60 BACTERTOLOaiCAL AND ENZYME CHEMISTRY 



was seen to be justified by the fact that only one monochlor- 

 benzene could be obtained. Many other consequences follow 

 from the ring formation, but the above is one of the simplest, 

 and suffices to distinguish benzene from another possible 

 arrangement of six carbon atoms and six hydrogen atoms to 

 form the molecule CgHe which might be conceived as follows, 

 the atoms of carbon forming a chain : — 



CHi^C— CHj— CH2— C=CH 



Such a substance does as a matter of fact exist, and is known 

 as dipropinyl; it differs, however, from benzene in that it forms 

 several monochlor derivatives according as chlorine is attached 

 to the first, second or third carbon atom from the end of the 

 chain. Moreover it will be seen that dipropinyl is an acetylene 

 hydrocarbon that readily combines with bromine, the bromine 

 being added to the compound, which then becomes saturated. 

 The first action of bromine upon benzene is one of substitution. 

 Two compounds such as benzene and dipropinyl, which 

 have the same number of atoms in the molecule but whose atoms 

 are differently arranged, are known as isomeric substances, and 

 the phenomenon is spoken of generally as isomerism. In order 

 to determine the arrangement of the atoms in the molecule 

 and thus to distinguish between isomeric substances, a syste- 

 matic study must be made of the typical reactions of such 

 substances. 



A simple case may be taken to illustrate the determination 

 of the constitutional formula in the case of two substances 

 having the molecular formula CjHjN. When these substances 

 are treated with potash one of them yields potassium acetate 

 and ammonia, while the other yields methylamine and 

 potassium formate. These reactions point clearly to the 

 conclusion that in the one case the two carbon atoms must 

 be closely connected, as they reappear together in the molecule 

 of acetic acid ; in the other case, one of them is separated from 



