92 MOLECULAR STRUCTURE 



To decide more closely, we may add the fact that 

 substituted benzenes are not capable of decomposition into 

 pairs of active isomers l . 



Kekule, in stating his view, relied empirically on the 

 available experimental data. Ladenburg 2 gave later the 

 strict proof, which will be repeated here. 



(a) Benzene gives only a single wwnosubstituted deriva- 

 tive. Let us distinguish the six hydrogen atoms in the 

 following way : 



C 6 H a H b H c H<fH 6 H/, 



and regard phenol (C G H 5 OH) as 



& H c H d H H. 



From phenol maybe derived abenzoic acid, C 6 H 5 COOH, 

 by replacing the hydroxyl group with chlorine (by means 

 of PC1-), the chlorine with methyl (by means of CH 3 I and 

 Na), the methyl with carboxyl (by oxidation) ; this benzoic 

 acid therefore has the COOH group in the a position. 

 The three oxy benzoic acids, C ( .H 4 OHCOOH, derived from 

 this acid, i. e. salicylic and meta- and para-oxybenzoic acids, 

 differ on account of the OH group having replaced different 

 atoms of hydrogen, and not that in a, therefore say those 

 in by c, d, yielding : 



C 6 (COOH) a (OH) t H^ f , C 6 (COOH) ft H b (OH) c H,_ /5 

 C fl (COOH) a H 6 , e (OH) d H ei/ , 



If now the calcium salts are distilled with lime, the 

 carboxyl group is replaced by hydrogen, and we get the 

 phenols : 



C 6 H a (OH) 6 H^ C c H a . 6 (OH) c H,,_ /F C e H a , & , (OH) ll H 1 . 1/ , 



which Ladenburg found to be identical with the original 

 C C (OH),,H 6 _, 



Thus substitution of the hydrogen atoms a to d in 



1 Van 't Hoff, Atornlagerung im Eaume (2nd ed.), p. 92. 



2 Theorie der aromatischen Verbindutigen, 1876. 



