Structure of Simpler Organic Compounds. 143 



made from it, the number of isomerides obtainable from the 

 ortho-, meta-, and para-compounds differs. Take, for ex- 

 ample, an ortho-derivative, C 6 H 4 X 2 , and suppose this is 

 treated with nitric acid to yield a nitro-derivative. The 

 following are the possible formula : 



X X X X 



It will be observed here that the formulae I. and IV. are 

 identical, as also are II. and III. From the 0r/to-derivative, 

 therefore, only two nitro-derivatives can be obtained. 

 Take now the case of the metfa-compound. The following 

 are the possible formulae : 



X; X X X 



i n ra rv 



Of these only IV. and II. are identical. The meta-com- 

 pound can yield, therefore, three isomerides. In the case of 

 the para-compound, the following are the possible formulae 

 for the nitro-compound : 



X X X X 



N0 2 NO* 



