78 ORGANIC AGRICULTURi*.L CHEMISTRY 



the silver salt is reduced to metallic silver and a silver mirror 

 is formed. This reaction is similar to that of aldehyde upon 

 silver nitrate. 



Stereo-isomerism of Tartaric Acid. — The tartaric acid 

 obtained from grapes, as above described, is optically active, 

 dextro-rotatory. There is also present with it in grapes the 

 inactive form, known as racemic acid, which is also produced 

 when tartaric acid is prepared synthetically from succinic acid. 

 Racemic acid, like the inactive lactic acid, can be split into its 

 two optical components whereby both dextro tartaric acid and 

 levo tartaric acid are obtained. This, however, is not all, 

 for when tartaric acid is prepared synthetically, or when it is 

 heated with water to 165°, a fourth tartaric acid is obtained. 

 This tartaric acid is also inactive, but differs from the racemic 

 acid in that it cannot be split into its two opposite optical com- 

 ponents. It is known as meso-tartaric acid. 



We have then four stereo-isomeric tartaric acids, as follows : 



(i) Dextro tartaric acid in grapes, 



(2) Levo tartaric acid, 



(3) Racemic acid (inactive tartaric acid) found also in grapes, 

 can be split into dextro and levo acids, 



(4) Meso-tartaric acid (inactive), cannot be split into two 

 components. 



The explanation of these four forms of tartaric acid is analo- 

 gous to that of the three lactic acids. The difference between 

 tartaric acid and lactic or malic acids, or any other optically 

 active substance which we have considered thus far, is that 

 while lactic acid has one asymmetric carbon atom, tartaric 

 acid has two and these two are directly joined to each other. 



OH 



H-C-COOH 

 HO - C - COOH 



H 



Tartaric acid 



