The Natural Organic Colouring Matters
By
Arthur George Perkin, F.R.S., F.R.S.E., F.I.C., professor of colour chemistry and dyeing in the University of Leeds
and
Arthur Ernest Everest, D.Sc., Ph.D., F.I.C., of the Wilton Research Laboratories; Late head of the Department of Coal-tar Colour Chemistry; Technical College, Huddersfield
Longmans, Green and Co.
39 Paternoster Row, London
Fourth Avenue & 30th Street, New York
Bombay, Calcutta, and Madras
1918
Kaikki kuvat (kemialliset kaavat) puuttuvat // None of the illustrations (of chemical formulas) included.
There are several varieties of catechu or cutch bearing different names according to the country or plants from which they are obtained. The following are those principally employed by dyers and tanners: Gambier catechu, Bengal or Acacia catechu, Bombay or Areca catechu, and Mangrove cutch.
Although catechu has perhaps received greater attention than other natural dyes, the results have been so varied that the chemistry of the subject was until recently in a most unsatisfactory condition. Some confusion has arisen from the non-appreciation of the fact that the main constituents of gambier and Acacia catechu are not identical, and some uncertainty has also apparently existed as to the botanical origin of the commercial varieties. Bombay catechu is, for instance, occasionally referred to as originating from the Acacia catechu, and Bengal catechu from the Areca catechu (Linn.); and in many cases, at the present time, it is impossible to ascertain the botanical derivation of commercial brown cutch preparations.
Gambier catechu.
Gambier, yellow cutch, cubical cutch, cube gambier, or terra japonica, is obtained from the Uncaria gambier, an extensive scandent bush which is met with, both wild and cultivated, in Malacca, Penang, and Singapore. The catechu is isolated by extracting the leaves and twigs with hot water until the liquid becomes syrupy, the insoluble matter being removed from time to time by means of a strainer. On cooling, the pasty mass is cut into cubes with sides i inch in length and dried on bamboo trays.
Catechin, C15H14O6, 4H2O, the crystalline principle, was first described by Nees van Esenbeck (Annalen, 1832, 1, 243), was subsequently examined by Berzelius in 1837 (Jahres., 14, 235), and more recently by numerous chemists. To isolate catechin, Löwe (Zeitsch. anal. Chem., 13, 113) devised the following method: catechu is washed with cold water to remove catechutannic acid, well pressed, allowed to dry, and dissolved in hot acetic ester. The filtered solution is evaporated and the residue crystallised from hot water.
According to Perkin and Yoshitake (Chem. Soc. Trans., 1902, 81, 1162), a combination of this method and that of Berzelius (loc. cit.) gives good results.
The finely powdered catechu is extracted with i o times its weight of boiling ethyl acetate, the solution is evaporated, and the residue crystallised from 10 times its weight of water. The product is again dissolved in boiling water, and lead acetate solution added, drop by drop, until a coloured precipitate is no longer formed, and the filtrate is almost colourless; the latter, while hot, is treated with sulphuretted hydrogen, the lead sulphide removed, and the crystals, which separate on cooling, are collected, washed, and allowed to dry at the ordinary temperature. The substance is now practically colourless, and the yield greater than if the purification had been effected by frequent crystallisation from water.
The formulæ which have been assigned to catechin are very numerous, and it has only recently been shown by v. Kostanecki and Tambor (Ber., 1902, 35, 1867), and simultaneously by Perkin and Yoshitake (loc. cit.), that it is correctly represented as C15H140, 4H2O
Catechin forms colourless needles, and when crystallised from water the air-dry product melts at 96° (Clauser, Ber., 1903, 36, 101). After standing over sulphuric acid, it possesses the formula C15H14O6, H2O, and melts at 176-177°, and this is also the melting-point of the anhydrous substance. Catechin is readily soluble in boiling water and cold alcohol, and gives with lead acetate solution a colourless precipitate, and with ferric chloride a deep green liquid. With pine wood and hydrochloric acid, it gives the phloroglucinol reaction.
On fusion with alkali, protocatechuic acid, phlorogucinol, and probably acetic acid are produced, and it is interesting to note that catechu has been considerably employed for the commercial preparation of the former compound.
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Catechutannic acid.
In addition to catechin, gambier catechu contains a small amount of catechutannic acid, and this substance is present in considerable quantity in the browner varieties of cutch. To isolate it the following method has been employed:
A hot aqueous extract of catechu is allowed to stand until no more catechin is deposited, and the clear liquid is evaporated to dryness. The residue is dissolved in alcohol, the solution treated with ether to precipitate impurities and then evaporated to dryness.
Catechutannic acid consists of an amorphous reddish-brown powder, readily soluble in water and alcohol, insoluble in ether. According to Löwe (Fr., 13, 121), it possesses the formula C21H18O8, and gives a lead salt 3P?bO, 2C21H18O8, but this most probably requires revision.
According to Etti (Annalen, 186, 332) catechutannic acid is an anhydride of catechin, and is derived from this substance by elimination of water. It is said to be produced when an aqueous solution of catechin is heated to 110 (Löwe, ibid.) 12, 285), or by boiling catechin with solutions of the alkali carbonates. Again, catechin is decomposed at its melting-point with evolution of water and formation of a product resembling catechutannic acid, and aqueous solutions of catechin on long standing become browncoloured with apparent formation of this compound. That the products obtained by these methods resemble catechutannic acid is certain, but the subject has not been fully investigated.
Catechutannic acid solution gives a precipitate with lead acetate, and also resembles catechin in giving a green coloration with ferric chloride, and the phloroglucinol reaction with pinewood and hydrochloric acid. It is a powerful tanning agent, and appears to differ but little from the so-called "catechol" tannins.
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Catechin, though largely employed for tanning purposes, does not precipitate a gelatine solution, and is not itself a tannin matter. On the other hand, it is absorbed by the hide, and there gradually passes into catechutannic acid.
Dyeing Properties.
On cotton, catechu is largely used. for the production of the well-known "catechu brown," which is exceedingly fast to light, acid and alkaline solutions, and also to bleaching powder. To obtain this, cotton is steeped in a hot solution of catechu (1-2 per cent.) to which has been added about 6 per cent, of copper sulphate, reckoned on the weight of the catechu employed. The material is allowed to remain in the bath as it cools, and without washing is then treated in a warm or boiling second bath containing i or 2 grams of bichromate of potash per litre. According to Hummel and Brown (J. Soc. Chem. Ind., 1896, 15, 422), in this operation the copper sulphate probably converts the catechin into catechutannic acid, and this is subsequently oxidised to japonic acid by the action of the bichromate. The colour is apparently intensified by the formation of a basic copper chromate. Gambier catechu is also employed in dyeing compound shades with logwood, fustic, and alizarine in conjunction with bichrome, and with bismarck brown, magenta, etc. In the latter case catechu-tannin forms the mordant for the basic colour.
Wool may be dyed with catechu in a similar manner to cotton. In silk dyeing, catechu is largely used for weighting purposes. The silk is steeped in basic ferrous sulphate solution, then in potassium ferrocyanide and hydrochloric acid, which causes the production of prussian blue on the fibre. It is then worked in a strong solution of gambier to which stannous chloride solution has been added.
Bengal or Acacia Catechu.
Bengal catechu is derived from the Acacia catechu, a tree 15 to 20 feet high, which is common in most parts of India and Burma. To isolate the catechu which is present in the red heartwood, the tree is cut down while it is fullest of sap, and the internal portion is sawn into small sticks and extracted with boiling water. The liquid thus obtained is concentrated over a fire, and then allowed to evaporate spontaneously in shallow dishes. The extract thus obtained comes into the market as Pegu catechu, brown cutch, and brown catechu (Crookes, "Dyeing and Calico Printing").
A purer substance, kath, or the pale catechu of India, is prepared by suspending twigs in the hot concentrated extract and collecting the crystals which thus separate.
Acacatechin, C15H14O6, 3H2O, can be obtained from the acacia kath by methods identical with those which are employed for the isolation of catechin from gambier catechu. It melts at 204-205°, is somewhat more sparingly soluble in water than catechin, and when fused with alkali gives phloroglucinol and protocatechuic acid. The reactions of acacatechin and catechin, in so far as they have been observed, are identical, and there is no difference in the composition of their derivatives when prepared under similar conditions. On the other hand, the melting-points of the substances themselves, and also of their derivatives, differ widely, as is evident from the following table (Perkin and Yoshitake):
Penta-acetyl derivative... Acacatechin 158-160° Catechin 124-125°
Pentabenzoyl ... Acacatechin 181-183° Catechin 151-153°
Azobenzene ... Acacatechin 198-200° Catechin 193-195°
Tetramethyl ... Acacatechin 152-154° Catechin 144-146°
Acetyl ... Acacatechin 135-137° Catechin 92-93°
It appears certain, therefore, that catechin and acacatechin are isomerides, and it is possible that the latter may, on the basis of v. Kostanecki and Lampe's formula for catechin, be represented thus (Perkin, private communication): [KUVA PUUTTUU] a point which should not be difficult to determine.
Bombay or Areca Catechu.
This variety is obtained from the fruit of the Areca catechu or betel-nut palm, a tree which is common in tropical Asia. Though its chief constituent resembles catechutannic acid, catechin itself has not been isolated from this product. It possesses a bright chocolate and sometimes an orange-brown colour, and yields, on dyeing, very similar results to the ordinary cutches.
Mangrove cutch is obtained from the bark of the mangrove Ceriops candolleana, and is of somewhat recent employment. In its preparation it is preferable to extract the fresh bark, which is of a light colour internally, rather than the stored product which has become red, or is said to have "sweated ". The extract, on evaporation, becomes deep-red in colour, and the object of the manufacturer is to prevent this anhydride formation from going too far, otherwise a certain portion of the extract is rendered insoluble in water.
By fusion with alkali, it gives protocatechuic acid, but at present no catechin has been isolated from it (Perkin, private communication). When dissolved in sodium bicarbonate solution, acetic ester extracts an almost colourless tannin, which possesses the properties of a catechol tannin.
For many purposes, mangrove cutch is competing closely with the other varieties of catechu, and this competition will no doubt become keener, if its quality can be further improved.
Mahogany.
According to Caseneuve (Ber., 8, 828) mahogany wood contains a catechin. (See also under Rhubarb tannic acid, p. 449.)
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