350 SMITH'S INTERMEDIATE CHEMISTRY 



The esters form the sweet-smelling constituents of plants. 

 Many are now produced synthetically as substitutes for natural 

 and fruit essences. 



A few additional classes of hydrocarbon derivatives will be 

 given later (p. 353). 



Unsaturated Hydrocarbons. In addition to the saturated 

 series of hydrocarbons, several other series are known in which 

 smaller proportions of hydrogen are present. Thus, ethylene 

 C 2 H 4 , to which illuminating gas largely owes the luminosity of its 

 flame, belongs to a series CH 2n , all the members of which contain 

 two atoms of hydrogen less than the corresponding compounds 

 of the first series. Again, acetylene C2H2 is the first member of a 

 series C n H 2n _ 2 , and benzene C 6 H 6 begins a series C n H 2n -6. These 

 are all unsaturated because the full valence of the carbon is not 

 in use, and these compounds, therefore, unite more or less readily 

 with hydrogen, chlorine, bromine, and concentrated sulphuric 

 acid. The hydrocarbons of all the series are mutually soluble, 

 but none of them dissolve in water. 



Members of the ethylene and acetylene series are found in 

 petroleum, and are formed also to some extent by decomposition 

 during the distillation. As oil containing them acquires dark- 

 colored products by chemical change, the oils are always refined 

 before being sold. They are agitated with concentrated sulphuric 

 acid, which unites with the unsaturated substances and, being 

 insoluble in the oil, collects in a layer below it. The oil is finally 

 washed free from the acid with dilute alkali and with water. 



Ethylene C 2 H 4 . The formation of one molecule of ethyiene 

 from two molecules of methane with elimination of two molecules 

 of hydrogen suggests its graphic formula: 



H H H H H H 



I I II II 



H-C-H + H-C-H->H-C-C-H or H-C = C-H 

 I I II 



H H 



