Rhubarb, Rhein, Rheinolic acid, Alizarin(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
Fourth Avenue & 30th Street, New York
Bombay, Calcutta, and Madras
1918

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

RHEIN.

Rhein was isolated from rhubarb by Hesse (Annalen, 1899, 309, 43), whereas Oesterle and Babel (Schweiz. Woch. Chem. Pharm., 1904, 42, 329) showed that Tilden's aloe-xanthin prepared by the oxidation of barbaloin, and to which he had assigned the formula C₁₅H₁₀O₆, was a mixture of aloe-emodin and rhein. More recently its presence in rhubarb root has been confirmed by Tutin and Clewer (loc. cit.). It crystallises from pyridine in glistening orange-yellow needles melting at 318° (Tutin and Clewer), 314° (Oesterle), and is soluble in dilute alkaline solutions with a red colour. According to Oesterle it possesses well-defined tinctorial property and gives on fabrics mordanted with aluminium and chromium yellow shades.

The relationship between aloe-emodin and rhein and the constitution of these compounds has been elucidated by Robinson and Simonsea (Chem. Soc. Trans., 1909, 95, 1085). Whereas rhein yields only a diacetyl compound, and this is soluble in sodium acetate solution, further proof of the presence of a carboxyl group is afforded by the fact that this substance on hydrolysis and esterifi cation readily yields an ester, which by saponification is again converted into rhein. As aloe-emodin C₁₅H₁₀O₅ containing three hydroxyl groups is converted by oxidation with chromic acid into rhein C₁₅H₆O₄(OH)₂ in which but two are present, it is clear that one of the hydroxyls of aloe-emodin must be contained in a CH₂OH group and the oxidation process be represented as follows:
C₁₄H₅O₂(OH)₂CH₂OH + O₂ = C₁₄HC>₂(OH)₂COOH + H₂O

Aloe-emodin is thus the carbinol and rhein the corresponding carboxylic acid of a dihydroxy-anthraquinone.

To obtain an insight into the nature of this latter, Robinson and Simonsen prepared its dimethoxy derivative by converting dimethyl rhein amide, C₁₄H₅O₂(OMe)₂CONH₂, into amino-dimethoxy-anthraquinone, C₁₄H₅O₂(OMe)₂NH₂, and then eliminating the amino group in the usual way. The properties of this dimethoxy compound, according to these authors, indicate it to consist of iso-chrysazin dimethyl ether.

The constitutions of rhein, aloe-emodin, and chrysophanic acid may therefore be expressed thus: [KUVA PUUTTUU]

The following derivatives of rhein have been obtained:
Diacetyl-rhein, C₁₅H₆(C₂H₃O)₂O₆, needles, melting-point 247-248° (Oesterle), 258° (Tutin and Clewer), Tetranitro-rhein (Leger), C₁₅H₄O₆(NO₂)₄, short efflorescent prisms, Rhein ethyl ester, C₁₄H₅O₂(OH)₂COOEt, needles, melting-point 159°, Diacetyl-rhein ethyl ester, plates, melting-point 170°, Rhein dimethyl ether, C₁₄H₅O₂(OMe)₂2COOH, light-brown needles, melting-point 283-284°, Dimethyl rhein ethyl ester, C₁₄H₅O₂(OMe)₂COOEt, sulphur-yellow needles, melting-point 185-187°, Dimethyl rhein chloride, C₁₄H₅O₂(OMe)₂COCl, yellow prisms, melting-point 190°, Dimethyl rhein amide, C₁₄H₅O₂(OMe)₂CONH₂, plates, melting-point 287° (Robinson and Simonsen). The aminodimethoxy- anthraquinone and dimethoxy-anthraquinone alluded to above melt respectively at 243° and 204-205°.

RHEINOLIC ACID.

This compound, which is present in rhubarb root only in very small amount, was discovered by Tutin and Clewer (loc. cit.) in the mother liquor obtained during the purification of their emodin. Crystallised from pyridine it forms dark-red lustrous needles, which after drying at 130° melted between 295-297°.

Rheinolic acid, C₁₇H₁₀O₆, is a carboxylic acid containing at least one hydroxyl group, and when acetylated yields acetylrheinolic acid, small orange-coloured needles, melting at 236°, which readily dissolve in cold aqueous sodium carbonate. It is evidently an anthraquinone derivative, probably structurally related to rhein, and according to the above authors may contain a bridged ring. Alkaline solutions and concentrated sulphuric acid dissolve it with an intense red colour, and it is distinguished from rhein by the fact that addition of water does not precipitate it from its solution in the latter solvent.

ALIZARIN.

The presence of this substance in rhubarb root has been demonstrated by Hugo Muller (Chem. Soc. Trans., 1911, 96, 967) who isolated it from the residue which accumulates during the preparation of the officinal aqueous extract of rhubarb. This was extracted with benzene, and the product thus obtained fractionally crystallised from the same solvent. The alizarin in these circumstances is much less soluble than the emodin and chrysophanic acid which accompany it. The quantity present in the root is apparently minute.