132 ORGANIC AGRICULTURAL CHEMISTRY 



carbons which readily take up elements hy addition to the mole- 

 cule in such a manner as to indicate that in them the carbon 

 atoms are not fully saturated. Ethylene or ethene gas, the 

 simplest member of the series, has the composition C2H4 and 

 when treated with bromine it easily takes up two atoms as 



follows : 



C2H4+ Br2 — > C2H4Br2 



Ethylene Ethylene bromide 



The compound formed is known as ethylene bromide and also as 

 symmetrical di-brom-ethane. It is the same compound as is 

 obtained from ethane gas, C2H6, by substituting two bromine 

 atoms for two hydrogens. This latter reaction, as discussed 

 previously (p. 20), we represent as follows: 



H H H H 



H-C-C-H + 2Br2 -> Br-C-C-Br + 2HBr 



H H H H 



Ethane Di-brom-ethane (symmetrical) 



As this same product is obtained by adding two bromine atoms 

 to ethylene we are led to represent the reaction and the con- 

 stitution of ethylene as follows : 



H H H H 



II II 



C = C + Br2 -> Br- C - C - Br 



H H H H 



Ethylene Di-brom-ethane, or 



Ethylene bromide 



In ethylene we believe that the two carbons are only partly 

 saturated, each one having one free valence or bond, but that 

 this bond instead of remaining free is joined to the other carbon, 

 thereby making the two carbon atoms doubly joined, instead of 

 singly, as in the methane hydrocarbons. This double linkage 

 of two carbon atoms is not a point of strength, but rather of 

 weakness due to strain, and thus is easily broken and converted 

 into singly bound carbons while the broken bond of each carbon 



