MIXED COMPOUNDS 



79 



It is considered possible then that the two halves of the 

 compound may be alike in asymmetry and optical power or 

 may be opposite. If the two halves of the compound are alike 

 they may both be dextro, giving dextro tartaric acid, or they may 

 both be leva, making levo tartaric acid. These two in equiva- 

 lent amounts would make inactive tartaric acid, which could 

 be split into its two components again ; that is, racemic acid. 

 If, however, the two halves are diferent, one dextro and the 

 other levo, an inactive compound would be formed which 

 cannot be split into a dextro and a levo form. This would be 

 meso-tartaric acid. 



Thus the existence of four stereo-isomeric tartaric acids is 

 possible in accordance with the van't Hoff - Le Bel theory. 

 These will be more readily comprehended if we examine the 

 four formulas as drawn on a plane surface, or better by an exam- 

 ination of models. 



OH 



H 



OH 



Racemic 



The stereo-isomerism of these acids is especially interesting 

 historically because it was with racemic acid salts that Pasteur 

 made the first separation of an optically inactive compound 

 into its two opposite optical components and also isolated the 

 two enantiomorphic (object and image) forms of crystals, thus 

 demonstrating the nature of inactive compounds of this type. 

 Such inactive compounds which can be split into optical isomers 

 are known now by the general name of racemic compounds. 



