UNSATURATED COMPOUNDS 9 



The relationship between the five series of acids, which 

 differ from each other successively by two atoms of hydrogen, 

 as shown by the formulae 



C n H 20 2 , CnH 2n _ 2 2 , C Q H 2n _ 4 2 , C n H 2n _ 6 2 , and C n H 2n - 8 O 2 



is similar to that subsisting between the three series of hydro- 

 carbons having the general formulae : 



CnH 2n +2 C n H 2n , C n H 2n _ 2 



The hydrocarbons of the first or Paraffin series are said to be 

 saturated, by which is meant that each of the four valencies of 

 their carbon atoms are fully satisfied, as shown by the follow- 

 ing graphic formulae : 



H H H H H 



H C C-H H C C C H 



Ethane C 2 H 6 Propane C 3 H 8 



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 



4 A i 



Ethylene C 2 H 4 Propylene C 3 H 8 



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 s CH 



J 



Acetylene C 2 H 2 Allylene C 3 H 4 



All such substances containing two carbon atoms united 

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



