8 FATS, OILS. AND WAXES 



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 



1 

 H 



Acetylene C^H^ 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 sub- 

 stances, 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 + Br^ = CjHsBr + HBr 

 Ethyl bromide 



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



halogen directly, 



C2H4 + Bra = CjH^Br, 

 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 

 saturate a compound containing two double bonds. In this 

 way it is shown that since the acids of the oleic, linolic, and 

 linolenic series require two, four, and six atoms of halogen 

 respectively for saturation, they must contain respectively one, 

 two, or three double bonds. 



A measure of the degree of unsaturation of a given acid 

 may accordingly be obtained by determining how much 

 bromine it will absorb ; as, however, the interaction with 

 bromine is liable to be violent, it is found more convenient 

 to employ iodine, which, in addition to being less violent in 

 its action than bromine, is also easier to handle. 



A description of the method employed in determining what 

 is known as the " iodine value " of fats is given below (p. 23). 



