Electronic Effect of Substituents on Phenoxyacetic Acids 



433 



character was clearly shown by the tact that 2,3,4,5-tetrachlorophe- 

 noxyacetic acid was very active. 



If simple lipophilic adsorption of the ring is not enough to ex- 

 plain its function, one might argue that adsorption of the ring 

 through its pi electrons to give a weak pi complex would help ra- 

 tionalize the situation. Considering the nature of the phenoxy, 



Table 1 . Activity of phenoxyacetic acids compared to indole-3-acetic acid. * 



* All of the data reported in this table have been previously reported (26) except 

 for 4-fluoro-, 4-acetyl-, and 3-trifluoromethylphenoxyacetic acids. 4-Hydrogenphe- 

 noxyacetic acid is simply phenoxyacetic acid. 



t Indole-3-acetic acid = 100. 



phenyl, naphthalene, and indole rings, adsorption to an electron-defi- 

 cient site would seem more reasonable than the assumption of an 

 electron-rich site. The relative activating effect of substituents in 

 the 4-substituted phenoxyacetic acid as shown in Table 1 is exactly 

 the opposite from what one might expect (F>Cl>Br>CH30>I) for 

 such a mechanism. More difficult to rationalize in terms of a non- 

 specific pi complex is the great difference in effect of given substi- 

 tuents at various points on the ring. Thus, for example, 2,4-dichloro- 

 phenoxyacetic is very active, while the 3,5-dichloro-analogue is inert. 

 As mentioned above, the difference in these two molecules cannot 

 be rationalized as being due to steric factors (35). Many other such 

 examples can easily be shown from data published on the phenoxy- 

 acetic acids. 



If nonspecific adsorptions involving van der Waals forces or pi 

 complex formation do not offer satisfactory solutions, adsorption or 



