Wdkeman—Pigments of Flowering Plants. 
889 
Rhein.—Dihydroxy-3, 4' -carboxyl-l-anthraquinone. 
Rhein occurs in Rheum officinale; 1 English rhubarb ; 2 Rheum 
rhaponticum ; 3 Rheum palmatum ; 4 and Aloe vulgaris . 5 It may 
be formed by oxidation of Aloeemodin which occurs with it in 
several species of Rhubarb. 
Rhein crystallizes in small yellow needles which melt at 
321°-322°. It is difficultly soluble in most ordinary solvents. 
It is soluble in concentrated sulphuric acid with a red color, 
also soluble in ammonia with a red color, upon exposure to the 
air this color goes through violet into blue. In dilute alkaline 
solutions it is readily soluble. Acids precipitate it from these 
solutions as a yellow mass. It forms esters 6 and ethers, 6 the 
former with alcohol, the latter with dimethyl sulphate in the 
presence of potassium hydroxide. It dyes wools mordanted 
with chromium a yellow color. 7 
The structural formula given is that suggested by Robinson 
and Simonsen. 6 
II. B. 2.) Pigments referable to methyl-h-anthracene. 
a. Trihydroxides of methyl-2-anthraquinone. 
One pigment which is a trihydroxide of methyl-2-anthra- 
quinone is known to exist in plants, this is morindon. 
b. Hexhydroxides of methyl-2-anthraquinone. 
One pigment, pseudo purpurin, which is a hexhydroxide of 
methyl-2-anthraquinone, is known to exist in plants. 
1 Pharm. Post., 37, p. 233, Arch. Pharm., 245, p. 150. 
3 Arch. Pharm., 245, p. 141. 
3 Arch. Pharm., 243, p. 443. 
* Schweiz. Wochenschs. Pharm., 1904. Nr. 40. 
5 Arch. Pharm., 247, p. 413. 
"Jr. Chem. Soc., 95, p. 1085. 
7 Rupe. Natuerliche Farbstoffe, 2, p. 143. 
