144 Biological Chemistry. 



It will be noticed that all these formulae are identical. 

 The para-derivative can yield, therefore, only one nitro- 

 derivative. In introducing another group, such as the 

 nitro group, into the benzene nucleus, the production of one 

 isomeride predominates over another, the positions of sub- 

 stitution following certain definite rules, some groups 

 having a tendency to occupy one particular position 

 relatively to the other groups. In general, however, 

 mixtures of isomerides are obtained, although the amount 

 of one isomeride will usually greatly predominate over that 

 of another. 



So far the determination of the constitution of only di- 

 substitution products has been discussed. The constitution 

 of more complex derivatives is often not easy. The above 

 illustrations will serve, however, to emphasize the reasons 

 for adopting the ring formula for benzene, and in showing 

 how, generally, the relationship of the various isomerides 

 to one another can be explained. It is of importance to 

 grasp the meaning of the isomerism of benzene derivatives 

 in view of the great differences of the isomerides in their 

 physiological action. 



GENERAL PROPERTIES OF AROMATIC COMPOUNDS. 



The benzene or aromatic derivatives containing the 

 ring nucleus differ from the so-called "chain" or "ali- 

 phatic " compounds, as they are sometimes called, in many 

 particulars. The chief of these may be summarized as 

 follows : 



I. The Difference in the Action of Halogens. Atten- 

 tion has been already called to this point, and it was then 

 mentioned that benzene, unlike its aliphatic isomeride 

 dipropargyl, does not readily form addition, but rather 

 substitution products. In the absence of sunlight and 

 at ordinary temperatures, substitution products, such as 



