258 R. M. Muir and C. Hansch 



siderable steric hindrance. Usually, in these circumstances the re- 

 placement rate becomes very small. 



Dr. Muir: The selection of the ortho position for the attack as 

 shown in the reaction of 2,3,6-trichlorobenzoic acid is determined by 

 the S/<^'> values. AVhere there is only one halogen-substituted position, 

 that one is it. In the cases where there are two ortho substituents, as 

 Fukui, Nagata, and Yonezawa have indicated, the one with the greater 

 S/^N> would be most likely to be displaced in the reaction. Their cal- 

 culations of superdelocalizability show that one may be of a different 

 value from the other, depending upon the substituents in the ring. 



Dr. Wain: We are not really concerned necessarily with reaction 

 at the set 2 or 6 position. We were under the impression from all of 

 your papers that it is the 2 or 6 position which was involved. But I 

 now take it that any other position will do, providing that it is suit- 

 ably activated. Is that right? 



Dr. Muir: Partially. Where there is steric hindrance, even though 

 the electronic value or density is favorable, the steric hindrance T\ill 

 make the molecule inactive. 



Dr. Ray: In relation to the idea that 4-ethyl-3-mercaptobenzoic acid 

 is reacting with the same site as do the auxins, I wonder if 

 there is any w^ay that you can explain why you get reversal of this 

 inhibition by some auxins but not by others? It seems that, from 

 the data you show, in the case of some auxins the inhibition was quite 

 independent of the auxin concentration. 



Dr. Muir: My explanation would be that in not all cases did we 

 have the appropriate range in which to demonstrate competition. In 

 some instances this was outside of the ranges in which the reversi- 

 bility is most easily shown. 



Dr. Wain: I would like to ask whether 4-ethyl-3-mercaptobenzoic 

 acid reacts chemically with any of these auxins. For instance, have 

 melting point curves been constructed to show whether you get com- 

 pound formation or a simple eutectic? 



Dr. Hansch: The answer is no. 



Dr. Wain: One further point. The pH is very important in rela- 

 tion to all thiol compounds which are readily oxidized to disulfides 

 under alkaline condition. The pH here was on the acid side, wasn't 

 it? 



Dr. Muir: It was controlled at 5.6. 



Dr. Hansch: The values for superdelocalizability which vou have 

 just seen and which Professor Fukui was so kind to fiunish us repre- 

 sent only a small number of those he has made. A more extensive list 

 has been published (Jour. Amer. Chem. Soc. 80: 2267. 1958). 



