Rhubarb, Aloe-Emodin(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.

ALOE-EMODIN.

Aloe-emodin appears to have been first isolated from rhubarb by Hesse (Pharm. J., 1895, iv., 1, 352, and Annalen, J 899, 309, 32), who described it as rhabarberon isomeric with emodin, C₁₅H₁₀O₅, and subsequently by Eyken who, though considering it as probably identical with Hesse's rhabarberon, termed it iso-emodin. More recent work by Oesterle has shown (Arch. Pharm,, 1899, 88) that historically the first preparation of this compound, though in an impure condition, is due to Tilden (Chem. Soc. Trans., 1877, 267), who under the name of aloe-chrysin obtained it by the oxidation of aloin (see later).

Tutin and Clewer (ibid., 1911, 948), on the other hand, first recorded aloe-emodin as a constituent of rhubarb root, and at the same time confirmed the identity of rhabarberon, iso-emodin, and aloe-emodin. According to Oesterle (loc. cit.) the emodin existing in senna is also aloe-emodin. Aloeemodin crystallises in pale brown orange needles melting at 223-224°, soluble in dilute ammonia to form a red-coloured liquid and in sulphuric acid with the same tint. These properties are identical with those of frangula-emodin, but on heating their solutions in sulphuric acid the product thus given by aloe-emodin dissolves in ammonia with a violet, whereas that from frangula-emodin gives a red coloration.

Acetylaloe-emodin, C₁₅H₅O₅(C₂H₃O₃, yellow needles, melts at 177-178° (Oesterle, Arch. Pharm., 1899, 237, (2), 81, 699), tripropionyl aloe-emodin, needles, at 152-153° (Oesterle), and tribenzoyl aloeemodin, greenish-yellow needles, at 23°4 (Robinson and Simonsen, Chem. Soc. Trans., 1909, 95, 1091). According to Leger (Comptes rend., 1910, 151, 128) tetranitroaloe-emodin, C₁₅H₆O₅(NO₂)₄, forms golden needles, melting-point 285°.

By the reduction of aloe-emodin with tin and hydrochloric acid in the presence of acetic acid, Oesterle (Schweiz. Wochenschr. f. Chem. u. Pharm., 1900, 21) obtained a compound crystallising in yellowish-green leaflets, melting-point 181-187°, which possessed the formula C₁₅H₁₂O₃ and was regarded as a dihydroxymethylanthranol.

By oxidation with chromic acid aloe-emodin is converted into rhein (Oesterle, Arch. Pharm., 1903, 241, 604), and when reduced chrysophanic acid is obtained (Oesterle, Arch. Pharm., 1911, 249, 455). The constitution of aloe-emodin is given on page 59.