304 TANNINS 



DEPSIDES. 



The term depside (derived from the Greek word Seificiv = 

 to tan) was suggested by Fischer to designate chain compounds 

 analogous to the peptides produced by Unking together the 

 carboxyl group of one phenoHc acid with the hydroxyl group 

 of a similar one such as — 



HO— ( \— COOH + HO— < >— COOH 



■ >HO<^ \— CO.O— <^ J>COOH 



According to the number of constituent groups so linked 

 together, the resulting products were termed di-, tri-, etc., 

 depsides. Digallic acid according to this nomenclature would 

 be a didepside. Although digallic acid precipitates gelatine 

 from solution and thus justifies the term depside, many other 

 depsides are known which do not possess this property. 



Naturally occurring depsides are found chiefly among the 

 lichens where they are represented by the lichen acids, lecanoric 

 and evernic acids. 



Lecanoric Acid. 



This substance is a didepside of orsellinic acid — 



COOH 



CH3— r \— OH 



and has the constitution represented by the formula- 



CH3 CH3 



HO— <' \_CO— O— <^ >— COOH 



\ \ 



OH OH 



Lecanoric acid forms colourless crystals which melt at 

 166° ; it gives a purple colour with an alcoholic solution of 

 ferric chloride, and with dilute bleaching powder a blood-red 

 colour. 



