Thionaphtholsulphonic acid, when acted upon by formaldehyde, yields a condensation product of the following const.i.tution:--

HSO_4 ^ ^ SH SH ^ ^ HSO_4 | | | | | | | | |_____CH_2_____| | | v v v v

This is a light yellow powder which, dissolved in water, yields an opaque solution; the latter only exhibits any tanning properties when it is not neutralised and even slightly acidified and then precipitates gelatine, aniline hydrochloride and barium chloride; dissolved in alkali, it forms a clear, yellow solution devoid of tannoid properties. Leather tanned with the acidified solution is very similar to those tanned with the phenolsulphonic acid condensation products; its colour, however, is more p.r.o.nouncedly yellow.

b-Naphthol condensed with hydrochloric acid and formaldehyde yields a methylenedinaphthol, which is insoluble in water; the sodium salt, however, easily dissolves. The same condensation, however, takes place in alkaline solution with direct formation of the sodium salt. The condensation product gives a slight precipitate with gelatine, and a bluish-grey precipitate with ferric chloride; acids re-precipitate the insoluble methylene compound. Towards pelt it exhibits tanning properties, whereby the insoluble product referred to above is deposited, and soft, full, and white leather is obtained, possessing, however, but little tensile strength.

4. Condensation of the Anthracene Group

Anthracene heated with excess sulphuric acid yields the water-soluble anthracenesulphonic acid; the latter, when heated with formaldehyde, yields water-soluble, reddish-brown condensation products, which remain soluble on prolonged heating with formaldehyde. The aqueous solution of the condensation product shows no particular reactions; it gives a flocculent precipitate with gelatine and a green precipitate with copper sulphate, soluble with blue colour in excess of the reagent.

The partly neutralised solution tans pelt--to which it imparts a brown colour--in eight days, but on the surface only; the inner layers are merely pseudo-tanned (white colour). When dried, pelt thus treated yields a full and soft leather with brown grain and flesh possessing but little tensile strength. Hence, this condensation product exerts a pickling rather than a tanning effect.

Anthraquinone heated with sulphuric acid and treated with formaldehyde in the usual manner, yields a substance which, when mixed with water, forms an opaque, milky solution. This is not altered by excess of caustic soda. The aqueous solution precipitates gelatine and aniline hydrochloride; all other tannin reagents give no reaction.

The partly neutralised solution of the condensation product exerts, in the main, a pickling action on pelt; only the surface of which is tanned, with brown colour, the remainder being merely pickled (white colour). During "tannage," bakelite is formed in the liquid, and practically all solubles originally present are deposited. The tannage completed, a light brown, fairly soft and full leather, possessing little tensile strength, results; this leather can be washed only with great difficulty and approaches more the character of a pickled pelt.

1-Hydroxyanthraquinone, 1,5-dichloroanthraquinone, l,5-diaminoanthraquinone, 1-methylaminoanthraquinone, 1-benzoylamino, 6-chloranthraquinone, 1-_m_-toluidoanthraquinone, when treated with sulphuric acid and formaldehyde, all yield condensation products which are but little soluble in water, and which do not at all precipitate gelatine. Tanning experiments with these condensation products in alcoholic solution yielded empty leathers of p.r.o.nounced pickle character.

If, however, 1-methylamino-4-bromanthraquinone is condensed with sulphuric acid and formaldehyde, a condensation product is obtained which is but slightly soluble in water, but which precipitates gelatine.

When phenanthrequinone is heated with excess of sulphuric acid for some time, a water-soluble, reddish-yellow coloured condensation product results. The latter, when treated with formaldehyde in the cold and then finally heated, gradually fixes the formaldehyde and forms a substance soluble in water. If the heating, however, is prolonged, insoluble bakelites are formed, which are neither soluble in alkali nor in alcohol.

An aqueous solution of these condensation products gives no reactions with the usual tannin reagents, though it completely precipitates gelatine. When acting upon pelt, the partly neutralised dilute solution of the condensation product pickles the former, and after a few days the pelt is converted into a light brown, full, and rather soft leather possessing good tensile strength.

When the condensation product is acted upon by bromine in hot aqueous solution, an additive compound is formed and the resulting product is soluble in water. The aqueous solution of the brominated product gives no special reactions with the usual tannin reagents, but precipitates gelatine completely. Its tanning action upon pelt is much slower than that of the original condensation product; the surface of the pelt only is tanned with brown colour, the inner pelt being only pickled (light brown colour). When dried, a hard and empty leather of good tensile strength is obtained, possessing mainly the properties of a pickled pelt.

CO OH ^ ^ ^ QUINIZARENE, | | | | , treated with sulphuric acid | | | | v v v CO OH

and formaldehyde, yields a condensation product which is but little soluble in water and which does not precipitate gelatine.

QUINOLINE, when sulphonated and condensed with formaldehyde, yields a dark coloured condensation product, completely soluble in water; the solution does not precipitate gelatine.

OXYQUINOLINE exhibits similar behaviour.

On the other hand, the use of _retene_ (methylisopropylphenanthrene),

CH_3 ^ ___________ ^ | | | | | |___CH:CH___| | C_3H_7 v v

for the production of synthetic tannins, is protected by Ger. Pat., 290,965 [Footnote 1: _Cf_ Austr. Pat., 69,194]

5. Di- and Triphenylmethane Groups

If DIPHENYLMETHANE, (C_6H_5)_2CH_2, is heated with excess sulphuric acid, a dark blue ma.s.s, easily soluble in water, is obtained. The product gently heated with formaldehyde yields a brown, water-soluble condensation product; once condensation is complete, the product will stand stronger heat. If, on the other hand, more formaldehyde is added, brown, water-insoluble bakelites are formed. The water-soluble condensation product precipitates gelatine, but not aniline hydrochloride. Dissolved in water, it possesses tannoid properties: the pelt is, however, tanned on the surface only, the intermediary layers being merely pickled; after four days in the solution, the pelt after drying was found to be converted into a greyish-brown, badly coloured leather, which was empty, hard, and possessed but little tensile strength.

CARBAZOLE (dibenzopyrrole),

^ _____ ^ | | | | | |__ __| | v v v N_3

on the other hand, was found a suitable base for the commercial production of synthetic tannins; its use is protected by Ger. Pat, 290,965.

TRIPHENYLMETHANE, (C_6H_5)_3CH, heated with excess sulphuric acid, yields a nearly black ma.s.s which, when condensed with formaldehyde in the cold, and subsequently heated, yields a ma.s.s which is soluble in water. With gelatine and aniline hydrochloride it exhibits reactions similar to those given by the diphenylmethane condensation products; its tanning properties also are similar to those of the latter. The resultant leather is black, but is soft and full and possesses good tensile strength.

Baeyer's observation, [Footnote: _Ber_., 1872, 5, 280, 1096.] that pyrogallol on condensation with formaldehyde yields an amorphous body soluble in water, which precipitates gelatine and is very similar to tannin, was confirmed by Caro [Footnote: _Ibid_., 1892, 25, 947.] and Kahl. [Footnote: _Ibid_., 1898, 31, 114.] These investigators found that by the condensation of phenols and hydroxybenzoic acids with formaldehyde, diphenylmethane derivatives were formed; pyrogallol yields hexahydroxydiphenylmethane--

C_6H_2(OH)_3 CH_2{ C_6H_2(OH)_3

Nierenstein [Footnote: _Collegium_, 1905, 221.] repeated these experiments, and found that in addition to the insoluble diphenylmethanes, water-soluble bodies were formed, which latter precipitate gelatine. The condensation product yielded by gallic acid was identified as hexahydroxyaurinecarboxylic acid--

_C_6H(OH)_3COOH C{-C_6H(OH)_3COOH | }C_6H(OH)_2COOH O

which is formed in addition to hexahydroxydiphenylmethane-dicarboxylic acid--

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 const.i.tution--

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 const.i.tution 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