CONSTITUTION OF BENZENE 



95 



CH 



CH 



type C G (a b c d ef), i.e. with five hydrogen atoms replaced by 



different groups, C, HX 1 X 2 X 3 X 4 X 5 , 



have a symmetrical constitution ; for asymmetry implies 



a non-identical reflected image, so that we should expect 



two isomers differing as we shall see later by their 



opposite rotation of polarized light, their so-called optical 



activity. 



This condition is only satisfied if benzene has a plane of 

 symmetry, in which all the hydrogen atoms lie ; in other 

 words, all the hydrogen atoms are in one plane, with 

 respect to which the carbon atoms are symmetrically 

 placed. The suggested spatial arrangements of the 

 hydrogen atoms are thus 

 excluded. With Laden- 

 burg's prism formula, e.g., 

 even the diderivatives 

 C 6 H 4 X 2 would suffer de- 

 composition into optical 

 antipodes, since the two 

 symbols in Fig. 13 are not 

 symmetrical, and there- 

 fore not identical reflected 

 images ; the assumption 

 of the regular octahedron with the hydrogen atoms at the 

 corner points carries one a little further, but there again 

 the triderivatives would suffer decomposition. 



How the hydrogen atoms are arranged in the plane 

 follows partly from the first principle, that there is only 

 a single monoderivative, according to which the hydrogen 

 atoms must be similarly situated. Two possibilities occur : 



H 



and 



,H 



The second is the arrangement at the angles of a regular 

 hexagon ; the first at those of a hexagon with alternately 



