ASYMMETRIC CARBON ATOM 107 



reflected images, that cannot be superposed on one 

 another. 



The similarity of the antipodes in other respects implies 

 the necessity of a separation, when a body with an asym- 

 metric carbon atom is prepared in the laboratory. For 

 starting with an inactive, symmetrical body, such as 



when a fourth different group R 4 is introduced in place of 

 one R 3 , the two R 3 groups, being symmetrically contained 

 in the molecule, are replaced with equal facility, and so an 

 inactive mixture of the two isomers, CR 1 R 2 R 3 R 4 , in equal 

 amounts is produced, in which the components are yet to 

 be separated. It is well known that in the organism, on 

 the other hand, the antipodes occur separately, e. g. tartaric 

 acid obtained from grapes is the active dextro-rotary 

 modification. 



The methods of separation available may be grouped 

 as follows : 



(a) Methods based on the phenomena of solution. 

 (a) So-called spontaneous separation. 

 (/3) Separation by means of active compounds. 



(6) Methods based on chemical behaviour, depending on 

 the action of enzymes and organisms. 



(a) Methods based on the Phenomena of Solution. 



(a) Spontaneous separation. Separation of the optical 

 antipodes is rendered difficult by their similarity of behaviour 

 in almost every respect, especially their equal solubility; 

 moreover, the isorners mostly unite to form a so-called 

 racemic body, as in the classical example of the racemic 

 acid precipitated on mixing concentrated solutions of the 

 two oppositely active tartaric acids. 



The fact discovered by Pasteur, that the substances do 

 not always appear together in this way, but that, e. g., from 

 the solution of the sodium-ammonium salt of racemic acid 

 laevo- and dextro-sodium-ammonium tartrates crystallize 



