In the late 1930s, the Hammett equation marked a breakthrough in the understanding of organic chemistry. It describes rate and equilibrium constants of the reactions of aromatic acids, phenols and anilines, as well as other compounds, in a quantitative manner, by using reaction and substituent parameters, о and a In the same manner, the lipophilicity parameter л, derived from /i-octanol/water partition coefficients by Corwin Hansch, led to a breakthrough in quantitative structure-activity relationships (QSAR) in biology. Like о, л also is an additive constitutive molecular property. Their combination, later also with molar refractivity values MR, Taft's steric Es values or the Verloop steric parameters, allowed the derivation of quantitative models for many biological in vitro activity values. Nonlinear lipophilicity models describe in vivo biological activities, where substance transport through membranes and distribution within the biological systems play an important role.

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