104 MOLECULAR STRUCTURE 



CH a6 (N0 2 ) c H ci . Renewed introduction of the carboxy] 

 group, chlorination, and nitration yielded the fourth nitro- 

 methane, CH abo (N0 2 )^. The four nitromethanes thus 

 arrived at were found, as already remarked, to be identical. 



Further, we may briefly discuss the possible explanation 

 of isomerism not falling within the theory of constitution, 

 by differences in atomic movements. Berthelot l suggested 

 it for the case of tartaric and lactic acids. The attempt, 

 however, remained without success, as it was not possible 

 to formulate the view clearly. It should be observed in 

 this connexion that any attempt at explanation based on 

 atomic movements requires that the phenomenon explained 

 should vanish at the absolute zero, since there atomic and 

 molecular movements cease. Fall of temperature should 

 therefore lead to a gradual assimilation of the differences 

 between isomers, if difference of atomic movement is the 

 cause, and of this the isomers in question do not show the 

 smallest indication. 



The third alternative, ilsomerism due to different positions 

 of the atoms in space, has - shown itself capable of meeting 

 the case, and led to the development of stereochemistry. 

 We will bring the subject forward in the following 

 divisions : 



(1) The asymmetric carbon atom and separation into 

 optical antipodes. 



(2) Single carbon linkages, and compounds with more 

 than one asymmetric carbon atom. 



(3) Double linkages and rings. 



(4) Stereochemistry of other elements. 



i. The Asymmetric Carbon Atom and Separation into 

 Optical Antipodes. 



Stereochemistry is based on the principle (p. 91) of 

 determining configuration by the number of isomeric sub- 

 stitution products, supported and extended by application 



1 Bull, de la Soc. Chim. 1875. 



2 Walden, Journ. russ. Phys.-Chem. Ges. 30. 483. 



