A series of substituted aryltriazolinones, known to inhibit protoporph
yrinogen oxidase, were prepared and their structure-activity requireme
nts at positions 4 and 5 of the aromatic ring investigated. A QSAR equ
ation obtained for substituents at the 5 position identified the hydro
phobicity term pi and the Sterimol minimum width B-1 as the two parame
ters affecting in-vitro biological activity. Greenhouse pre-emergence
activity correlated with in-vitro activity and the hydrophobicity term
pi of the substituent at that position. It was found that the phenoxy
-4-oxyacetate group at aromatic position 5 was an outlier and had to b
e considered separately. SAR analysis of substituents at aromatic posi
tion 4 revealed that two different models were required to explain all
observed substituent effects. In the first model, where the 5 positio
n was occupied by hydrogen, the 4-chlorobenzyloxy group at aromatic po
sition 4 gave the best compound. The second model, where the 5 positio
n of the aromatic ring was occupied by a group other than hydrogen, re
sulted in a OSAR equation, previously derived, which links substituent
effects at position 4 with pi and with the electronic para inductive
term F-p. In this model the chloro group provides optimum biological a
ctivity. The need to separate the aryltriazolinone herbicides into sev
eral different classes in order to explain their substituent effects a
t aromatic positions 4 and 5 could be rationalized if more than one bi
nding conformation, within the same binding site, is possible.