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.
It is usual to subdivide the great family of yellow colours derived from flavone into two classes, flavone and flavonol, and the latter group is distinguished by the fact that the hydrogen in the γ-pyrone ring of these compounds is substituted by hydroxyl, whereas in the former it is not.
Flavonol) so designated by v. Kostanecki, was synthesised by v. Kostanecki and Szabranski (Ber., 1904, 37, 2819) in the following manner:
By the action of amyl nitrite and hydrochloric acid in alcoholic solution on flavanone, isonitrosoflavanone, melting-point 158-159°, is produced, and this by means of boiling dilute acids splits off hydroxylamine and is converted into flavonol.
Flavonol crystallises from alcohol in yellow needles, melting-point 167-170°. When warmed with aqueous sodium hydroxide it forms a yellow liquid, and on cooling the sodium salt separates in the form of yellow needles. Its solution in sulphuric acid exhibits an intense violet fluorescence. Acetylflavonol, colourless needles, melts at 110-111°.
According to Auwers and Müller (Ber., 1908, 41, 4233) benzylidenecoumaranones can be converted into flavonols. Thus benzylidene 4 methylcoumaranone dibromide when treated with potassium hydroxide gives 2 methylflavonol. The reaction may be thus expressed : [KUVA PUUTTUU]
The hydrolysis of flavonol into 0-hydroxybenzoylcarbinol and benzoic acid may be expressed by the following equations [KUVA PUUTTUU] and this reaction, which is typical of the behaviour in these circumstances of the whole series of these compounds, has in general been employed to ascertain their structure. It is best effected by digesting the fully methylated flavonols with boiling alcoholic potash for some hours, for owing to the occurrence of secondary reactions it cannot be satisfactorily carried out with the unmethylated compounds.
For the synthesis of numerous flavonols, many of which occur naturally, v. Kostanecki and his co-workers have employed as a general method that found serviceable for the preparation of flavonol itself, and many instances of this are given in the sequel. The flavonols, with the exception of morin, which curiously enough is colourless, are yellow crystalline substances, soluble in alkaline solutions with a yellow colour, and yield with ease in the presence of acetic acid orange crystalline oxonium salts. According to Perkin, whereas as a rule hydroxyflavones are not oxidised by air in alkaline solution and can be precipitated therefrom unchanged by acids, flavonols on the other hand are readily decomposed in this manner with the formation of water-soluble products.
Interesting is the fact that though certain colouring matters of this group do not possess two hydroxyls in the ortho-position relatively to one another, they are nevertheless strong dyestuffs, and of these the tetrahydroxyflavonol morin may be taken as an example [KUVA PUUTTUU]
That this peculiarity arises from the presence of the pyrone hydroxyl is evident if the structure of morin is compared with the lotoflavone of Dunstan and Henry (loc. cit.) the tinctorial properties of which are exceedingly feeble. It seemed possible that this dyeing effect was to be attributed to the fact that this compound contains the hydroxyl (i) in the peri-position to the chromophore and which is present in most of the natural dyes of this group. Such a suggestion, however, became untenable on the synthesis of resomorin by Bonifazi, v. Kostanecki, and Tambor (Ber., 1906, 39, 86), which dyes the same shades as morin but does not contain the peri-hydroxyl in question. Evidently therefore the tinctorial properties of these hydroxy flavonols can only be accounted for by their possession of the grouping [KUVA PUUTTUU] the effect of which is considerably strengthened by the presence of hydroxyls in other positions in the molecule, and this has received support from the observation of v. Kostanecki and Szabranski that flavonol itself dyes on aluminium mordant a pale yellow shade. Though ortho-hydroxyl groups are not essential to the dyeing property of hydroxyflavonols, their presence, at least in certain positions, has considerable influence, not only in deepening the tone, but also in reddening the shade. Thus, whereas morin dyes bright yellow shades, quercetin gives a brown-orange shade on aluminium mordant, and the effect of the pyrone hydroxyl is very evident on comparing quercetin with luteolin which gives in the same way only a bright yellow colour. A multiplication of hydroxyls does not effect any general alteration of shade given by these compounds, as is so well known to take place in the anthraquinone group, and this affords support to the theory of Watson previously mentioned.
The shades given by the flavonols are not so fast to light as those given by the flavone luteolin, and this may arise in part owing to the greater susceptibility of their salts (or lakes) to oxidation. In this respect they vary again among themselves, quercetin being a somewhat faster colour than fisetin, and morin than quercetin.
On the other hand, the character of the shade given by the natural dyestuff varies in tone, as to whether the colouring matter is present as glucoside or in the free condition. Thus in dyeing with quercitron bark, quercitrin and not quercetin is the dyestuff, whereas in old fustic no glucoside is present, and the tinctorial effect is due to morin itself. The shade again given by a glucoside is naturally dependent on the position of the sugar nucleus, and thus the quercetin glucoside, quercimeritrin (see Cotton Flowers) has quite distinct properties in this respect from quercitrin itself. Again, a glucoside may be almost devoid of tinctorial property, as in the case of the kaempferol glucoside robinin and the alizarin glucoside ruberythric acid. The idea formerly held that glucosides in general were not true dyestuffs, and that during the dyeing operation by the action of the mordant they were hydrolysed with production of the colour lake of the free colouring matter, is incorrect. This evidently arose from the fact that in certain of these dyestuffs, as, for instance, madder and Persian berries, the glucoside is accompanied by its specific enzyme, which in case the temperature of the dye-bath is gradually raised from the cold upwards, effects the hydrolysis of the glucoside before the dyeing operation has really commenced.
In the following pages the natural dyestuffs containing flavonols, or their glucosides, are as far as possible arranged as to the number of hydroxyls present in the colouring matter, commencing with those which contain least. As, however, in many plants more than one flavonol is present, it has obviously not been possible to adhere strictly to this method of classification.
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