268 Tcubner, Wittwer, and Shen 



ferent from those found for the substituted benzoic, phenoxyacetic, 

 and phenylacetic acids. In the benzoic acids it was originally con- 

 sidered that the para position must not be substituted (13, 28, 29, 36). 

 It has been shown, however, that with the highly active 2,3,6-tri- 

 chlorobenzoic acid further substitution of either chloro- or fluoro- at 

 the 4-position reduces, but does not abolish, activity (19, 30). This is 

 similar to the effects of para substitution of chloro- or methyl- groups 

 on the activity of several active A'^-phenylphthalamic acid derivatives. 

 Unfortimately the 2,3,4-trichloro derivative which would best illus- 

 trate this effect was not available for these tests. The high activity 

 obtained with 2,3-dichloro substitution in A'^-phenylphthalamic acid 

 is also similar to that found with phenylacetic acid and to a lesser 

 degree with benzoic acid (20). The resemblance between the benzoic 

 and phenylacetic acid auxins and the N-arylphthalamic acids, how- 

 ever, is dissimilar when di-ortho or 2,6-substitution is considered. 

 Normally, these derivatives are active both in the phenylacetic and 

 benzoic acid series (2, 19, 20, 28, 36). In contrast, di-ortho substitution 

 residts in inactive derivatives of N^-phenylphthalamic acid. Here again 

 it will be of considerable importance to establish if this is a total 

 effect or if a second ortJw substituent will merely reduce the activity 

 of the highly active Ar-2,3-dichlorophenylphthalamic acid. As yet, the 

 2,3,6-trichloro derivative is not available for study. The loss of ac- 

 tivity with di-ortho substitution is similar to that found for the sub- 

 stituted phenoxyacetic acid auxins (9, 14, 22) . However, this is an 

 effect which is subject to some qualifications (25, 27, 32). 



The benzoic acid and phenoxyacetic acid auxins have in common 

 the inactivity of their 3,5-disubstituted derivatives. Here again, how- 

 ever, the effect of di-meta substitution is not an absolute one since 

 slight activity is exhibited by 2,3,5-trichlorobenzoic acid (28), 2,3,5,6- 

 tetrachlorobenzoic acid (19) , and by the 2,3,6-trichloro-, the 2,3,5-tri- 

 methyl- and trichloro-, the 3,4,5-trichloro-, and the 2,3,4,5-tetrachloro- 

 phenoxyacetic acids (20, 27) . This is in distinct contrast to the high 

 activities of the 3,5-dichloro- and the 2,3,5- and 3,4,5-trichlorophenyl- 

 phthalamic acids (Table 3). It will be of interest to learn if the studies, 

 which Pybus et al. (20) refer to, on nuclear substitution in the phenyl- 

 acetic acids show similar structure-activity relationships. 



SUMMARY 



The activities of several chloro- and methyl- derivatives of N- 

 jjlienylphthalamic acid have been evaluated with respect to tomato 

 flower formation, parthenocarpic development of tomato ovaries, and 

 straight growth of Avena sections. 



Modifications in molecular structure had almost identical effects 



