ROBERT M. MUIR 



University of Iowa 



CORWIN HANSCH 



Pomona College 



Chemical Structure and Growth- Activity 

 of Substituted Benzoic Acids 



Since the discovery by Zimmerman and Hitchcock in 1942 (12) that 

 substituted benzoic acids would induce growth responses in plants, 

 many studies have been made of the effects of these compounds in a 

 wide variety of plant responses. In these investigations the usual find- 

 ing is that unless the benzoic acid has a halogen substituent in one 

 or both ortho positions it is inactive except in the case of a few com- 

 pounds with methyl substituents in the ortho position (7, 8, 9, 11). The 

 activating effect of an electronegative substituent in the ortho posi- 

 tion may be explained by the electronic theory of organic reactions in 

 that the low electron density of the benzene ring would promote dis- 

 placement of the electronegative radical in reaction with a nucleo- 

 philic substance. This reaction may be represented for 2,8,6-trichloro- 

 benzoic acid which Bentley (1) found to be very active in promoting 

 cell elongation as follows: 



COOH COOH 



Ck 1 /CI CK J\ /X 



+ ci- 



ci ^/ ^ci 



In this interpretation the activity of the benzoic acid structure is 

 dependent upon a suitable electron density at the ortho position. Re- 

 cently Fukui et al. (2) have calculated the pi-electron distribution 

 for various benzoic acid derivatives by the molecular-orbital method. 

 The frontier electron density in the lowest vacant orbital of the 

 ground state is calculated as the approximate superdelocalizability 

 (S/[^']) for a reaction with a nucleophilic reagent. Some of these values 



[249] 



