PHENOLIC CONSTITUENTS 283 



GALLIC ACID. C6H,(OH)3COOH 

 OH 



A OH 

 HOOcl J OH 



Gallic acid 



Gallic acid was first prepared by Scheele in 1786 by leaving 

 an aqueous extract of gall nuts which contain tannin to stand 

 in a warm place, and from time to time removing the layer of 

 mould which formed on it ; the crystalline precipitate which 

 deposited from the solution was purified by recrystallization 

 from water. 



Within recent years this change has been studied anew 

 by Fernbach,* who isolated a tannin splitting enzyme, tan- 

 nase, from Penicillium, and also by Pottevin,t who isolated a 

 similar enzyme from the mould Aspergillus. 



This change, which may be represented by the equation — 

 C^HioOc, + HP = zCjHeOj 



may be effected more rapidly by boiling gallotannic acid with 

 dilute sulphuric acid. 



Gallic acid, besides occurring in gall nuts, both free and 

 in the form of its anhydride gallo-tannic acid, is also found free 

 in sumach, divi-divi, the fruits of Ccesalpinia coriaria, in the 

 leaves of Arctostaphylos Uva-ursi, and in tea and wine. 



Gallic acid crystallizes in silken needles, and melts at 

 220°, forming pyrogallol and evolving carbon dioxide ; it is 

 sparingly soluble in cold water, but dissolves readily in hot 

 water and in alkalis ; alkaline solutions, like those of pyro- 

 gallol, absorb oxygen from the air, becoming brown in colour ; 

 they also reduce metallic solutions of silver or gold and 

 Fehling's solution. 



Reactions. 



I. Gallic acid is precipitated from solution by lead acetate ; 

 on adding caustic potash a carmine-coloured precipitate is 

 formed, which dissolves in excess to a raspberry-red solution. 



* Fernbach : " Compt. rend.," 1900, 131, 1214. 

 f Pottevin : id., 1900, 131, 1215. 



