20 ORGANIC AGRICULTURAL CHEMISTRY 



Di-halogen Ethanes. — The di-halogen ethanes are significant 

 as introducing to our consideration a new type of isomerism. 

 Analogous to di-chlor-methane and di-brom-methane, we have 

 the corresponding di-halogen ethanes : 



CH2CI2 C2H4CI2 C2H4Br2 



Di-chlor-methane Di-chlor-ethane Di-brom-ethane 



Isomerism of Di-substituted Ethanes. — Not only do we 

 know di-brom-ethane, but there are two isomeric compounds 

 of this same composition, C2H4Br2. We should mention the 

 fact that only one mono-substituted ethane of any kind is 

 known, and from this we gain the idea that all six of the hydrogen 

 atoms in ethane are alike. When, however, we consider the 

 entrance into the ethane molecule of two substituting elements 

 in place of two hydrogen atoms, we see at once, from our con- 

 stitutional formula of ethane, that two products are possible. 

 The fact as already stated is that two are known. By examin- 

 ing the formula for ethane, we see that it would be possible to 

 substitute two hydrogens united to the same carbon atom or two 

 hydrogens one of which is united to one carbon and the other 

 united to the second carbon. Expressing this idea by our struc- 

 tural formulas we have : 



H H H H H H 



H-C-C-H; H-C-C-Br; Br-C-C-Br 



II II II 



H H H Br H H 



Ethane Z>»-brom-ethanes 



This, then, explains the existence of two isomeric di-brom- 

 ethanes. From the fact that one structure represents a sym- 

 metrical compound while the other represents an unsymmetrical 

 one, the two compounds are known as symmetrical di-brom- 

 ethane and unsymmetrical di-brom-ethane. The symmetrical 

 di-brom-ethane is directly related to a new kind of hydrocarbon 

 called ethylene (C2H4), and the bromide is therefore known 

 as ethylene bromide. The unsymmetrical compound is also 

 known as ethylidene bromide. We shall mention ethylene again 



