Synthetic Tannins, Their Synthesis, Industrial Production and Application Part 6

is obtained which, when acidified, exhibits tannoid properties.

On condensing sodium phenolsulphonate with nitrotoluenesulphonic chloride, reducing the condensation product and condensing the latter with _p_-toluenesulphonic chloride, a compound similar to the above is obtained--

---O---SO_2--- ^ ^ ^ CH_3 | | | | | | | | | | | | v v ---NH---SO_2--- v NaSO_3 CH_3

Again, a similar product is obtained when _p_-toluenesulphamin.o.benzenesulphonic chloride or its h.o.m.ologues or isomers are condensed with sodium-_o_-cresylsulphonate--

---SO_2---NH--- SO_3Na ^ ^ ^ | | | | | | | | | | CH_3| | v v ---SO_2---O--- v CH_3

The chloride of this compound may again be condensed, for instance, with sodium aminotoluenesulphamin.o.benzene-sulphonate, and yields the compound--

---NH---SO_2--- ^ ^ ^ ---NH---SO_2--- ^ | | | | | | | | | | | | | | | | v v ---NH---SO_2--- v v CH NaSO_3 CH_3

The three latter compounds, when dissolved in water and the solution acidified, exert tanning action.

It is also possible to employ mixtures of arylsulphamin.o.benzylsulphonic acids in acidified aqueous solution for tanning purposes. According to Ger. Pat., 297,188, such mixtures are obtained by nitrating benzylchloride and heating with an equimolecular amount of sodium sulphite; the sodium nitrobenzylsulphonate thus obtained is reduced to amin.o.benzylsulphonic acid with iron and acetic acid, and finally condensed with the calculated amount of _p_-toluenesulphonic chloride. A mixture _o_- and _p_-toluenesulphamin.o.benzylsulphonic acid [Footnote 1: Cf. also Ger. Pat, 319,713 and 320,613.] thus results.

Amongst _aromatic alcohols_ the dihydric alcohols show characteristic behaviour; the latter combine with sulphonic acids with the elimination of water, condensation taking place without formaldehyde, and the resulting products being soluble in water and possessing tannoid properties. [Footnote 2: Ger. Pat., 300,567, of 20th September 1917.]

In addition to phenolic mono- and disulphonic acids (and higher sulphonation compounds), the h.o.m.ologues, cresols, xylenols, and naphthols enter into reaction. The two components condense with great ease, liberating heat; dilute solutions (of the components) are heated to about 100 C., the process being complete in a few minutes. The products obtained are exceedingly pure and are easily crystallisable. Employing 1, respectively 2, molecules of sulphonic acid, the reactions take place according to:--

OH CH_2.OH OH OH }C_6H_4 + HO.C_6H_3{ = H_2O + }C_6H_3-CH_2-C_6H_3{ HSO_3 CH_2.OH HSO_3 CH_2.OH

OH OH CH_2.OH CH_2.C_6H_3{ }C_6H_4 + HO.C_6H_3{ = 2(H_2O) + HO.C_6H_3{ HSO_3 HSO_3 CH_2.OH | OH CH_2.C_6H_3{ HSO_3

OH CH_2.OH OH OH }C_6H_3.CH_3 + HO.C_6H_3{ = H_2O + }C_6H_2.CH_3.CH_2.C_6H_3{ HSO_3 CH_2.OH HSO_3 CH_2.OH

OH OH CH_2.OH CH_2.C_6H_2.CH_3{ }C_6H_3.CH_3 + HO.C_6H_3{ =2(H_2O) + HO.C_6H_3{ HSO_3 HSO_3 CH_2.OH | OH CH_2.C_6H_2.CH_3{ HSO_3

OH CH_2.OH OH OH }(C_10)H_6 + HO.C_6H_3{ = H_2O + }(C_10)H_5.CH_2.C_6H_3{ HSO_3 CH_2.OH HSO_3 CH_2.OH

OH OH CH_2.OH CH_2.(C_10)H_5{ }(C_10)H_6 + HO.C_6H_3{ =2(H_2O) + HO.C_6H_3{ HSO_3 HSO_3 CH_2.OH | OH CH_2.(C_10)H_5{ HSO_3

The condensation products above enumerated were tested with regard to their tanning power, both non-neutralised and partly neutralised (1:10, 1:20, and 1:30 c.c. N/10 NaOH) samples being examined. In all cases rapid tannage was observed yielding firm and soft leathers of light brown colour and varying degrees of swollenness.

Relatively to their reactions, all the products strongly precipitate gelatine, whereas only the condensation products of phenol, cresol, and xylenol derivatives give a characteristic coloration with iron salts.

The tannin contents of the non-neutralised condensation products lie between 72-80 per cent.--figures which clearly indicate the purity and efficiency of these substances.

Notable amongst _aromatic acids_ is salicylic acid, C_6H_4.OH.COOH, which at higher temperatures is easily sulphonated with concentrated sulphuric acid; the sulphonation product represents a white solid, which easily dissolves in water forming a clear liquid. The sulphonic acid, when mixed with about one-third of its weight of water and heated to about 120 C., is easily condensed with formaldehyde. Towards the end of the reaction, considerable frothing sets in, but in spite of the high temperature required by this reaction no insoluble bakelites are formed. A reddish-brown fluid is obtained easily soluble in water, to which it imparts a brown colour. An aqueous solution of the product completely precipitates gelatine, gives a strong opalescence with aniline hydrochloride and a deep violet coloration with ferric chloride.

Neutralised as usual, the product, in a 3 Be solution, converts pelt within three days into a white, full leather of good tensile strength.

This process has been patented by the Deutsch-Koloniale Gerb und Farbstoff Gesellschaft (German-Colonial Tanning and Colour Extracts Ltd.) in Karlsruhe, the letters patent also including the ring h.o.m.ologues of salicylic acid. Similar results are obtained when cresotinic acid (hydroxy-toluic acid), OH.C_6H_3.CH_3.COOH, is employed as base.

If the phenyl ester of salicylic acid, _Salol_,

HO.C_6H_4.CO.O.C_6H_5

is sulphonated, a product is obtained which is easily soluble in water, but which is identified as a mixture of the sulphonation products of salicylic acid and phenol, the salol being dissociated on sulphonation. The temperature must not exceed 80 C. by condensation with formaldehyde, or insoluble bakelite will be formed from the phenol; the aldehyde must also be added gradually. An aqueous solution of the partly neutralised condensation product has a p.r.o.nounced tanning effect on pelt, and converts the latter into leather in one to two days; the leather being very similar to that produced by the salicylic acid condensation product. The qualitative reactions of the product in aqueous solution are the same as those given by the salicylic acid condensation product.

Salicylic acid may, however, also be condensed with formaldehyde without first being sulphonated; in this case, a little hydrochloric acid should be present. A product slightly soluble in water is obtained, which may be looked upon as being methylenedisalicylic acid. In alkaline solution it is easily soluble,

[Footnote 1: Its solubility in alcohol and alkalies renders this product an effective and cheap subst.i.tute for sh.e.l.lac.--_Transl._]

the liquid possessing an intensely bitter taste. The sodium salt gives a deep violet coloration with ferric chloride, a slight precipitate with gelatine, and slight opalescence with aniline hydrochloride. In contact with pelt, however, it exhibits no tanning effect, but when dissolved in alcohol, a pickling effect may be observed.

[Footnote 2: A similar reaction is observable in the case of the sodium salts of METHYLENEDISALICYLIC acid brommated or iodised, which form a clear solution varying from red to reddish-brown.]

The attempt at preparing a condensation product from sodium-_m_-hydroxybenzoate by means of formaldehyde and bisulphite is worthy of attention. A dark brown, viscous liquid is obtained which is perfectly soluble in water, and the aqueous solution of which gives opalescence with gelatine, a precipitate with aniline hydrochloride, and a bluish-black coloration with ferric chloride. Its behaviour towards pelt is very similar to that of phenolsulphonic acid, and it yields a similar leather.

A very similar condensation product was obtained by condensing sodium-_p_-hydroxybenzoate with formaldehyde and subsequent sulphonation with sulphuric acid. From a practical standpoint, however, these substances cannot be employed, since their tanning action is only effective in acid solutions of such concentration of acid as would gelatinise the pelt.3

If, on the other hand, non-condensed methane derivatives of phenol, _e.g._, hydroxyphenylmethanesulphonic acid, are partly neutralised and a solution of the product thus obtained used for tanning experiments, no tanning action is observable. The acidified solution does not precipitate gelatine, and gives a dark brown coloration only with ferric chloride.

GALLIC ACID, C_6H_2(OH)_3COOH, when heated with sulphuric acid, is easily converted into the insoluble rufigallic acid, which is also insoluble in alcohol. If, however, gallic acid is heated with an excess of sulphuric acid, the product cooled and treated with formaldehyde, a deep brown condensation product is obtained which is soluble in alcohol, and in this state is capable of converting pelt into a substance similar to leather which, though rather hard, possesses good tensile strength. This water-insoluble condensation product is also soluble in alkalies, the solution exhibiting properties similar to that described above. Gallic acid, therefore, is not a suitable base for the production of synthetic tannins soluble in water.

Phthalic acid also is difficult to sulphonate: the sulphonated compound treated with formaldehyde gives only water-insoluble condensation products.

3. Condensation Of Naphthalene Derivatives

The simplest method of condensing [Greek: b]-naphthalene-sulphonic acid is to heat it at 135 C. at a pressure of 20 mm. for several hours.[Footnote: Austr. Pat., 61,061, of 10th September 1913.] The resulting product is a cheesy ma.s.s which reacts strongly acid. By reducing the acidity of the substance to 1 gm. = 10 c.c. N/1O NaOH, a grey, cheesy ma.s.s results, which easily dissolves in water, the solution being coloured a light yellow-brown and precipitating gelatine aniline hydrochloride; no coloration, however, appears on adding ferric chloride.

The condensation of [Greek: b]-naphthalenesulphonic acid, however, proceeds with much greater energy in the presence of formaldehyde. In practice, for instance, 10 kilos of naphthalene is heated with the same weight of concentrated sulphuric acid (66 Be), when the mixture is converted into [Greek: b]-naphthalenesulphonic acid by heating for several hours at 150-160 C; the sulphonation completed, the sulphonic acid is cooled to about 85 C., and 4 kilos of formaldehyde (30 per cent, by weight) slowly added; finally, the product is stirred at the temperature mentioned till all formaldehyde has combined.[Footnote: Austr. Pat., 69,194, of 25th June 1915; Ger. Pat, 290,965.]

Tanning experiments with this product yielded, in a short time, a nearly white coloured leather (see later).

In addition to formaldehyde, there are other substances which induce condensation of naphthalenesulphonic acid; if, for instance, sulphur chloride is allowed to act upon [Greek: b]-naphthalenesulphonic acid, a light brown solid of p.r.o.nounced acidic character is obtained; if the latter is partly neutralised with caustic soda, a greyish-brown solid results, which dissolves in water with a light brown colour, the solution precipitating gelatine and aniline hydrochloride, but giving no coloration with ferric chloride.[Footnote: Austr. Pat., 96,194.]

Tanning experiments with this product in aqueous solution gave a light brown, rather soft leather, and this, in addition to the qualitative reactions of the substance, prove that this method of condensation hardly alters the character of the product from a tanning point of view. The brown coloration imparted to the leather tanned with this condensation product owes its existence to coloured intermediary products.

Attempts at condensing chloronaphthalenesulphonic acid and nitronaphthalenesulphonic acid resulted in soluble condensation products which gave some of the reactions given by the tannins (precipitation of gelatine and aniline hydrochloride), but which were incapable of tanning pelt, a light tannage being effected on the surface only.

[Greek: a]-Naphthol dissolved in hot concentrated sulphuric acid and heated for some time on the water bath, yields the light brown, water-soluble [Greek: a]-naphtholsulphonic acid. A dilute solution of the latter, when treated with formaldehyde in the cold, undergoes no change; on heating the mixture on the water bath a brown precipitate is thrown down. If gelatine solution is added to the opaque liquid, a yellow flocculent precipitate separates. If caustic soda is added to the opaque liquid containing the condensation product described above, a clear solution results from which no deposit separates on the addition of acetic acid. Gelatine is precipitated by this solution.

The concentrated hot a-naphtholsulphonic acid, upon addition of sufficient formaldehyde, effervesces strongly and yields a dark brown condensation product insoluble in water, but soluble in caustic soda. If acetic acid is added in excess to the alkaline solution, the resultant solution strongly precipitates gelatine.

A suspension in water of the insoluble condensation product does not precipitate gelatine.

b-Naphthol, dissolved in hot concentrated sulphuric acid and heated for some time, yields the light brown, viscous b-naphtholsulphonic acid. A dilute solution of the latter, mixed with formaldehyde, remains clear; when heated on the water bath, however, it a.s.sumes a dark, reddish-yellow colour, and remains soluble in water and precipitates gelatine strongly. This condensation product, on adding excess of caustic soda, a.s.sumes a deep blue coloration, the alkaline solution giving no precipitate with gelatine; on adding acetic acid the solution turns brown, remains clear, and now precipitates gelatine.

The concentrated b-naphtholsulphonic acid heated with formaldehyde on the water bath yields as condensation product a dark, reddish-yellow ma.s.s, soluble in water, which precipitates gelatine. A dilute solution, when allowed to act upon pelt, gave in a few days a light brown leather, the properties of which are very similar to those possessed by vegetable tanned leathers.

The use of naphtholsulphonic and aminonaphtholsulphonic acids for the manufacture of synthetic tannins is protected by Ger. Pats., 293,640, 293,693, 293,042, and 303,640. [Footnote: _Cf._ Austr. Pat., 70,162.]

It is a remarkable fact that non-condensed methane derivatives of naphthol, _e.g._, b-naphthol-a-methanesulphonic acid, dissolved in water and partly neutralised, are devoid of tanning character when allowed to act upon pelt. Neither does this substance precipitate gelatine, but it does give a deep blue coloration with ferric chloride.

The condensation product of b-naphthol above referred to precipitates gelatine and aniline hydrochloride and gives a brown coloration with ferric chloride.