The molecular weight of this substance is 4,021, and probably represents the highest molecular organic body obtained in any chemical synthesis.

From a physiological standpoint the recognition of tannins as esters of glucose and hydroxybenzoic acids, possessing characteristics similar to those of tannin, is of great importance. Especially interesting appears the fact of plants utilising sugars for the esterification of acids, just as glycerol or monohydric alcohols may be employed for the same purpose. Free acids, as a rule, are only tolerated in certain parts of the organism, the latter usually striving to neutralise acidic groups which may be brought about by salt formation; formation of amino compounds (proteins) or esterification (fats); and, lastly, esterformation by means of sugars.

Why Nature should always build up substances of very complex const.i.tution can only be explained by biochemical investigations, but it may, at any rate, be a.s.sumed that by this means any substance poisonous to the living organism is rendered inactive. The function of the tannins present in plants may thus be explained; if, for instance, phenols are formed by the oxidation of corresponding sugars, [Footnote: Mielke, "Ueber die Stellung der Gerbstoffe im Stoffwechsel der Pflanzen"

(Hamburg, 1893).] the poisonous character of the former would be lessened by the introduction of the carbonic acid esters and subsequent coupling of the substances (depside formation). The depsides thus formed would serve as vehicle of the sugars and transport the migrating tannins, [Footnote: Kraus, "Grundlinien zu einer Physiologie der Gerbstoffe" (1889).] and, after subsequent deposition of the sugars, would then be eliminated from the plant organism, either by oxidation into ellagic acid and phlobaphenes or by condensation with the formation of cork.

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.

[Ill.u.s.tration: Chart Showing the Decomposition of Products of Tannin.]

SECTION II

SYNTHESIS OF TANNING MATTERS

1. AROMATIC SULPHONIC ACIDS

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-called _sulphonic 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 the _m_-disulphonic acid by gently heating with fuming sulphuric acid; stronger heating converts the _m_- into the _p_-disulphonic acid, and at 190 C. the trisulphonic acid is formed. Toluene treated with fuming sulphuric acid first yields _o_- and _p_-sulphonic acids, finally _o_- and _p_-disulphonic acids, ethylbenzene at the boiling point _p_-ethylbenzene-sulphonic acid. Of the three isomeric xylenes _o_- and _m_-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 160-170C. 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 pota.s.sium 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 subst.i.tution 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 a.s.sumed that the sulphur is hexavalent, and it is hence possible to consider the sulphones to be esters of sulphinic acid.

==O R--S==O --H

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 subst.i.tuted 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 into _p_-phenolsulphonic acid. In the absence of free sulphuric acid the conversion of _o_- into _p_-phenolsulphonic acid is brought about by heating the aqueous solution. Phenol-2,4-disulphonic acid is prepared from _o_- or _p_-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, yields _o_-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 subst.i.tuted 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 p.r.o.nounced 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 cla.s.s 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 ma.s.s 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, const.i.tutes an excellent insulating material; it is exceedingly resistant towards most chemical reagents even in concentrated forms of the latter. Its p.r.o.nounced refractivity, and the ease with which it may be worked, makes bakelite a favourite subst.i.tute 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 const.i.tution 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===CH H_2C:C{ }CO CH===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 35C. 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 Be.) are heated with stirring for two hours at 105-106C., cooled to about 35C., and 463 kilos 30 per cent. formaldehyde added during three hours, the temperature thereby not exceeding 35C.; 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 subst.i.tuted for phenol. There are three isomers of cresol, viz., _o_-, _m_-, and _p_-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 quant.i.ties of the _o_-, _m_-, and _p_-compounds, in the manufacture of these tanning matters. [Footnote: Gen Pat, 291,457.]

The tar obtained from the Rochling coal-gas generator contains considerable quant.i.ties of phenols (B.P.=200-250C.), and the author has protected the use of these for the production of synthetic tannins by Ger. Pat, 262,558. A deep brown viscous ma.s.s is obtained which, when partly neutralised, yields similar results to those given by the product above referred to.

It may be antic.i.p.ated that by a.n.a.logy from the chemical reactions taking place in the condensation of phenols on the one hand and cresolsulphonic acid on the other, that all other h.o.m.ologues of phenol, its polyvalent derivatives, subst.i.tution 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 cla.s.sification 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 Towards Substances 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. " "

acid Bromophloroglucinol " " No tannage Galloflavine Slight ppte. " "

Bromosalicylic acid " " "