The Role of Ortho-Substitution in the Synthetic Auxins

Abstract

The paper describes expts. to test recent theories on the relation between structure and activity in substances active as auxins. 3 derivatives of phenoxyacetic acid in which both positions ortho to the side chain are substituted were found active as auxins in the pea test. These are 2,6-dichloro-, 2,4,6-trichloro- and 2-methoxy-4,6-dichloro phenoxyacetic acids. The 3,5-dichloro-derivative, however, is quite inactive. In phenylacetic acid, while 2,4-dichloro-substitution increases activity, the corresponding 2,4-dinitro-substitution decreases it. 2,3,6-Trichloroben-zoic acid is highly active, more so than indoleacetic acid, but the activity of the aldehyde is very much lower and does not vary uniformly with concn. For this reason its slight activity is considered to be due to its conversion to the acid in pea stem tissue. Two other chlorinated benzoic acids were found inactive. The activities of several other compounds are reported. Several compounds, including some with and some without auxin activity, are shown to have synergistic action in the pea test. The results are discussed in the light of 2 proposed new rules for the relation between structure and activity. It is concluded that neither of these rules, namely, (a) that activity requires 2 free positions para to one another, and (b) that activity requires one free position ortho to the carboxyl, is consonant with all the facts, though some evidence, both here and in the literature, favors each of them. There is also some indication that benzoic acid derivatives may obey an opposite rule to that for phenoxyacetic and phenylacetic acid derivatives. Arguments in favor of the conclusion that a definite chemical reaction does take place at one or more positions on the ring are summarized. It is considered that this general conclusion is consonant with all the data. A detailed theory to account for the powerful effects of specific substitutions is proposed.