62 Biological Chemistry. 



Part II. The Halogen Derivatives of the Hydrocarbons. 



These substances have already been repeatedly referred 

 to in the discussion of the hydrocarbons, and can be pro- 

 duced from the latter by " substitution " in the case of the 

 saturated members of the class, and by " addition " from 

 the unsaturated. When these halogen derivatives are 



O 



treated with an alcoholic solution of sodium hydroxide an 

 unsaturated hydrocarbon is produced (see preparation of 

 ethylene, p. 57). If, however, a dilute aqueous solution 

 of the alkali is employed instead of the alcoholic solution, 

 the halogen is replaced by the elements oxygen and 

 hydrogen, and a characteristic substance belonging to the 

 class known as alcohols is produced. These reactions may 

 be illustrated by the following reactions of mono- 

 bromethane : 



CH 3 CH 2 



| + KOH (alcoholic) = KBr + H 2 + - - 

 CH 2 Br CH 2 



CH 3 CH 3 



| + KOH (aqueous) = KBr + | 



CH 2 Br CH 2 (OH) 



In the latter case, the halogen is replaced by a " hydroxyl " 

 group, the properties of which will be discussed later. 

 Conversely, if an alcohol is treated with a halogenating 

 reagent, the hydroxyl group can be replaced by a halogen. 

 Thus, if ethyl alcohol is treated with phosphorus penta- 

 chloride, or with dry hydrogen chloride in the presence of 

 a dehydrating agent, such as zinc chloride, it yields mono- 

 chlorethane. 



CH, CH 2 OH + PC1 5 = CH 3 CH 2 C1 + POC1 3 + HC1 

 CH 8 CH 2 OH + HC1 ;r CH 3 . CH 2 C1 + H 2 O 



If a dehydrating agent is not used in the latter case, and 

 the water formed is not removed from the sphere of action, 

 the reverse action may take place, and this is indicated in 



