298 TANNINS 



Stahelen and Hoffstetter,* in 1844, to a red-brown substance 

 obtained by them by adding water to an alcoholic extract 

 of bark which had previously been extracted with ether to 

 remove fats or waxes. It has since been shown that aqueous 

 extracts of oak bark, deposit from solution a substance known 

 as oak-red or phlobaphene, and that this substance is more 

 rapidly produced by warming concentrated solutions of tannin 

 with sulphuric acid. 



Inasmuch as phlobaphenes are produced by any process 

 which tends to remove water, such as heating tannins to a high 

 temperature or prolonged boiling or heating under pressure, 

 they are regarded as anhydrides of the tannins ; besides being 

 thus produced artificially, they occur also in nature side by 

 side with the tannins from which they can be produced. 



They are red-coloured substances and are practically in- 

 soluble in water though they dissolve in solutions containing 

 tannic acid ; also they dissolve in alcohol and in alkaline 

 solutions. 



Practically nothing is known concerning the chemistry of 

 the phlobaphenes. The term is not confined only to the 

 products artificially produced by acid hydrolysis and oxida- 

 tion, but is also applied by Freudenberg to the naturally 

 occurring coloured decomposition products of the tannins 

 found in the plant. 



As stated above the formation of phlobaphenes by treat- 

 ment of a tannin with acid is characteristic of pyrocatechol 

 tannins (p. 288) in just the same way as ellagic acid is pro- 

 duced from pyrogallol tannins. 



A number of different phlobaphenes are known, such as 

 kino-red, catechu-red, oak-bark red, etc. 



RELATIONSHIP BETWEEN CATECHOL TANNINS AND 

 FLAVONOLS, Etc. 



Freudenberg has drawn attention to the close relationship 

 existing between the catechol tannins and the plant pigments 

 belonging to the group of anthoxanthins and anthocyanins. 

 It will be remembered that the potash fusion of anthocyanins 



* Stahelen and Hoffstetter : " Annalen," 1844, 51, 63. 



