212 TANNINS 



1. Turkish or Aleppo galls, produced by the gall wasp 

 Cynips gallcB, which lays its eggs in the buds of Quercus in- 



fectoria. These contain from 50 to 60 per cent of gallotannic 

 acid. 



2. Chinese galls, produced by the burrowing of Aphis 

 chinensis in the leaf-stalks of young twigs of Rhus semialata. 

 These galls may contain up to 70 per cent of gallotannic acid. 



Gallotannic also occurs in sumach {Rhus Coriaria), in tea 

 and in many other plants. 



Until recently there was some difference of opinion as to 

 whether this tannin occurs in the plant combined with 

 glucose in the form of a glucoside, or whether the sugar which 

 is frequently found associated with it is merely an impurity.* 



E. Fischer and Freudenberg.f on reinvestigating the ques- 

 tion, have found that " tannin," even after repeated careful 

 purification, yielded about 7 to 8 per cent of glucose on hy- 

 drolysis with sulphuric acid ; from this it is concluded that 

 "tannin" or gallotannic acid is in reality a compound of five 

 molecules of digallic acid with one molecule of glucose, in 

 which the five hydroxyl groups of the sugar are esterified by 

 five imolecules of acid. Such a compound would be a penta- 

 digalloyl glucose of the formula — 



CflH,06[C6H2(OH)3CO.O.C6H2(OH),.CO]5 or CjeH^Ae 



The authors have, moreover, been able to synthesize a substance 

 having a similar structure by esterifying glucose by means of 

 tricarbomethoxy-galloyl chloride — 



QiHiaOg+sCeHaCOCOOCRgX^COCl = C8H,Oe[C6H2(OCOOCH3)3CO]5+ 5HCI 



and saponifying the resulting compound with caustic soda: — 



C6H706[CbH2(OCOOCH3)3CO], -> C6H.Og[C6H,(OH)3CO], 



This pentagalloyl glucose closely resembles "tannin" as 

 regards optical activity, feebly acid character, solubility, taste, 

 colour with iron salts and ability to precipitate gelatine or 

 alkaloids. This furnishes a striking confirmation of the cor- 

 rectness of Fischer's suggested constitution of "tannin," and it 



* Cf. Strecker : " Annalen," 1852, 81, 248 ; 1854, 90, 328 ; Pottevin : " Compt. 

 rend.," 1901, 132, 704. 



+ Fischer and Freudenberg : "Ber. deut. chem. Gesells.," 1912, 45, 915 and 

 2709. 



