250 



R. M. Muir and C. Hansch 



Table 1 . Chemical reactivity and auxin activity indices 

 of substituted benzoic acids. 



* Approximate superdelocalizability at ortho position for reaction with a nucleo- 



philic reagent [After Fukui et al. (2)]. 

 t Molar concn. of indole-3-acetic acid inducing an elongation of 0.15 mm. 



Molar concn. of growth regulator inducing an elongation of 0.15 mm. 



are given in Table 1 together with oin- vahies for the activity of the 

 various benzoic acids in promoting elongation at low concentrations 

 compared with indole-3-acetic acid at 100. 



Compoimds with S/*-"^'^ values less than 0.6693 at the ortJio posi- 

 tion do not promote elongation while those with a value of 0.6693 

 or greater do except for some such as those substituted in the para 

 position. The inactivating effect of substitution in the para position 

 has been recognized for a long time. Veldstra (10) has suggested that 

 since 2,3,6-trichloro-4-fluorobenzoic acid is active, the inactivation by 

 substitution at the para position is due to the size of the substituent 

 and the small size of fluorine permits activity. Since the S/<^'' values 

 for 2,4-dichloro- and 2,4,6-trichloro- benzoic acids are high but the 

 compounds are inactive, some steric hindrance of the chlorine atom 

 at the para position must exist. However, fluorine in the para posi- 

 tion may also result in an inactive compound, for we have found 

 2-chloro-4-fluorobenzoic acid (IX) to be completely inactive in pro- 

 moting elongation. 



In general the introduction of fluorine into the benzoic acid struc- 

 ture has an inactivating effect as far as the promotion of elongation 

 is concerned. We have examined a number of fluorine-substituted ben- 

 zoic adds and all except 2-fluoro-6-chlorobenzoic acid have been in- 

 active. Of ]:)nrti( ular significance is the effect of replacing the chlorine 



