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.
Though there are but few data available concerning the tinctorial properties of the anthocyan pigments, there can be little doubt that they are capable of acting as mordant colours, and it is interesting to find that, more than fifty years ago, the pigment of the black hollyhock was used in Germany particularly in Bavaria both for dyeing and printing. It is stated that the Bavarian Government encouraged the investigation of the chemical nature of this colouring matter, and that the work of Buchner, Eisner, and Kopp was the result (Bull, de la Soc. d'encouragement, 1860, 332; Polyt. Zentr., 1860, 1540). It was observed that an aqueous extract of the black flowers dyed cotton mordanted with iron blue-black shades; the same material mordanted with alum gave violet-blue, whilst with a tin mordant a blue-violet colour resulted. According to Kopp, these colours were more fast to light and air than the colours produced from logwood, but they gradually lose their intensity with age, and are not fast to soap. Recent work has shown that though the natural pigments of this series which are capable of giving dyeings are fast to light, they are all very fugitive to reagents (Willstatter and Mallison, Annalen, 1915, 408, 29).
The colours are taken up well by the fibre, it being stated that excepting in the case of pelargonidin the bath is exhausted even. at low temperatures. The colours are strong even ¼ per cent, dyeings being satisfactory and the tones good though somewhat dull. The best dyeings are given with tin mordant on wool, or on tannined cotton, though in some cases the colours are taken up direct by unmordanted wool.
In regard to their dyeing properties the glucosides of the series react very similarly to the sugar-free pigments, and in this Willstatter and Mallison see evidence that the sugar groupings are not attached to the OH groups of the hydroxy-phenyl ring.
The dyeing properties of some of these pigments are collected together in the table on the next page.
For comparative purposes a bath was made up thus: 0,0025 gr. of the pigment was dissolved in a mixture of 10 c.c. alcohol and 40 c.c. water, the solution being made acid with 5 drops of 10 per cent, sulphuric acid (acetic acid is not used as it is not capable of preventing isomerisation with formation of the pseudo base nor the hydrolytic dissociation of the colour salt). To this bath 1 gr. of wool, or cotton, is added and left therein for one hour1 at a temperature of about 25° C,
| Pigment. | Wool - no mordant | Wool - Tin mordant | Tannined cotton |
| Pelargonidin. | Does not dye. | Purple-red | Red, with bluish tinge. |
| Cyanidin. | Fine rose. | Blue-violet | Violet |
| Delphinidin. | Violet | Blue, with violet tinge. | Blue-violet |
| Myrtillidin. | - | Violet-blue | Violet |
| Oenidin. | - | Violet-blue | Violet |
The dyeings thus produced are fast to light, but not to soap or water. Heating with water causes decolorisation, doubtless due to isomerisation with formation of the pseudo base, as is the case with dilute aqueous solutions of these pigments. Treatment of the dyeings with ammonia causes them to become blue, whereas mineral acids change them to red.
From the above data it will be observed that passing from pelargonidin → cyanidin→ delphinidin, i.e. by the substitution of additional OH groups in the hydroxy-phenyl residue, the colour of the dyeings becomes bluer.
*A. G. Perkin, Chem. Soc. Trans., 1902, 589.
** Willstätter and Mallison, loc. cit.In order to appreciate the alteration in shade produced as the result of the structural change in the molecule on passing from flavonol to anthocyan, the following table is introduced:
| Flavonol.* | On Wool (Tin) |
| Kaempferol | Lemon-yellow. |
| Quercitin | Strong orange. |
| Myricetin | Strong orange-red. |
| Pelargonidin | Purple-red |
| Cyanidin | Violet |
| Delphinidin | Blue-violet |
It should further be noted that the great increase of the basicity of the oxygen atom of the pyrone ring that occurs in passing from flavonol to anthocyan is presumably the cause of the latter having the power of dyeing on tannin as well as on basic mordants.
The tinctorial properties of certain substitution products of the anthocyans have been studied by Watson (Chem. Soc. Trans., 1915, 1477). He prepared a series of compounds by the interaction of Grignard reagents with flavonol derivatives in which the H atom in the 4 position of the pyran ring is replaced by a variety of groups. The majority of his products are derivatives of cyanidin obtained from quercetin and the variation in the shade and properties of the dyeings produced by change in composition in this series of compounds is of considerable interest. In the table on the next page the most important points in this connection are brought together for comparison. The shades are those given on wool with various mordants.
A point of interest lies in the effect of methylation on the dyeing properties of these colours. It has been mentioned above, that as those anthocyanins of which the tinctorial properties were examined behaved similarly to the sugar-free pigments, Willstatter and Mallison concluded that the sugar groups in these compounds are not attached to the hydroxy-phenyl nucleus. Again, it should be possible, in certain cases, to deduce the position of the methyl groups by the effect of methylation on the tinctorial properties, and Watson refers to this in the case of two products obtained by him, and included in the above table. The great difference, both in shade and fastness to reagents, observed between the trimethyl ether prepared direct from quercetin-trimethyl ether by means of magnesium-ethyl iodide, and the triethyl ether obtained by partial de-ethylation (with aluminium chloride or sulphuric acid) of the product of the reaction of magnesium-ethyl iodide on quercetin-penta-ethyl ether is very interesting, and cannot be considered as being due to mere replacement of methyl by ethyl. It appears necessary, as Watson points out, to assume that it is caused by the attachment of alkyl to different OH groups in these two compounds (nos. 4 and 5 in above table). [KUVA PUUTTUU]
In the natural anthocyan pigments methylation causes the shade of the dyeings on wool (tin mordant) to be shifted towards the red; thus Willstatter and Mallison record that delphinidin gives blue (with violet tinge), whilst its monomethyl ether (myrtillidin) gives violetblue, and its dimethyl ether (oenidin) a blue-violet colour. Further, they state that these methyl derivatives have weaker dyeing properties than the free delphinidin, and, moreover, that in these cases the dyebath is not exhausted. This latter effect is also noticed in the case of pelargonidin where there is only one OH group in the hydroxypheny) residue.
Everest has pointed out (loc. cit.) that as flavone derivatives (as distinct from flavonols) are fairly widely distributed in the plant world, and that flavonols also occur in which the OH groups in the oxyphenyl nucleus are in the meta-position to one another, e.g. morin, and that as these, on acid reduction, yield red pigments resembling known anthocyans, it is probable that further investigation may lead to the isolation of natural anthocyans related to them in the same way that the known pigments of the series are to quercetin, etc. Up to the present no such bodies have been discovered, but, on the other hand, Watson has prepared (loc. cit.) a small number of products that are related to them in just the same way as his other compounds are related to the known anthocyans. His startingpoints were morin, luteolin, and apigenin, from which, by the use of methyl- or ethyl-magnesium iodide, he obtained compounds represented by the formulæ (1), (2), and (3) respectively. The results of his examination of their dyeing properties are recorded beside their formulæ, and those of the corresponding derivative of cyanidin (4) added for comparison.
The use of anthocyanin pigments as indicators has been suggested by various authors, e.g. Watson (Amer. J. Pharm., 1913, 85, 246), extract of blueberry pigment; Walbum (Biochem. Zeit, 1913, 48, 291), red cabbage pigment; but it should be noted that the colour change of many of these pigments, e.g. cyanin, is very seriously affected by neutral salts if present in any considerable quantity.
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