Structure of Simpler Organic Compounds. 57 



dehydrating agent such as concentrated sulphuric acid or 

 phosphoric acid. This gas is known as ethylene, and 

 possesses the formula C 2 H 4 . It can also be obtained from 

 a monohalogen substitution product of ethane when it is 

 treated with a caustic alkali in alcoholic solution. These two 

 reactions may be represented by the following equations : 



C 2 H 6 - H 2 (combines with dehydrating agent) = C 2 H 4 



C 2 H 5 Br + KOH = KBr + H 2 + C 2 H 4 



Ethylene is a gas which liquefies at 10 under a pressure 

 of 60 atmospheres. Other members of the series are pre- 

 pared by an analogous method, higher "alcohols" (see 

 p. 65 and seq.) or halogen derivatives yielding hydro- 

 carbons with large numbers of carbon atoms. 



Now there is a marked chemical distinction between 

 the hydrocarbons of this series and those of the paraffin 

 group. They are, in the first place, far more readily 

 attacked by chemical reagents such as oxidizing mixtures, 

 and there is, in the second place, a very marked differ- 

 ence in their behaviour towards the halogens. Whereas 

 the paraffins react with these substances to form " sub- 

 stitution " products, the substances of the ethylene group 

 react with the formation of " addition " products. When, 

 for example, ethylene is treated with bromine, it reacts 

 to form ethylene dibromide or " Dutch liquid," a colour- 

 less crystalline substance with melting point 9*5 and 

 boiling point 131, and high specific gravity. The reaction 

 may be represented by the equation 



It is to be noticed that no hydrogen bromide is evolved 

 during this reaction. In view of the fact that an " addi- 

 tion " and not a " substitution " product is formed in 

 reactions of this type, ethylene is stated to be an "un- 

 vsaturated " compound in contradistinction to " saturated " 



