The Substituent Constants of Hansch and Fujita

The first approach used to calculate partition coefficients from molecular structure was the simple adaptation of the Hammett equation to the oc-tanoUwater partitioning system as pioneered by Hansch and Fujita. Indeed, the Hammett equation (Eq. [9]) uses three quantities to predict kinetic (or equilibrium) constants of congeners (RX) (a) the reactivity of the parent com- 

By analogy with the Hammett equation, Hansch and Fujita proposed Eq. [10] to calculate lipophilicity  

the Hansch-Fujita approach allows one to calculate the partition coefficient of any substituted compound knowing the partition coefficient of the parent compound (RH) and the hydrophobic constants of substituents X, (Eq. [11])  

The simplicity of this method is illustrated with xylene (Eq. [12] experimental value, 3.20) and p-nitrotoluene (Eq. [13] experimental value, 2.37)57  

Although useful in simple cases (congeneric series), the Hansch-Fujita system suffers from several limitations, as amply discussed in the literature. 

---