Structure of Simpler Organic Compounds. 139 



number of benzene derivatives, has shown that this is really 

 the case. These are the (1, 2), (1, 3), and (1, 4) derivatives, 

 which are known respectively as the ortho-, meta-, and 

 para-derivatives, or, more shortly, the o-, m-, and ^-de- 

 rivatives 



ORTHO META X 



PARA 



[Example : o-dinitro benzene would be represented by 

 formula No. (1), in which, in place of X, the group NO 2 

 would be given. This substance could also be indicated 

 by the formula C 6 H 4 (N0 2 ) 2 (1, 2).] 



Of the tri-substitution products, when the three sub- 

 stituting groups are all the same, three isomerides should 

 be possible, namely 



XXX 



X 



1, 2, 3, or 1, 2, 4, or 1, 3, 5, or 



Adjacent Asymmetrical Symmetrical 

 derivative. derivative. derivative. 



A larger number of isomerides is possible when the three 

 substituting groups are not all the same. All these supposi- 

 tions, which are based on the acceptance of the hexagonal 

 ring formula for benzene, are borne out by ample experience. 



Determination of the Constitution of Benzene De- 

 rivatives. Accepting all these facts as a basis, the question 

 next arises : How is the constitution of any given benzene 

 derivative ascertained ? How is it possible, for example, 



