Rhamnus frangula(CHAPTER I. The Anthraquinone Group.)

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
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Bombay, Calcutta, and Madras
1918

Kaikki kuvat (kemialliset kaavat) puuttuvat // None of the illustrations (of chemical formulas) included.

The bark of the alder buckthorn, R. frangula (Linn.), a shrub widely distributed throughout Europe, is employed in medicine chiefly in the form of a fluid extract as a mild purgative. Since the beginning of the last century the bark of this plant has been submitted to repeated examination, but an account of the earliest researches need not be recorded here. Casselmann (Annalen, 1857, 104, 77) was the first to analyse its main crystalline constituent which he termed frangulin, and this he extracted from the bark by means of ammonia. Faust (Annalen, 1872, 165, 229) by the hydrolysis of frangulin with hydrochloric acid obtained, in addition to a sugar, a substance he termed frangulic acid, and as by the distillation of this compound with zinc-dust anthracene was produced he concluded it to be a dihydroxy-anthraquinone isomeric with alizarin.

Liebermann and Waldstein did not prepare frangulin (Ber., 1876, 1775) but studied the colouring matter obtained from the bark by means of caustic soda solution, evidently the same substance as Faust's frangulic acid. This they isolated by sublimation and found to consist of emodin, and suggested that the hydrolysis of frangulin with acids might probably be represented by the following equation:
C₂₁H₂₀O₁₀ + H₂O = C₁₅H₁₀O₃ + C₆H₁₂O₆

Faust (Pharm. Zeitschrift fur Russland, 17, 257), on the other hand, considered that Liebermann and Waldstein's compound was not in reality emodin but rather a trihydroxydimethyl or a trihydroxyethyl- anthraquinone.

The subject was next studied by Schwabe (Arch. Pharm., 1888, 26, 259), who devoted special attention to frangulin. To this he assigned the formula C₂₁H₂₀O₉, and obtained from it by hydrolysis emodin and a sugar. This latter, which he did not isolate in a pure condition, he regarded as the rhamnodulcite of Liebermann. Finally Thorpe and Robinson (Chem. Soc. Trans., 1890, 57, 38) and Thorpe and Miller (ibid., 1892, 61, 1) submitted frangulin to detailed examination, and showed that, as Schwabe had stated, its correct formula is C₂₁H₂₀O₉. When hydrolysed it yields emodin, and rhamnose, which was isolated in a crystalline condition, and as a result the reaction may be thus expressed:
C₂₁H₂₀O₉ + H₂O = C₁₅H₁₀O₅ + C₆H₁₂O₅

For the preparation of frangulin the crushed bark is first treated with ligroin to remove wax and chlorophyll and subsequently extracted with alcohol. The extract is evaporated to dryness with sand, the powdered product extracted with ether, and after distilling off the ether the residue is treated with a little alcohol. On standing the solution deposits a mixture of emodin and frangulin together with a small quantity of a more sparingly soluble substance, and these can be separated by fractional crystallisation from alcohol.

Frangulin is an orange-yellow crystalline powder sparingly soluble in alcohol and melts at about 225°. It is readily susceptible to hydrolysis, and this indeed occurs to a slight extent by mere boiling with alcohol.

The sparingly soluble substance which contaminates the crude frangulin, the presence of which was also observed by Schwabe, crystallises from alcohol in golden-yellow needles, melting at 202 203. It dissolves in sulphuric acid with a red coloration, in dilute alkalis with the same tint, and is almost insoluble in ammonia. It appears to possess the formula C₁₅H₁₀O₅, being thus isomeric with emodin, though Perkin (Chem. Soc. Trans., 1895, 67, 1084) has suggested its possible identity with the emodin methyl ether C₁₆H₁₂C₅ of the Ventilago madraspatana and Polygonum cuspidatum, with which in melting-point and property it closely agrees.