104 



I. M. KLOTZ 



Fig. 13. Molecular model of 

 H3C 



(CH3)2N- 



^-N=N-'^ 



-C 



/- 



o 



o- 



CH, 



carboxylic arrangement and A the ortho. You can see that there is very defi- 

 nitely a hindrance of access to this azo group in A. We reasoned then, as is 

 shown in the next illustration (Fig. 13), that if we could block access to this 

 azo group by a different method, keeping the carboxylic acid group out in the 

 para position, by putting in some methyl substituents, which I think you can 

 see at C (Fig. 13), these should block the access to the azo group, certainly as 

 much as the carboxyl did (Klotz, Burkhard, and Urciuhart, 1952). 



Then, if the azo group is the critical point, blocking access to it is what causes 

 the disappearance of the interaction and, therefore, in this particular molecule 

 (Fig. 13) we should also block the interaction. In other words, this should act 

 like an ortho carboxylic acid despite the fact that the — C02~ group is availa- 

 ble in the para position. On the other hand, if the group involved is not the azo 

 group but, perhaps, is the (CH3)2N — nitrogen, then the fact that we have 

 blocked access at the azo should make no difference as long as we have the car- 

 boxyl group back in its para position. Actually, this molecule with the methyl 

 groups in the ortho position acts exactly like the molecule which does not have 

 methyl groups in the ortho position. It acts like the para carboxylic acid and 



