BENZENE SERIES. AROMATIC COMPOUNDS. 559 







that all the hydrogen atoms are equivalent ; in other words, the mole- 

 cule C 6 H 6 is perfectly symmetrical. 



In view of these and many other facts the conclusion is that the 

 six carbon atoms in benzene are united into a cycle or ring, and that 

 each carbon atom is in combination with one hydrogen atom. This 

 view was first put forth by August KekulS in 1865. Graphically 

 the closed carbon chain and also benzene (usually referred to as 

 Kekul6's benzene hexagon) are represented thus : 



1 H 



c / 2 H \ C A C / H 



3 H/ \C^ \ 



i 



This formula for benzene accounts for the facts mentioned above. 

 Moreover, if two hydrogen atoms are replaced by substituting atoms 

 or radicals, three isomeric products are obtained. 



For instance, we know three different substances which have been 

 obtained by replacement of two hydrogen atoms in benzene by two 

 hydroxyl groups. This would indicate that it makes a difference, as 

 far as the properties of a compound are concerned, in which relative 

 position the introduced radicals stand to one another, and as a result 

 of a great deal of investigation it was found that the following 

 formulas represent the three relative positions which the two replac- 

 ing groups may occupy in a benzene molecule : 



OH OH OH 



H^ tf^ X)H H v ,C. 



A > -* 



\TT TT/ \C\4 > 



H H OH 



Ortho-position. Meta-position. Para-position. 



1:2. 1:3. 1:4- 



Designating the hydrogen atoms in benzene with numbers, thus : 



122456 



C 6 H H H H H H, the above 3 compounds show that in one case 

 the hydrogen atoms 1 and 2, in the second 1 and 3, in the third 1 

 and 4 have been replaced by OH. The compounds formed in this 

 way are distinguished as ortho-, ineta-, and para-compounds. 



