Georg Grasser

“ If the carboxyl of the first molecule combines with a hydroxyl of the second molecule (ester formation) , then a substance possessing character similar to that of a hydroxybenzoic acid is formed, which is capable of combining up with a further molecule in the same way. It is natural to assume that this ester form is much more prevalent in Nature than a combination of two carboxyls by the elimination of water. From the point of view of the chemistry of the tannins, therefore, the starting-point would naturally be that of synthesising the ester anhydrides of hydroxybenzoic acids. ”

“ For the purpose of condensing phenol with formaldehyde, it is not essential to first convert the phenol into the water-soluble phenolsulphonic acid, since it is possible to convert the condensation products of phenol and its derivatives, which are soluble in alkali, into water-soluble form by either heating the condensation products with concentrated solutions of formaldehyde and neutral sulphites, or by dissolving the condensation products in alkali and inducing reaction by means of formaldehyde bisulphite. ”

“ A suitable and reliable reagent is indigotine (B.A.S.F.) , which clearly distinguishes tanned and untanned layers of the pelt. If, for instance, a 1-2 per cent, solution of indigotine is brought into contact with a fresh cut on a pelt, and the latter subsequently washed with warm water, the indigotine is only retained by the untanned parts; a leather tanned with Neradol D is therefore only coloured by indigotine to the extent to which it has combined with the Neradol. ”

“ If the solution is kept below 45° C., by artificial cooling, the light colour is maintained, but a gelatinous precipitate is soon formed, the viscosity of which increases on stirring, and finally is converted into an insoluble, tough, gummy mass. If, on the other hand, the mass is heated at the beginning of the reaction, or if the amount of formaldehyde is increased and the mass cooled during reaction, effervescence occurs, and a cheesy, dirty-coloured mass results, which, on cooling, rapidly becomes solid and yields a very firm, elastic, rubbery mass, which is absolutely insoluble in water. ”

“ In a formaldehyde-tanned leather, however, no trace of tannin can be detected; and the yield (of leather, based on the pelt employed) , which, from a practical standpoint, is so important, is so very low that it is hardly possible to speak of it as a tannin in the ordinary sense of the word. Formaldehyde must, therefore, be termed a pseudo-tannin.
The tanning effect of formaldehyde is, according to Thuau, [Footnote: Collegium, 1909, 363, 211.] increased by those salts which bring about colloidal polymerisation of the formaldehyde, the resultant compounds being absorbed by the hide fibre. ”

“ 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. ”

“ 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. ”

“ As soon as the tanning matter has completely penetrated the pelt, the total time of tannage is noted, and the velocity with which the tanning matter converts the pelt into leather at that particular concentration is thus obtained. The tannage completed, the leather must be well washed in running water to remove excess of synthetic tannin and then dried. On examining the dry leathers, the colour may then be observed, and a cut will give an idea of the tensile strength and the length of fibre of the leather. ”

“ Hydrolysis of leucogallic acid yields gallic acid and gallic aldehyde; oxidation by means of hydrogen peroxide yields ellagic acid and luteic acid, and oxidation with potassium persulphate and sulphuric acid, in acetic acid solution, yields purpurotannin (see below) [Footnote: Liebig's Ann., 1912, 386, 318.] .
Another distinct difference between digallic acid and leucodigallic acid is the fact that the formaldehyde condensation product of the former resembles gallic acid, whereas that of the latter resembles tannin ”

“ 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 quantities of the o-, m-, and p-compounds, in the manufacture of these tanning matters. ”

“ The condensation products obtained by the interaction of dilute solutions of phenolsulphonic acid and formaldehyde at moderately high temperature, which form slimy masses and are insoluble in water, are soluble in alcohol. An alcoholic solution of such a product was used in a tanning experiment, and a piece of pelt immersed in the solution was tanned through in a few days; the resultant leather being rather firm, springy, and slightly hard, and the colour was a light brownish-grey. ”

“ If, therefore, natural tannins are mixed with this product and the solution used for tanning purposes, the resultant leather will possess a dark colour owing to the presence of solubilised phlobaphenes; if, on the other hand, a dark coloured leather, which has been tanned with natural tannins, is washed over with a 5° Bé solution of this synthetic tannin, or immersed for some time in the solution, the leather assumes a lighter colour owing to the phlobaphenes being dissolved and removed from the leather by the synthetic tannin. ”

“ A special characteristic of Neradol D tannage is the sensitiveness of the latter to the action of iron and alkalies. The active principle of Neradol D being free dicresylmethanedisul-phonic acid, which is easily neutralised by lime, ammonia, and amino-acids and hence rendered inactive for tanning purposes, it is essential that the pelt prior to tannage with Neradol D should be completely delimed, bated, and freed from all constituents possessing alkaline reaction. ”

“ The difficulty encountered by the o-position is eliminated when the carboxyl is not directly linked to the benzene nucleus, e.g., o-cumaric acid. Many hydroxybenzoic acids require an excess of chlorocarbonic methyl ester, which then also, to some extent, attacks the carboxyl group; but on dissolving the product in acetone and treating it with bicarbonate the carboxyl group as such is again restored without splitting off the carbomethoxy group. ”

“ Cresols, naphthalenes, and higher hydrocarbons are used as starting materials in the production of synthetic tannins; the former substances or their oxidation products are sulphonated by means of concentrated sulphuric acid, and the tanning matter produced by condensing the sulphonic acids with formaldehyde. The crude synthetic tannin thus obtained has yet to be diluted and partly neutralised before it can be applied in practice, and this is carried out by mixing the crude product with strong caustic lye. ”

“ The sensitiveness to alkalies of Neradol D is considerably greater than in the case of natural tannins, and it appears that a vegetable tan liquor neutralised with lime will not even surface-tan when acting upon pelt and will neither impart a dark colour to the leather nor remove from it any appreciable amount of protein. Similarly, a Neradol D liquor neutralised with lime exerts no tanning action, but in contradistinction to the vegetable tan liquor similarly treated, will impart a blue or blackish-blue colour to the pelt, from which it removes larger quantities of protein. ”

“ After about thirty hours a reddish powder deposits, from which ellagic acid may be extracted with pyridine; the mother-liquor on being concentrated yields luteic acid. It is also obtained by oxidising tannin with hydrogen peroxide, the other oxidation product being ellagic acid, and the two may then be separated as indicated above. Luteic acid forms reddish needles which are decomposed, with evolution of gas, at 338°-341° C. Heated with 10 per cent. caustic soda solution it yields ellagic acid. ”

“ It remains yet to be determined whether the sulphonic acid influences the leather substance to the extent that sulphuric acid does; it must, however, be borne in mind that Neradol D in addition to free sulphonic acid also contains sulphonates and sulphates, which may enter into the leather and thus increase the sulphur contents of the latter. A method must hence be devised which estimates the free acid only and provides the means of distinguishing this from all other acids of organic and inorganic acids. ”

“ 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 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. ”

“ 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 ”

“ If the acidity does not fall within these limits it must be corrected by soaking the powder before chroming for twenty minutes in ten to twelve times its weight of water, to which the requisite calculated quantity of standard alkali or acid has been added. The hide powder must not swell in chroming to such an extent as to render difficult the necessary squeezing to 70-75 per cent. of water, and must be sufficiently free from soluble organic matter to render it possible in the ordinary washing to reduce the total solubles in a blank experiment with distilled water below 0.005 gm per 100 c.c. ”

“ Their basic idea was the absence of carboxylic groups in tannin, and that hence the total gallic acid must be present in ester form. These conditions are fulfilled if one views tannin as being an ester compound of 1 molecule of glucose and 5 molecules of digallic acid, of similar construction as, for example, pentacetyl glucose. Fischer and Freudenberg succeeded in preparing the former by shaking a mixture of finely powdered glucose, chloroform, and quinoline with an excess of tricarbomethoxygalloyl chloride for twenty-four hours and precipitating the resulting product with methyl alcohol ”

“ Like other "strong" and "weak" acids this sulphonic acid exercises a strongly swelling influence on pelt. Whereas the effect of acid present in solutions of Neradol D of medium concentration and its tanning effect both influence the pelt and are fairly well balanced, this is not the case as regards highly concentrated and very dilute solutions. If, for instance, a very dilute solution of crude Neradol (about 0.25° Bé.) is used, the tanning effect of this solution is exceedingly small and does not show itself till after several hours. ”