48 BACTERIOLOGICAL AND ENZYME CHEMISTRY 



unsaturated, we then get the series of the olefine hydrocarbons 

 of the general formula C 7l H 2/l , e.g. : 



Ethylene, C 2 H 4 , 

 Propylene, C 3 H 6 , 

 Butylene, C 4 H 8 , etc. 



The initial member of this series should of course be 

 methylene CH 2 , but all efforts to prepare it result in the 

 formation of ethylene or dimethylene. 



A further elimination of hydrogen results in the series 

 C n H 2n _ 2 , the initial member of which is acetylene, C 2 H 2 . 



The next great series resulting from Kekule's generalisa- 

 tion are the ring hydrocarbons, of which the best known 

 member is benzene. Kekule represented benzene by the 

 following formula : 



CH 



HC 



-CH 



CH 



The proof of the ring formation in benzene is a very 

 beautiful instance of the method of determining what is 

 known as the constitutional formula of an organic compound. 

 Inasmuch as the structure of benzene as indicated by Kekule's 

 formula is a symmetrical one, it should follow that whichever 

 of the hydrogen atoms is replaced by chlorine the same 

 monochlorbenzene should result. As a matter of fact, how- 

 ever monochlorbenzene is prepared, only one monochlor- 

 benzene has ever been obtained. It has indeed been possible 

 by a series of reactions, too complex to be here considered, 

 systematically to replace one atom of hydrogen after another 

 in benzene, and, as has been stated, whichever atom is re- 

 placed only one monochlorbenzene results. 



A formula such as Kekule's formula for benzene, which 



