Substituted Benzoic Acids 255 



derivative is not as effective in inhibiting the elongation induced by 

 indole-3-acetic acid, even though being less polar it should penetrate 

 more rapidly to the reaction site. Thus the reaction at the thiol group 

 must be the primary element in the inhibition effect. The aromatic 

 thiol group itself, however, even in the meta position, gives rise to 

 only slight inhibitory effect for the benzoic acid structure. The in- 

 hibition by jbara-substituted benzoic acids is, in general, of the same 

 order for thiol, chlorine, and ethyl groups and only about one-fourth 

 or less of the inhibition brought about by EMBA. Thus the action 

 of EMBA appears to be quite specific and to depend on the effect of 

 the ethyl substituent on the thiol group. This effect is probably elec- 

 tronic in character, with the ethyl substituent releasing electrons to 

 the ring and rendering the thiol group S- with a greater potential 

 for reaction. 



On the basis of this study of the inhibitory action of 4-ethyl-3- 

 mercaptobenzoic acid on elongation, the following inferences may be 

 formed: (1) Since the degree of inhibition in interaction with the 

 various auxins is related to the activity of the auxins, the EMBA mole- 

 cule reacts at the substrate site where the auxins react and induce 

 growth. (2) Since the reaction involves the thiol group, it is possible 

 that the substrate site is a sulfhydryl, as has been suggested on the 

 basis of considerations involving only the auxin structure. 



SUMMARY 



The growth activity of substituted benzoic acids appears to be de- 

 pendent primarily upon electronic characteristics determined by the 

 substituents, but these electronic characteristics may be counteracted 

 in part by steric factors. 



ACKNOWLEDGMENT 



The authors express their profound gratitude to Professor Fukui 

 of the University of Tokyo for making available his calculations of 

 pi-electron distribtitions and thus making possible for the first time 

 the quantitative appraisal of the electronic factors in the activity of 

 growth regulators. 



LITERATURE CITED 



1. Bentley, J. A. Growth-regiilating effect of certain organic compounds. Nature. 

 165: 449. 1950. 



2. Fukui, K., Nagata, C, and Yonezawa, T. Electronic structure and auxin ac- 

 tivity of benzoic acid derivatives. Jour. Amer. Cliem. Soc. 80: 2267-2270. 1958. 



3. Hansch, C, Muir, R. M., and Metzenberg, R. L., Jr. Further evidence for a 

 chemical reaction between plant growth-regulators and a plant substrate. Plant 

 Physiol. 26: 812-821. 1951. 



