Diagrammatically, the following would represent the physiology of the tannins:—[Footnote: Nierenstein, "Chemie der Gerbstoffe" (Stuttgart, 1910).]
Sugar—>Phenol—>Hydroxybenzoic Acid—>Depside—>
|Phlobaphene—>Migrating Depside—>Glucoside—>Free Depside—>-{Ellagic Acid|Cork.
[Illustration: Chart Showing the Decomposition of Products of Tannin.]
In organic chemistry distinction is made between sulphonic acids of the aliphatic and the aromatic series, the characteristic group of these acids being the so-calledsulphonic acid group, HSO_3.
When sulphides or mercaptans in glacial acetic acid solution are heated with permanganate, the resulting sulphonic acid compounds exhibit great similarity to compounds containing free carboxyl groups. The sulphonic acid group may also be directly introduced either by concentrated, or by fuming sulphuric acid, or by elimination of halogen by the action of sodium or silver sulphite on the halogen derivatives of the aliphatic compounds. Saturated hydrocarbons do not react with sulphur trioxide, but unsaturated hydrocarbons are readily attacked by SO_3. Similarly, halogenated compounds and alcohols react with concentrated or fuming sulphuric acid forming sulphonic and hydrosulphonic acids respectively. The aromatic compounds form, as a rule, sulphonic acids with much greater facility. Benzene, for instance, is easily converted into them-disulphonic acid by gently heating with fuming sulphuric acid; stronger heating converts them- into thep-disulphonic acid, and at 190° C. the trisulphonic acid is formed. Toluene treated with fuming sulphuric acid first yieldso- andp-sulphonic acids, finallyo- andp-disulphonic acids, ethylbenzene at the boiling pointp-ethylbenzene-sulphonic acid. Of the three isomeric xyleneso- andm-xylene dissolve in concentrated,p-xylene in fuming sulphuric acid only.
The action of sulphuric acid on naphthalene is stronger even than on benzene. Equal parts of naphthalene and sulphuric acid heated to 100° C. yield 80 per cent. [Greek: a] and 20 per cent. [Greek: b]-monosulphonic acid. At 160°-170°C. 25 per cent [Greek: a]- and 75 per cent. [Greek: b]-sulphonic acid is formed, and at higher temperatures [Greek: b]-monosulphonic acid only. If, on the other hand, 8 parts of naphthalene are heated with 3 parts of concentrated sulphuric acid to 180° C., two different naphthyldisulphonic acids are obtained.
Complete solution of the substance in sulphuric acid is, generally speaking, a criterion of complete sulphonation. A completely sulphonated compound should remain clear on dilution with water, or, in case precipitation occurs, the precipitate should be completely soluble in alkali or ammonia. It is necessary to submit the product to this test, since many organic substances are soluble in concentrated sulphuric acid without undergoing any alteration in composition.
Phosphoruspentoxide or potassium sulphate considerably increase the sulphonating property exhibited by fuming sulphuric acid.
The separation of the sulphonic acids from sulphuric acid is effected by salting out the former with common salt, or by removing the sulphuric acid with calcium, barium, or lead salts, provided that the sulphonic acid salts of these metals are soluble in water.
The sulphonic acid, in its chemically pure state, is best obtained from its crystalline barium salts, which are decomposed with the equivalent of sulphuric acid; another way is to decompose the calcium salts of the sulphonic acids with oxalic acid. The sulphonic acids are frequently hygroscopic and are easily soluble in water; the majority of their barium and lead salts are also soluble in water. The sulphonic acids are insoluble in ether. The halogens do not easily react with sulphonic acids, but when they do they usually replace the sulphonic acid group. In order to prepare the halogen substitution products, therefore, use is made of sulphonic chlorides. The latter are obtained by the action of chlorosulphonic acid on aromatic hydrocarbons; a simpler method, however, is to treat the dry alkali sulphonates with phosphorus pentachloride—
C_6H_5SO_3Na + PCl_5 = C_6H_5SO_2.Cl + NaCl + POCl_3
Derivatives of sulphonic chlorides are sulphonamides, which are easily prepared from the former by grinding with ammonium carbonate—
C_6H_5SO_2.Cl + (NH_4)_2CO_3 = C_6H_5.SO_2.NH_2 + NH_4Cl + CO_2 + H_2O
Sulphonic chlorides react with alkaline sulphides to form thiosulphonic acids—
C_6H_5SO_2.Cl + K_2S = C_6H_5SO_2.SK + KCl
Sulphonic chlorides, dissolved in ether, yield sulphinic acids on reduction with zinc dust or metallic sodium—
C_6H_5SO_2.Cl + H_2 = C_6H_5SO_2.H + HCl
In the sulphonic acid compounds it is assumed that the sulphur is hexavalent, and it is hence possible to consider the sulphones to be esters of sulphinic acid.
The sulphones are mostly solid bodies, which soften prior to melting when heated. They are very stable towards chemical reagents; for instance, saponification of a mono-sulphone very rarely yields sulphinic acid.
If a hydroxyl is substituted for a hydrogen atom in the aromatic hydrocarbons, the action of sulphuric acid is greatly facilitated; thus, by merely mixing phenol with sulphuric acid, the sulphonic acid is at once formed, whereby, in the cold,o-phenolsulphonic acid prevails which on heating for some time to 100°-110° C. is completely converted intop-phenolsulphonic acid. In the absence of free sulphuric acid the conversion ofo- intop-phenolsulphonic acid is brought about by heating the aqueous solution. Phenol-2,4-disulphonic acid is prepared fromo- orp-phenolsulphonic acid, whereas phenol-2,4,6-trisulphonic acid is prepared directly from phenol by heating with concentrated sulphuric acid in presence of phosphorus pentoxide. Phenolsulphonic acids are also obtained by fusing benzenedisulphonic acid with alkali.
Cresol is not so easily sulphonated as is phenol;o-cresol when heated eight to ten hours at 90° C. with one and one-half times its weight of concentrated sulphuric acid, yieldso-cresol-p-sulphonic acid.
The phenolsulphonic acids are strong, rather stable acids; their alcoholic hydroxyl-hydrogen atom may, similarly to that of the phenols, be substituted by a metal or an alkyl radical.
From [Greek: a]- and [Greek: b]-naphthol a number of sulphonic acids may easily be prepared; viz., mono-, di-, and trisulphonic acids. Nearly all these acids are important as basic materials in the dyestuff industry, especially 2,6-[Greek: b]-naphtholmonosulphonic acid (S-acid), 2,3,6-[Greek: b]-naphtholdisulphonic acid (R-acid) and 2,6,8-[Greek: b]-naphtholdisulphonic acid (G-acid).
2. Condensation of Phenols
Phenolsulphonic acids exhibit pronounced tendencies to condensation, for which purpose A. v. Baeyer (1872) employed aldehydes. The reaction is rather violent, and yields, in addition to well-defined crystalline substances, amorphous bodies resembling rosins. In addition to formaldehyde, paraformaldehyde, trioxymethylene, methylal, hexamethylene-tetramine, and other substances containing a reactive methylene group, as well as acetaldehyde, benzaldehyde and other aldehydes may be employed to induce reaction.
A number of these condensation products are derivatives of diphenylamine or hydroxybenzyl alcohols. When the latter are heated, either by themselves or in presence of acids, anhydrides and polymerisation products are formed producing hard, brittle, fusible substances, insoluble in water but fairly soluble in organic solvents. The same substances are formed when phenols are condensed with formaldehyde, especially in the presence of acid contact substances and excess of phenol by sufficiently long heating at certain temperatures. The substances referred to are termed "Novolak": similar to these are the so-called "Resols," insoluble and non-fusible substances, very resistant to chemical and physical action. Another member of the series is the so-called "Bakelite" or "Resitol," which does not fuse but softens when heated and swells in organic solvents. The ultimate product of this class of substances is "Resit" which is obtained when concentrated hydrochloric acid is allowed to act upon a mixture of phenol and formaldehyde; the temperature rises spontaneously, and a hard, porous, insoluble mass of great resistance is obtained. By heating resols, resitols are formed which, on further heating, are finally converted into resits. [Footnote:Ber.,1892, 25, 3213.]
Of all these products, bakelite (resitol) has found the greatest industrial application; in its purest form, this substance is a nearly colourless or light yellow body of sp. gr. 1.25 and, being a poor conductor of heat and electricity, constitutes an excellent insulating material; it is exceedingly resistant towards most chemical reagents even in concentrated forms of the latter. Its pronounced refractivity, and the ease with which it may be worked, makes bakelite a favourite substitute for amber (Ger. Pat, 286, 568). Similarly, the resols which can be easily moulded are used either as such or mixed with sand, pulverised cork, asbestos or wood, and the moulded substances then converted into the more highly resistant bakelite by heating.
The constitution of these bodies no doubt depends largely on their method of preparation; Baekeland [Footnote:Chem. Ztg.,1913, 73, 733.] considers resit a polymerised hydroxybenzylmethylene glycol anhydride; Raschig, a diphenylmethane derivative (e.g., dihydroxydiphenylmethane alcohol); Wohl [Footnote:Ber.,1912, 45, 2046.] considers them polymerisation products of methylene derivatives of tautomeric phenol.
CH===CHH_2C:C{ }COCH===CH[Note: Lower Right CH has double bond to CO]
This group possesses the characteristic property of being capable of converting animal hide into leather when suitably dissolved. The author has dissolved a number of these water-insoluble condensation products in alkali and alcohol and was able to demonstrate their tanning effects on pelt; bakelite is easily soluble in alkali; a faintly alkaline solution partially precipitates gelatine, and completely so when the alkali is neutralised. This latter solution gives a dirty brown precipitate with iron salts.
These condensation products gained extraordinary importance for the tanning trade when Stiasny [Footnote: Ger. Pat, 262,558; Austr. Pat, 58,405.] succeeded in preparing them in water-soluble form when they are enabled to directly exert their tannoid properties. This may be done by acting upon two molecules of concentrated phenolsulphonic acid with one molecule of formaldehyde, the temperature thereby not exceeding 35°C. By condensation, however, considerable heat is liberated, and hence the rise in temperature can only be limited by adding the diluted formaldehyde drop by drop, whilst stirring and cooling, to the phenolsulphonic acid. The original letters patent is worded as follows: 10 kilos each of crude phenol and sulphuric acid (66° Bé.) are heated with stirring for two hours at 105°-106°C., cooled to about 35°C., and 463 kilos 30 per cent. formaldehyde added during three hours, the temperature thereby not exceeding 35°C.; the stirring is continued for a couple of hours after the final addition of formaldehyde. This yields about 24 kilos of the crude condensation product. On a commercial scale, however, cresol (cresylic acid) is substituted for phenol. There are three isomers of cresol, viz.,o-,m-, andp-cresol, and it was naturally of interest to investigate whether one or the other of the isomers exerted any particular influence on the properties of the final product. It was found, however, that condensation products from the three isomers were distinguishable from one another neither in physical nor in tannoid properties. It is hence possible to employ crude cresol, which contains varying quantities of theo-,m-, andp-compounds, in the manufacture of these tanning matters. [Footnote: Gen Pat, 291,457.]
The tar obtained from the Rochling coal-gas generator contains considerable quantities of phenols (B.P.=200°-250°C.), and the author has protected the use of these for the production of synthetic tannins by Ger. Pat, 262,558. A deep brown viscous mass is obtained which, when partly neutralised, yields similar results to those given by the product above referred to.
It may be anticipated that by analogy from the chemical reactions taking place in the condensation of phenols on the one hand and cresolsulphonic acid on the other, that all other homologues of phenol, its polyvalent derivatives, substitution products and acids, would yield similar condensation products.
The particular position occupied by the aromatic hydroxy compounds in the chemistry of substance possessing tannoid character is not only evidenced by the natural classification of the tannins, tannin derivatives, and decomposition products so far isolated and investigated, but also by other chemical behaviour shown by these substances. Meunier and Seyewetz [Footnote:Collegium, 1908, 315, 195.], for example, were able to show that phenol,p-aminophenol, chlorophenol, trinitrophenol, catechol, resorcinol, hydroquinone, monochlorohydroquinone, orcinol, pyrogallol, and gallotannic acid precipitate gelatine from its aqueous solution, that is, to a certain extent possess tanning properties.
The author has extended this series somewhat and obtained the following results:—
Relative Behaviour TowardsSubstances Gelatine. Hide Powder. Pelt.Tribromophenol Slight ppte. Tans Surface tannage[Footnote: In alcoholic solution]o-Nitrophenol No ppte. " "Br-o-Nitrophenol Slight ppte. " "Tribromopyrogallic Ppte. " "acidBromophloroglucinol " " No tannageGalloflavine Slight ppte. " "Bromosalicylic acid " " "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 class of hydroxy-cymenesthymol,
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 mass 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° Bé. 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 assumes 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 quantity of insolubles whilst the liquid assumes 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 assume 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 mass, which is soluble in water and which had completely fixed the formaldehyde. If, however, this mass is heated for some time to 100°C, it assumes 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 potassium 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 thetrihydroxybenzenespyrogallol 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 mass, 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 assuming 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 mass 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 assumes 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.] assumes 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 analogy 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, assumes 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,guaiacolwas 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 assuming 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 washing, 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 washing 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 mass 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.
Aminobenzene, 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 class 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 aminotoluenesulphaminobenzenesulphonic acid is finally treated with p-toluenesulphonic chloride till the latter disappears. A compound of the composition
——-NH——-SO_2——-^ ^ ^| | | | | || | | |—-NH—-| |V V VSO_2Na CH_2
is thereby obtained, which, when acidified, is readily capable of being used for tanning purposes.
The intermediary product of the aminotoluenesulphaminobenzenesulphonic 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 vSO_3Na CH_3 CH_3
Ifp-toluenesulphaminobenzenesulphonic chloride is condensed with sodium sulphanilide, a compound,
—-SO_2—-NH—- NaSO_3^ ^ ^| | | | | || | | | | |v v —-SO_2—-NH—- vSO_3Na
is obtained which, when acidified, exhibits tannoid properties.
On condensing sodium phenolsulphonate with nitrotoluenesulphonic chloride, reducing the condensation product and condensing the latter withp-toluenesulphonic chloride, a compound similar to the above is obtained—
—-O—-SO_2—-^ ^ ^ CH_3| | | | | || | | | | |v v —-NH—-SO_2—- vNaSO_3 CH_3
Again, a similar product is obtained whenp-toluenesulphaminobenzenesulphonic chloride or its homologues or isomers are condensed with sodium-o-cresylsulphonate—
—-SO_2—-NH—- SO_3Na^ ^ ^| | | | | || | | | CH_3| |v v —-SO_2—-O—- vCH_3
The chloride of this compound may again be condensed, for instance, with sodium aminotoluenesulphaminobenzene-sulphonate, and yields the compound—
—-NH—-SO_2—-^ ^ ^ —-NH—-SO_2—- ^| | | | | | | || | | | | | | |v v —-NH—-SO_2—- v v CHNaSO_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 arylsulphaminobenzylsulphonic 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 aminobenzylsulphonic acid with iron and acetic acid, and finally condensed with the calculated amount ofp-toluenesulphonic chloride. A mixtureo- andp-toluenesulphaminobenzylsulphonic acid [Footnote 1: Cf. also Ger. Pat, 319,713 and 320,613.] thus results.
Amongstaromatic alcoholsthe 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 homologues, 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:—
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 amongstaromatic acidsis 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° Bé 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 homologues 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,
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 pronounced 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 substitute for shellac.—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 mass which reacts strongly acid. By reducing the acidity of the substance to 1 gm. = 10 c.c. N/1O NaOH, a grey, cheesy mass 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° Bé), 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 pronounced 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 assumes a dark, reddish-yellow colour, and remains soluble in water and precipitates gelatine strongly. This condensation product, on adding excess of caustic soda, assumes 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 mass, 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.
Thionaphtholsulphonic acid, when acted upon by formaldehyde, yields a condensation product of the following constitution:—
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 pronouncedly 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 pronounced 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 vCO 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 ofretene(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:CfAustr. Pat., 69,194]
5. Di- and Triphenylmethane Groups
If DIPHENYLMETHANE, (C_6H_5)_2CH_2, is heated with excess sulphuric acid, a dark blue mass, 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 vN_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 mass which, when condensed with formaldehyde in the cold, and subsequently heated, yields a mass 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—
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—
which is formed in addition to hexahydroxydiphenylmethane-dicarboxylic acid—
Baeyer's experiment with pyrogallol probaly also yields, according toNierenstein, another compound of the following constitution—
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:—
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 aurinecarboxylicin 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.
The reaction between phenols and aldehydes has been further studied by Michael, [Footnote:Amer.Jour., 5, 338; 9, 130.] who prepared a condensation product from phenol and resorcinol with benzaldehyde, and Russanow, [Footnote:Ber.9 1889, 22, 1944.] who also employed benzaldehyde and phenol. Lipp [Footnote: Diss., Bern., 1905.] investigated the action of benzaldehyde and piperonal on phenols, anisoles, cresols, cresylic ether, resorcinol, and the ether of the latter and phenol, and showed that when free phenols are condensed with benzaldehyde the hydroxyls occupy the same position as by the interaction between benzaldehyde and the corresponding phenolic ethers. The resulting dihydroxytriphenylmethane derivatives form beautiful crystals, which on oxidation are converted into benzaurines, the constitution of the latter probably being—
O= ^=_____ ^ OH| | | || |== __| |=v v vC|C_6H_5
In alkalies, the hydroxylated triphenylmethanes dissolve without imparting any colour to the solution; by concentrated sulphuric acid they are taken up with intense coloration.
If the hydroxyls occupy the ortho-position to methyl, they may form xanthenes by splitting off water—
O^ ^ ^| | | || | | |CH_3 v v v CH_3CH|C_6H_5
In the benzene series this reaction is difficult to establish, and has to be induced by distilling the particular dihydroxy-diphenylmethane at ordinary pressure. In the naphthalene series, on the other hand, the ring closes up by, for instance, the condensation of [Greek: b]-naphthol with benzaldehyde or paraldehyde, and yields the following compounds:—
These xanthenes are white, silk-glossy needles, which are soluble in water and in alkalies. In concentrated sulphuric acid, they are taken up with beautiful fluorescence.
6. Summary
From the qualitative reactions of the different condensation products described it may be seen that their tannoid properties are not dependent on whether they precipitate gelatine or are adsorbed by hide powder or not. Hydroxynaphthylmethanesulphonic acid, for instance, precipitates gelatine but does convert pelt into leather; on the other hand, sodium dicresylmethanesulphonate does not precipitate gelatine, and neither does it tan pelt; nevertheless it is adsorbed by hide powder as "tanning matter". The author discovered thato-nitrophenol does not precipitate gelatine, but has some tanning action on both hide powder and pelt.
Relatively to the possibilities of forming condensation products possessing tannoid properties, the following may be stated:—
All mono- and polyhydric phenols may be converted into true tanning matters by either condensing them as such, or after their conversion into the corresponding sulphonic acids, by substances capable of eliminating the elements of water. It makes no difference to the final product whether the condensation is the first step followed by sulphonation and consequent solubilisation of the intermediary insoluble product, or whether, vice versa, the sulphonic acid is subjected to condensation. Alkaline solution of phenols may also be condensed, the reaction products, when condensed, constituting tanning matters soluble in water.
Among the substitution products of the phenols, the thio-, chloro-, bromo-, nitro-, and aminophenols as a rule yield tanning matters similar in character.
The quinones are as such—i.e., without being condensed—substances possessing tannoid properties.
The aromatic dihydric alcohols are easily condensed with the different sulphonic acids and yield valuable tanning matters.
Of aromatic acids all those which yield water-soluble sulphonation products seem suitable for the industrial production of tanning matters. If the acids themselves do not yield water-soluble sulphonation products, the alkali salts of the latter may be condensed with formaldehyde, and the resulting products then constitute tanning matters provided their solutions can be neutralised or faintly acidified without the solute being thrown out of solution in insoluble form.
The diphenyl derivatives of the above groups often possess tannoid properties.
The same holds good of those compounds with condensed nuclei (naphthalene, anthracene, etc.), and all their derivatives which satisfy the above conditions.
The choice of condensing agent is, as a rule, of little significance. Elimination of the elements of water by the mere application of heat succeeds in few cases only, since the high temperature required to induce reaction in many cases causes decomposition of the substances. This difficulty is overcome by heatingin vacuo. Condensation with formaldehyde always succeeds, with acetaldehyde and benzaldehyde only partly.
The action on hide powder, pelt, and gelatine by these characteristicsubstances is tabulated below:—Relative Behaviour towardsSubstance. Gelatine. Hide Powder PeltFormaldehyde … … TanningPhenol Ppte. … …Chlorophenol " … …SurfaceTribromophenol Slight ppte Tanning tanningoNitrophenol No ppte " "Bromonitrophenol Slight ppte " "Trinitrophenol Ppte " TanningBromotrinitrophenol Slight ppte " "pAminophenol Ppte … …mDihydroxybenzene " … …Orcinol " … …pDihydroxybenzene " Tanning TanningMonochloropdihydroxybenzene " … …oDihydroxybenzene " … …Pyrogallic acid " … …SurfaceTribromopyrogallic acid " Tanning TanningGallic acid No ppte Not tanning Not tanningBromophloroglucinol Ppte Tanning "Gallotannic acid " " TanningGalloflavine Slight ppte " Not tanningQuinone " " TanningBromosalicylic acid " " Not tanningDinaphthylmethanedisulphonic acid Ppte " TanningDiphenylmethanedisulphonic acid " " "Dicresylmethanedisulphonic acid " " "Sodiumdicresylmethanedisulphonate acid No ppte " Not tanningDixylylmethanedisulphonic acid Ppte " TanningNaphtholdisulphonic acid " Not tanning Not tanningMethylenedinaphthol " Tanning TanningHydroxyphenylmethanesulphonic " " "acid Not tanningHydroxynaphthylmethanesulphonic Slight ppte " "acidDiaminonaphthylmethanedisulphonic Ppte Tanning Not tanningacidDihydroxynaphthylmethanedisulphonicacid " " "Dichloronaphthylmethanedisulphonicacid " " Surface tanningDinitronaphthylmethanedisulphonicacid " " "Dithionaphthylmethanedisulphonicacid " " TanningBromo[Greek: b]naphthol [1] Slight ppte " "Rosolic acid_ [1] Ppte " "