138 Biological Chemistry. 



When the di-substitution products are considered, certain 

 isomerides are possible. Starting from a mono-substitution 

 product (for example, monobrom benzene) 



(omitting in the formula the symbol for hydrogen and 

 numbering the carbon atoms in the way indicated), it is 

 conceivable that the following derivatives are possible if 

 a second hydrogen atom is substituted (for example, to 

 form a dibroino benzene, C 6 H 4 Br 2 ) 



X X X X X 



(I) (2) X(3) (4) (5) 



At first sight it would seem as if five di-substitution 

 products were possible. Suppose, now, that the figure 

 representing No. (4) is constructed from sticks, instead of 

 being drawn, and is then regarded from a certain point. 

 Let the figure then be turned through 180 about an axis 

 joining carbon atoms 1 and 4. If looked at again a 

 figure identical with No. (2) will be observed. In other 

 words, with the formulae as above, the relative arrange- 

 ments of the atoms in Nos. (2) and (4) are identical. 

 Substances represented by these formulae should also be 

 identical and not isomerides. From the same considera- 

 tions, substances denoted by the formulae (1) and (5) are 

 identical. It therefore follows, if the suggested benzene 

 formula is sufficient, that three, and only three, di-sub- 

 stitution products of benzene are capable of existence. 

 Experience, drawn from the preparation of an immense 



