Abstract

The quantitative approach of Hansch and his co-workers to structure-activity relationships has been extended to optical isomers of a series of phenoxyproprionic acids acting as plant growth regulators. We have treated the physicochemical constants of the two chiral substituents as independent variables. Pfeiffer’s rule, which states that the isomeric activity ratio of a more active compound is higher than that of a less active one, is explained in terms of different structural requirements for the substituents as measured by π and van den Waals volume. A positive dependence on the aromatic ring substituent constant σ suggests a drug-receptor interaction of electronic nature in addition to hydrophobic interactions.