Synthetic Tannins, Their Synthesis, Industrial Production and Application Part 5

Bromo-[Greek: b] " " Tans -naphthol [Footnote: In alcoholic solution]

Rosolic acid " " "

[Footnote: In alcoholic solution]

Gallic acid No ppte. No tannage No tannage

By the condensation of their sulphonic acids, it may be demonstrated experimentally how the tannoid properties of nearly every member of the series are intensified. Investigattion in this direction, however, has not been systematically undertaken, for which reason the author determined to examine this subject; but the enormous number of samples required, obtainable only with great difficulty during the war, made it impossible to conclude completely the researches in this field. What little has so far been done relatively to this subject should, when collected, indicate the way to be pursued in this wide field of investigation. What follows will hence comprise the conversion of a few of the most important members of this series of substances into their methylene-condensation products with a brief discussion of the qualitative and tannoid reactions of the latter.

The didepside of phenolsulphonic acid is obtained by condensing carbomethoxyphenolsulphonic chloride with sodium phenolsulphonate in the presence of the calculated amount of caustic soda. A product of the composition

CH_3.0.COO.C_6H_4SO_2.0.C_6H_4.SO_3Na

is first obtained, which on saponification with soda yields the pure didepside--

HO.C_6H_4.SO_2.C_6H_4.SO_3.Na

By acidifying the concentrated solution the didepside is obtained as a white crystalline substance; a solution of which precipitates gelatine without, however, exhibiting any tanning effect upon animal hide. If, on the other hand, the above ester is converted into the chloride

CH_3O.COO.C_4H_4SO_2.O.C_6H_4.SO_2Cl

by treatment with PCl_5, and the chloride thus obtained further condensed with sodium phenolsulphonate, saponified, and the solution acidified, the pure tridepside

HO.C_6H_4.SO_2.O.C_6H_4.SO_2.O.C_6H_4.SO_3Na

is precipitated as white crystalline needles which not only precipitate gelatine, but are capable of converting animal hide into leather.[Footnote: _Chem. Ztg._, 1919, 43, 318.]

Of the cla.s.s of hydroxy-cymenes _thymol_,

C_6H_3.CH_3.C_3H_7OH,

was converted into the water-soluble sulphonic acid by warming with concentrated sulphuric acid at 50 C., the sulphonic acid being subsequently easily condensed with formaldehyde by slightly heating the mixture. The condensation product thus obtained is a viscous brown ma.s.s which is easily soluble in water, precipitates gelatine completely, gives a bluish-black coloration with iron salts, and gives a precipitate with aniline hydrochloride. To investigate its tannoid properties, the mixture was brought to the acidity 1 gm = 10 c.c. N/10 NaOH and a piece of bated calf skin was then introduced into a solution measuring about 2 Be. After eighteen hours the pelt was nearly tanned through, and a further twenty-four hours completed the tanning process, after which a light fat-liquor was given. The dried leather was brownish-grey in colour, possessed soft and full feel and good tensile strength.

On account of their importance, the three dihydroxybenzenes were examined with a view to test their suitability for conversion into tannoid substances.

_o_-Dihydroxybenzene, catechol, yields a sulphonic acid easily soluble in water, which on the careful addition of formaldehyde a.s.sumes a blue colour. The compound thus obtained may be heated to 100 C., without depositing insolubles. A further addition of formaldehyde, however, results in the formation of a considerable quant.i.ty of insolubles whilst the liquid a.s.sumes a brown coloration. If, on the other hand, the sulphonic acid is diluted with twice its volume of water, formaldehyde added and the mixture heated on the water bath, the liquid immediately turns brown, the formaldehyde is completely fixed, and a condensation product soluble in water results. The latter gives a brownish-black coloration with ferric chloride, completely precipitates gelatine, but gives no opalescence with aniline hydrochloride. Tanning experiments with the partly neutralised (1 gm.= 10 c.c. N/10 NaOH) substance resulted in both grain and flesh being tanned with a black colour, whereas the interior of the pelt was pickled (white colour). After a further forty-eight hours, however, the black colour penetrated the pelt, and tannage was complete. The washed and lightly fat-liquored leather was soft, of full feel and good tensile strength, and was greyish coloured throughout.

With regard to the black colour possessed by leathers tanned with synthetic tannins the following should be noted. When sulphonating and especially when condensing substances, black dyestuffs or very finely divided carbon in the colloidal state are often formed. Such a substance does not deposit the black particles, even when filtered through kaolin, and hence convert pelt into leather possessing black colour on the surface. The hide in this case acts as a perfect filtration medium, whereby the surface layers retaining the coloured particles a.s.sume their colour; thus only the pure tanning matter enters into the interior, which then, according to the composition of the former, imparts a colour varying from white to light brown to the inner layers.

_m_-Dihydroxybenzene, resorcinol, is also easily sulphonated by concentrated sulphuric acid, the brownish-coloured sulphonic acid being easily soluble in water. If the sulphonic acid is diluted with three times its volume of water, cooled down, a few drops of formaldehyde added and the mixture heated on the water bath to completely fix the formaldehyde, and this process repeated till no more formaldehyde is taken up, a brown water-soluble condensation product results, the aqueous solution of which precipitates gelatine completely, aniline hydrochloride only partly and which gives a deep blue colour with ferric chloride.

A piece of calf skin immersed in a solution of the partly neutralised (as above) product was tanned through in twenty-four hours; when lightly fat-liquored, the resulting leather possessed a yellowish-green colour and good tensile strength, and was soft and full.

_p_-Dihydroxybenzene, hydroquinone, was converted into the water-soluble sulphonic acid by heating it with concentrated sulphuric acid at 100 C.; the sulphonic acid, mixed with formaldehyde at ordinary temperature, immediately solidifies to a white ma.s.s, which is soluble in water and which had completely fixed the formaldehyde. If, however, this ma.s.s is heated for some time to 100C, it a.s.sumes a light brown coloration and its solubility in water is diminished. A slight excess of formaldehyde and the application of heat result in dark violet insoluble condensation products. The aqueous solution precipitates gelatine, gives a deep blue colour with ferric chloride, but gives no precipitate with aniline hydrochloride; on the other hand, addition of pota.s.sium nitrite produces the yellow colour characteristic of hydroquinone.

The product effects a slower tannage (seven days) than the former product, when a brown, soft, but rather empty leather of good tensile strength is obtained.

Of the _trihydroxybenzenes_ pyrogallol and phloroglucinol only were included in these investigations.

When pyrogallol is sulphonated with concentrated sulphuric acid a violet-coloured sulphonic acid, soluble in water, is obtained, which, when treated with formaldehyde first in the cold and then when heated, yields a solid deep red-coloured ma.s.s, which precipitates gelatine but not aniline hydrochloride, and gives a blackish-brown colour with ferric chloride. The partly neutralised substance in aqueous solution tans pelt in twenty-four hours with black colour on the surface only, the intermediary layer being pickled (white colour) only, but the black-coloured tanning matter ultimately penetrates the pelt, which tanned through in seven days. The resultant leather is coloured black throughout, is full, soft, and possesses good tensile strength.

Sulphonation of phloroglucinol succeeds at higher temperatures only, the sulphonic acid being a solid which is scarcely soluble in water, the latter then a.s.suming a wine-red colour. The condensation product--prepared as described for resorcinol, but requiring higher temperature--is a brick-red powder, insoluble in water.

The same end-product also seems to be obtained by simply heating the sulphonic acid at a higher temperature; this also induces condensation with the formation of a reddish-brown ma.s.s insoluble in water. It is, of course, impossible to attempt any tanning experiments with this product in aqueous solution; attempts at dissolving the condensation product in alcohol proved barren of result, since only traces of impurities accompanying the substance dissolved, imparting a light reddish-brown colour to the solution. In highly concentrated alcohol, however, the condensation product is somewhat soluble, yielding a reddish-brown solution. A piece of pelt introduced into the alcoholic solution was surface tanned only after forty-eight hours, leaving the remainder of the pelt pickled; extending the experiment over a further four days produced no change in the pelt. The latter was therefore rinsed with water, lightly fat-liquored and dried, when a soft but empty leather of grey colour and good tensile strength was obtained. It appears, therefore, to be a case of pseudo-tannage, where an infinitesimal amount of synthetic tannin produces a tanning effect without, however, a true tannage being effected.

The Elberfelder Farbenfabriken have protected the use of the condensation products of di- and polyhydroxybenzenes by Ger. Pat., 282,313; owing to the high cost of the latter substances, however, it is doubtful whether synthetic tannins prepared from these materials would not be too expensive for any other than pharmaceutical purposes.

Before leaving the phenols, mention must be made of the quinones, the use of which for tanning purposes was first protected by Ger. Pat., 206,957 (30th April 1907). According to this patent, only 400 gm. of quinone are required for the conversion into leather of 400 kilos pelt, drum tannage being preferable. During the process the leather first a.s.sumes a reddish colour, changing through violet to brown; its resistance to water, acids, and alkalies is said to be considerably greater than that exhibited by all other kinds of leather.

The chemistry of the quinone tannage has been investigated, and an explanation given by Thuau [Footnote: _Collegium_, 1909, 363, 211.]

a.s.sumes a reaction between the quinone and the amino groups of the hide protein with the formation of hydroquinone--

+-O OH | | | 2R.NH_2 + 2C_8H_4 | = C_6H_4 + C_6H_4(O.NH.R)_2 | | | +-O OH (Pelt.) (Leather.)

Fahrion has shown that, during the tanning process, the quinone loses its active oxygen, and this can only be brought about by the amino group of the hide protein, the amino group only being capable of effecting reduction of the quinone. An a.n.a.logy is here offered by dianilinoquinone. A spent quinone liquor contains considerable amounts of hydroquinone. The tannage may also be effected by exposing pelt saturated with hydroquinone to oxidation by the air. The pelt, which is unaltered by the hydroquinone bath, on being removed from the latter, and in the presence of alkali, a.s.sumes a red colour at first, which changes into violet, blue, and finally brown, the pelt being thereby converted into a quinone-tanned leather.

It may be noted that quinone only effects pseudo-tannage; quinone mixed with water deposits, in time, a black amorphous substance practically insoluble in water. This substance is easily adsorbed by hide powder, but is not capable of converting the latter into that insoluble form into which it is converted by the natural tannins.

Amongst polyhydric alcohols, the behaviour of the methyl ester of catechol, _guaiacol_ was investigated. The sulphonic acid was prepared by heating guaiacol with concentrated sulphuric acid, the resulting water-soluble product possessing a light, brownish-green colour. On condensing the sulphonic acid with formaldehyde, the same precautions were observed as in the case of resorcinol, but complete fixation of the formaldehyde could only be obtained by finally heating the product for a short time over a free flame, at about 105 C. Condensation was indicated by the brownish appearance of the liquid. No insoluble products were formed. The condensation product easily dissolves in water, the solution a.s.suming a rich brown colour and exhibiting the following reactions: gelatine is completely precipitated, aniline hydrochloride produces opalescence, and ferric chloride a deep brown coloration.

Tannage, with the partly neutralised product, was rapid, the pelt being nearly tanned through in twenty-four hours, excepting a small white streak in the middle; after a further twenty-four hours this streak had vanished, and the completely tanned, dark grey-coloured leather, after was.h.i.+ng, fat-liquoring, and drying, was soft, full, and of good tensile strength, very similar to the leather yielded by the catechol-condensation product.

Of the nitro-compounds, trinitrophenol, C_6H_2(NO_2)_3OH (picric acid), was investigated. If a concentrated solution of picric acid is brought into contact with pelt it will penetrate the latter completely in a few days; it is, however, difficult to fat-liquor the resultant leather, since the fat is absorbed only with difficulty. If a pelt treated in this way be dried, a soft but rather flat leather results, the colour of which easily rubs off, the leather also tasting intensely bitter. These disagreeable qualities prevent a general use of this material for tanning purposes; in spite of them, however, picric acid, in admixture with boracic acid, salicylic acid, and glycerol, is used in the production of the so-called transparent leather. The latter is very flexible and possesses great tensile strength, but loses the latter quality when exposed to heat, and, when stored, also loses its flexibility. By simply was.h.i.+ng with water, the leather is reconverted into pelt.

When picric acid is treated with hot sulphuric acid and formaldehyde gradually added, a dark coloured water-soluble condensation product is formed which strongly precipitates gelatine. Exposed to the action of bromine, the condensation product yields a ma.s.s which is insoluble in water.

Experience has taught that the amino bodies--the basic N-derivatives of the phenols--do not yield substances possessing tannoid properties on condensation. On account of their importance, however, a few have been included in this series of investigations.

Amin.o.benzene, C_6H_5NH_2, aniline, treated with sulphuric acid, yields the water-soluble aniline sulphate, which, on cautious addition of formaldehyde, yields a reddish-coloured gel, insoluble in water, in addition to a small volume of a reddish-yellow liquid. The latter precipitates gelatine, but is not capable of converting pelt into leather. The insoluble gel is likewise insoluble in alcohol, so that tanning experiments with this substance are excluded.

Dimethylaniline, C_6H_5N(CH_3)_2, when treated with sulphuric acid, yields a product soluble in water which neither reacts with nor fixes formaldehyde. Hence the substance does not precipitate gelatine.

If, on the other hand, nitrosodimethylaniline,

NO | C_6H_4 | (CH_3)_2

is sulphonated, and the water-soluble sulphonation product heated with formaldehyde for some time, the product remains soluble in water and precipitates gelatine. No tanning effect could, however, be detected.

Arylsulphaminoarylsulphonic acids and arylsulphoxyarylsulphonic acids precipitate gelatine but are devoid of tannoid character. The latter is acquired by compounds belonging to this cla.s.s containing two or more sulphamino groups, or when they, in addition to one sulphamino group, contain a sulphoxy group and another sulphonic group. According to Ger. Pat., 297,187 (Society oc Chemical Industry, Basle), such compounds are obtained when, for instance, sodium sulphanilide in alkaline solution acts upon nitrotoluenesulphochloride, and the resulting nitrotoluenesulphamino compound is subsequently reduced with acetic acid and iron. The resulting aminotoluenesulphamin.o.benzenesulphonic acid is finally treated with p-toluenesulphonic chloride till the latter disappears. A compound of the composition

-----NH-----SO_2----- ^ ^ ^ | | | | | | | | | |---NH---| | V V V SO_2Na CH_2

is thereby obtained, which, when acidified, is readily capable of being used for tanning purposes.

The intermediary product of the aminotoluenesulphamin.o.benzenesulphonic acid obtained by this process may again be employed for the purpose of reacting with one-half molecule soda and 1 molecule nitrotoluenesulphonic chloride. The following compound is obtained--

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

If _p_-toluenesulphamin.o.benzenesulphonic chloride is condensed with sodium sulphanilide, a compound,

---SO_2---NH--- NaSO_3 ^ ^ ^ | | | | | | | | | | | | v v ---SO_2---NH--- v SO_3Na