-^^ 



CORWIN HANSCH 



Pomona College 



ROBERT M. MUIR 



University of Iowa 



Electronic Effect of SubstLtuents on the Activity 



of Phenoxyacetic Acids 



In the two-point attachment mechanism of action which has been 

 developed (14, 26, 27), it has been postulated that the plant growth 

 regulators react with a protein substrate in plants by means of a car- 

 boxyl group and a ring position ortho to the attachment of the car- 

 boxyl group. Since the work of Osborne et al. (28, 29) on 2,6-di- 

 substituted phenoxyacetic acids and that of Wain's group (30) on 

 phenylacetic acids, it appears that, stereoelectronic conditions permit- 

 ting, other points on the ring may function in place of an ortho posi- 

 tion. Despite further evidence for the validity of this mechanism (6, 

 9, 10, 22, 23), considerable doubt still remains (37), and more evidence 

 is necessary for complete understanding. 



In attempting to correlate the effects of various substituents on 

 the aromatic rings of the different growth regulators, there are three 

 important factors connected with each substituent which must be 

 considered: electronic, steric, and H/L factor (hydrophilic/lipophilic). 

 It is relatively easy to compare molecules with respect to the first 

 and third factors; however, the steric effect of substituents on reac- 

 tivity is extremely difficult to assess since nothing is known of the ge- 

 ometry of the reaction site. Also, the nature of the side chains is 

 such that assuming attachment by the side chain first, the ring with 

 its attached groups could be presented in many ways to a second site. 

 The purpose of this paper is to consider some monosubstituted phen- 

 oxyacetic acids which are simple enough that comparison of the elec- 

 tronic effects of the various groups is not too complicated. 



First, a quick summary of the more important evidence favoring 

 reaction vs. simple adsorption. Most important is the large amount 

 of evidence indicating that the ring associated with growth regulators 



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