C_6H(OH)_3COOH CH_2{ C_6H(OH)_3COOH
Baeyer's experiment with pyrogallol probaly also yields, according to
Nierenstein, another compound of the following constitution—
C_6H_2(OH)_3 C{-C_6H_2(OH)3 |}C_6H_2(OH)_2 O
Nierenstein considers these bodies confirmation of his hypothesis of the existence of a "tannophor,"—CO—, in the tannins.
This supposition was adopted by Stiasny [Footnote: Gerber, 1905, 233.] and Kauschke [Footnote: Collegium, 1906, 362.] and the latter points out that these easily soluable substances exhibit tanning properties. Nierenstein [Footnote: Ibid., 1906 424.] was further able to show that by all processes of condensation between phenols (or hydroxybenzoic acids) and formaldehyde, compounds of the character of hydroxyaurine (or hydroxyaurinecarboxylic acid) were formed in addition to the insoluble hydroxydiphenylmethanes (or hydroxydiphenylmethanecarboxylic acids), the former possessing the characteristic tannophor group and hence precipitating gelatine, i.e., exerting tanning action. If the formation of leather is viewed in the light of Schiff's base, [Footnote: Ibid., 1905, 159.] one may consider the constitution of a hexahydroxyaurinecarboxylic acid leather as follows:—
_C_6H_2(OH)_3.COOH C{-C_6H_2(OH)3.COOH |}C_6H_2(OH)_2.COOH R-N
In the preparation of these and similar condensation products, Nierenstein and Webster [Footnote: Ber., 1908, 41, 80.] observed a peculiar steric effect of the carboxyl group. Each 2.5 gm. of the phenol or the acid in question were dissolved in 30 c.c. of water, the solution brought to boil and 5 c.c. formaldehyde (20 per cent.) and 2.5 c.c. hydrochloric acid added drop by drop; the precipitate formed was filtered off after twenty-four hours, dried at 110° C. to constant weight, extracted (in a Gooch crucible) freely with water, and the residue again dried at 110° C. till constant. The following values were obtained:—
Total Insol. Aq. Sol. Aq. Oxy-
Precipitate Diphenylmethane aurinecarboxylic
in Grammes. Derivatives Acid.
Per Cent. Per Cent. Phloroglucinol 2.4002 100 … Hydroquinone 2.3716 100 … " 2.0542 100 … Pyrogallol 2.5150 100 … " 2.7940 100 … Pyrocatechol 2.9805 100 … " 2.9574 100 … Resorcinol 2.9954 100 … " 2.9725 100 … Gallic acid 2.0706 78.84 21.16 " 1.2240 83.18 16.82 " 1.1405 59.94 41.06 [Greek: b]-Resorcylic acid 2.1040 51.08 48.92 " " 2.2008 47.12 52.88 Protocatechuic acid … … … " " … … … Vanillic acid … … … Tannin 2.0599 … Nearly all sol. Digallic acid 2.1042 80.16 19.84 Leucodigallic acid 2.0041 1.94 98.06
With the introduction of the carboxylic group the tendency of condensation to diphenylmethane derivatives is lessened; by protocatechuic acid the tendency is nil. Nierenstein considers this reaction analogous to the formation of cork, to the genetic relation of which with the diphenylmethane formation Drabble and Nierenstein have referred in an earlier publication. [Footnote: Biochemical Jour.., 1907, 2, 96.] It is hence possible that the plants may employ formaldehyde as a methylation medium, and produce these insoluble condensation products for the purpose of ridding themselves of the poisonous phenols and aromatic hydroxy acids (and tannins), in addition to oxidising processes whereby phlobaphenes, ellagic acid, etc., are formed.