252 BENZENE DERIVATIVES. 



exist. When two or more hydrogen-atoms are dis- 

 placed, the properties of the compounds are materially 

 dependent upon the relative position of the displacing 

 atoms or atomic groups to each other. Of every disub- 

 stitution-product (for instance, C 6 H 4 C1 2 , C 6 II 4 (OH 2 ), 

 C 6 H 4 (CO.OH) 2 ) three isomeric varieties can exist, viz. : 



(1) Those in which the hydrogen-atoms of two neigh- 

 boring carbon-atoms are displaced (1 : 2 or 1 : 6, 2 : 3, 3 : 4, 

 etc.). These compounds have been designated by means 

 of the prefix ortho ; 



(2) Those in which the hydrogen-atoms of two car- 

 bon-atoms are displaced, which are separated from each 

 other by one group CH (1 : 3 or 1 : 5, 2 : 4, etc.). Meta- 

 compounds ; 



(3) Those in which the hydrogen-atoms of two car- 

 bon-atoms are displaced, which are separated by two 

 groups CH from each other (1 : 4, 2 : 5 or 3 : 6). Para- 

 compounds. 



A similar method of consideration shows that by 

 the displacement of three hydrogen-atoms by one and 

 the same kind of atoms or atomic groups, also three 

 isomeric compounds (1:2:3, 1:2:4 and 1:2: 5) can 

 be formed, and that the number of the possible cases of 

 isomerism becomes much greater when the three atoms 

 or atomic groups are unlike. 



In the homologues of benzene, cases of isomerism may 

 also be caused by the substitution taking place in the 

 group CH 3 , and not in the benzene-residue. From 

 methylbenzene (toluene) C 6 IF.CH 3 are derived thus the 

 two classes of isomeric compounds: 



C 6 H 4 C1.CH 3 . . . C 6 H 5 .CH 2 C1 



C 6 H 4 | 



... C 6 H 5 .CH 2 .OH 



C 6 H 4 Qg 3 CH 5 .CH 2 .CO.OH 

 C 6 H 4 j CH 3 ' ' ' C6H5 - CH2 - CH3 - 



