252 



R. M. Muir and C. Hansch 



the Sr'<^') value for ortho iodobenzoic acid is higher than 0.6693, the 

 compound is inactive, presumably because of the large size of iodine 

 and the consequent steric hindrance in displacement. If, however, the 

 Sj.'<'N') value is increased by additional iodine substitution in the 3 and 

 5 positions giving 2,3,5-triiodobenzoic acid, then the compound is very 

 active. 



Fukui et al. (2) have calculated approximate superdelocalizability 

 at the ortho positions for reaction of the benzoic acids with an electro- 

 philic reagent and with a radical reagent. No relation is found be- 

 tween these values and the auxin activity of the molecules. Thus our 



2693 



02893 



2 899 



2879 



HO, ^0 



0.5096 



HO^^O 



0.0 4 4 4 



00005 



0.00 07 



0.1429 



0.6 9 45 



Fig. 1. Approximate superdelocalizability values (Sr"^""^) as calculated by Fukui for 

 2-chloro-4-fluorobenzoic acid (left) and 2-fluoro-6-chlorobenzoic acid (right). 



suggestion of the nucleophilic character of the substrate with which 

 the growth regulators react (3, 6) has been substantiated by the the- 

 oretical chemistry of molecular orbitals. 



In the course of examining substituted benzoic acids for auxin 

 activity, a few have been found to inhibit the elongation of cells of 

 Avena coleoptiles at low concentrations. Of these, 4-ethyl-3-mercapto- 

 benzoic acid (EMBA) (4) has been the most interesting. It was found 

 to inhibit elongation by 40 per cent at lO'^M and to repress the effects 

 of growth regulators promoting elongation at concentrations as low 

 as lO'^M. The interaction of EMBA and these growth regulators is 

 shown in Table 2. The growth induced by the synthetic regulators 

 appears to be much more sensitive to the effect of EMBA than the 

 growth induced by the natural hormone, indole-3-acetic acid. In gen- 

 eral, the degree of the inhibitory action of EMBA appears to be in- 

 versely related to the relative activity of the regulators in promoting 

 elongation. Similar and lesser inhibition is demonstrated for the 



