RHUBARB 365 



The chief astringent constituent is gallic acid, in addition to which 

 a small quantity of tannin and (?) catechin is present. 



The chief laxative constituent is, according to Tutin and Clewer 

 (1911), an amorphous non-glue osidic resinous mixture. By hydrolysis 

 with dilute sulphuric acid this yields gallic acid, cinnamic acid, rhein, 

 emodin, aloe-emodin, emodin monomethyl ether, chrysophanic acid, 

 and a trihydroxydihydroanthracene. The gallic and cinnamic acids 

 are probably present as esters. 



In addition to the resin the drug contains rhein, emodin, aloe-emodin, 

 emodin monomethyl ether and chrysophanic acid in the free state and 

 also in the form of glucosides. These substances are all derivatives 

 of oxymethylanthraquinone and are 

 accompanied in the drug by rheinolic 

 acid. 



Further constituents are starch, 

 fat, dextrose, levulose, pectin, calcium 

 oxalate, &c. 



Chrysophanic acid, CH 3 .C 14 H 5 O 2 (OH) 2 , 

 is a dihydroxymethylanthraquinone, but 

 the positions of the groups are not known 

 with certainty. 



Emodin, CH 3 .Cj 4 H 4 O 2 (OH) 3 , is hydroxy- 

 chrysophanic acid and is therefore a 

 trihydroxymethvlanthraquinone. FIG. 191 -Chinese Rhubarb (Canton 



Emodin monomethyl ether, round) showing granular fracture. 



CH 3 .C 14 H 4 2 (OH) 2 O.CH 3 , 

 is the corresponding methoxychrysophanic 



acid ; it has been found in Rumex Ecklonianus, Meissner, and may be prepared 

 from emodin. 



Aloe-emodin, C 14 H 5 O 2 (OH) 2 .CH 2 .OH, is the primary alcohol (hydroxymethyl- 

 dihydroxyanthraquinone) corresponding to chrysophanic acid ; as its name 

 indicates, it is found in aloes. 



Rhein, C 14 H 5 2 (OH) 2 .CO 2 H, is the corresponding carboxylic acid, and may 

 be obtained by the oxidation of aloe-emodin. 



The total quantity of oxymethylanthraquinones, free and combined, present 

 in rhubarb has been estimated to vary from 2 to 4 per cent. Only chrysophanic 

 acid and aloe-emodin are laxative. The mixed glucosides of the oxymethyl- 

 anthraquinones crystallise readily together ; this crystalline mixture, termed 

 by Gilson rheopurgarin, is, however, destitute of purgative action. The sug- 

 gestion that has been made that the comparative medicinal value of rhubarb 

 may be determined by estimating the total oxymethylanthraquinones present 

 would appear, therefore, to be unacceptable. 



According to Gilson, the gallic acid is present as a glucoside (glucogallin), 

 and gallic acid, cinnamic acid, and rheosmin as another glucoside (tetrarin), 

 but this is improbable. 



The amount of calcium oxalate contained in rhubarb varies considerably, 

 averaging about 7-3 per cent. (Fluckiger). The ash is subject to a similar varia- 

 tion ; it has been found as low as 3'5, and as high as 43'27 ; usually good Chinese 



