CONSTITUTION OF BENZENE 97 



that the fourth valencies of the carbon atoms do not 

 saturate each other singly, but neutralize in their 

 entirety. 



2. Determination of Position in the Benzene Derivatives. 



After the constitution of benzene was so far settled as to 

 give an explanation of the existence of only one mono- 

 derivative, but of three isomeric diderivatives, the question 

 arose, which of the three possible constitutions i . 2 (ortho), 

 ] . 3 (meta), i . 4 (para) to attribute to any given dideriva- 

 tive. This is the problem known as absolute deter- 

 mination of position. Kekule considered the answer as 

 only partly possible, regarding experiment as limited to 

 determining whether two compounds belong to the same 

 series or not. E. g. which dichlorbenzene is to be grouped 

 with any one of the three dioxybenzenes say hydro- 

 quinone can be found from the conversion of hydroquinone 

 into dichlorbenzene by means of PC1 5 , 



taking care as far as possible to exclude an intramolecular 

 displacement (p. 88). This is accordingly a relative deter- 

 mination of position. 



The further problem of absolute determination is now to 

 settle whether these two compounds, or rather the whole 

 group they belong to, has the constitution i . 2 or another. 



Korner answered this question also by the method of 

 the number of isomeric derivatives. He remarked that 

 a diderivative, such as C G H 4 Cl 2 (i .2), leads on introduc- 

 tion of a third group, say NO 2 , to two possible formulae, 

 1.2.3 an< ^ i 2 . 4, as is shown by the diagrams : 



whilst 1.2.5 = 1.2.4 and 1.2.6 = 1.2.3. But C 6 H 4 C1 2 



G 



