Hammett Constants An Electronic Parameter

Hammett defined a parameter, o-R, as the logarithm of the ratio of KR to Ku, the equilibrium of unsubstituted benzoic acid (Equation 12.9). 

All equilibrium constants are defined relative to benzoic acid substituted with the simplest R-group, a hydrogen (R = H). Equation 12.9 can be rewritten as a difference in log K values. Substitution of Equation 12.3 shows that crR is a measure of standard free energy change caused by the R-group (Equations 12.10 and 12.11). Because o-R is a logarithm (Equation 12.9), o-R is unitless. 

SCHEME 12.2 Resonance and inductive effects in substituted benzoic acids 

An effective QSAR parameter should correlate only to its specific property, be it electronics, lipophilicity, sterics, or something else, a- Values for R-groups in the meta and para positions have been found to account for only electronic effects. Substitution at the ortho position, however, is another story. Because ortho substituents are physically so close to the carboxylic acid, ortho R-groups affect the acid-base equilibrium through both electronic and steric effects. Therefore, the ortho tr-value ( r0) is not useful as an electronic parameter for QSAR studies. Despite their limitations, Hammett constants are by far the most commonly encountered electronic parameter in Hansch analysis. 

---