UNSATURATED COMPOUNDS 9 



When, however, the graphic formulae of the corresponding 

 members of the second or Olefine series are written, it is found 

 that if the tetravalency of carbon is maintained, there are not 

 enough hydrogen atoms to satisfy all these valencies, and, in 

 order not to leave any unsatisfied, the remaining valencies 

 must be united to each other, thereby joining two carbon 

 atoms to each other by more than one bond : — 



H H H H H 



C = C H — C — C = C — H 



Ethylene C2H4 Propylene CjHg 



In the next series of hydrocarbons, the acetylenes, by the 

 loss of two more hydrogen atoms, the process has been carried 

 a step farther, with the result that two carbon atoms are united 

 by a triple bond : — 



H 

 HC=CH H— C — C=CH 



I 



H 

 Acetylene C^Hj Allylene C3H4 



All such substances containing two carbon atoms united 

 together by more than one bond are said to be unsaturated, 

 and are able to form additive compounds with many substances, 

 notably the halogens. 



Thus, while the saturated hydrocarbon will only react with 

 chlorine or bromine by the replacement of one atom of hydro- 

 gen for each atom of halogen introduced into the molecule, 



C^Hg + Brj = CaHgBr + HBr 

 Ethyl bromide 



an unsaturated compound, such as ethylene, will add on the 

 halogen directly — 



C2H, + Br^ = CaH.Bra 

 Ethylene 

 dibromide 



the resulting additive compound being saturated. 



It will thus be seen that it requires two atoms of bromine 

 to saturate an unsaturated compound containing one double 

 bond, and similarly it requires four atoms of halogen to satu- 



