.COO.C_6H_4.COO.C_6H_4.COOH CH_3O

The former has been prepared from carbethoxyhydroxy-benzoyl-p-hydroxybenzoyl chloride and p-hydroxybenzoyl-p-hydroxybenzoic acid in alkaline solution; the second tetradepside was prepared from carbomethoxyvanilloyl-p-hydroxybenzoyl chloride and p-hydroxybenzoyl-p-hydroxy-benzoic acid.

The preparation of these compounds is rendered difficult by the slight solubility of the substances and their slight affinities for entering into reaction. Both tetradepsides were obtained in crystalline form, and are but very little soluble in most organic solvents. They decompose on being fused.

Tannoid Substances of the Tannin Type

The preparation of pentagalloyl glucose has proved this compound to be nearly identical with tannin obtained from galls (tannin); a few other natural tannins belong to this type which Fischer terms acyl compounds of sugar with hydroxybenzoic acids. The method of preparation employed in the synthesis of pentagalloyl glucose may be easily applied to other hydroxybenzoic acids, e.g. penta[p-hydroxybenzoyl] glucose [Footnote: Fischer and Freudenberg, Ber., 1912, 45, 933.] was prepared in this way. Similar characteristics are exhibited by pentasalicylo glucose. Mention must also be made of the corresponding derivative of pyruvic acid and the compound with pyrogallolcarboxylic acid, penta-[pyrogallolcarboyl]glucose. [Footnote: Fischer and Rapoport, Ber., 1913, 46, 2397.] The latter is isomeric with pentagalloyl glucose and possesses similar properties; there is, however, a vast difference in the solubility of the two. Whereas the galloyl compound is easily soluble in cold water, its isomer is hardly soluble in hot, and completely insoluble in cold water. Considering the very similar structure of these two tannins, such differences appear surprising, but an analogy may be readily found in the existence of colloidal solutions of tannin and the (nearly) identical pentagalloyl glucose. These properties clearly show how dependent is the colloidal state on small differences in the structure of two substances. On the other hand, the formation of hydrosols is of the greatest importance relatively to the part played by these substances in Nature as well as relating to their chemical characteristics; thus it is extremely difficult to make a solution of penta-[pyrogallolcarboyl]-glucose, at the same time ascertaining its astringent taste and its property of precipitating gelatine.

The experience gained by the methyl glucosides makes it exceedingly probable that the simpler polyhydric alcohols also are suitable substances to employ in these syntheses; as a matter of fact, glycerol has been condensed with gallic acid. [Footnote: Fischer and Freudenberg, _Ber., 1912, 45, 935.]

One of the chief characteristics of synthetic tannins is their high molecular weight; for instance, the molecular weight of penta-[tricarbomethoxygalloyl]-glucose is 1,810, that of penta-[pentamethyl-m-digalloyl]-glucose 2,051. Employing gallic acid derivatives, especially the tribenzoyl compounds, coupled with glucose, e.g., mannite, yielded a neutral ester of molecular weight 2,967.

The determination of the elementary composition of compounds of high molecular weight is greatly facilitated by employing their halogen derivatives; so, for instance, is p iodophenyl maltosazone very suitable. Coupling the latter with tribenzoylgalloyl chloride yielded hepta-[tribenzoyl-galloyl]-p-iodophenyl maltosazone, the structure of which is represented by—

The molecular weight of this substance is 4,021, and probably represents the highest molecular organic body obtained in any chemical synthesis.