Equilibria, extrathermodynamic approach

The next step in the development of the extrathermodynamic approach was to find a suitable expression for the equilibrium constant in terms of physicochemical and conformational (steric) properties of the drug. Use was made of a physicochemical interpretation of the dissociation constants of substituted aromatic acids in terms of the electronic properties of the substituents. This approach had already been introduced by Hammett in 1940 [14]. The Hammett equation relates the dissociation constant of a substituted benzoic acid (e.g. meta-chlorobenzoic acid) to the so-called Hammett electronic parameter a ... 

The Hansch-Fujita approach is also called the linear free-energy relationship (LFER) or extrathermodynamic approach, since most of the descriptors are derived from rate or equilibrium constants. 

Apply extrathermodynamic approach to determining the equilibrium properties. 

The correlation of biological activity with physicochemical properties is often termed an extrathermodynamic relationship. Because it follows in the line of Hammett and Taft equations that correlate thermodynamic and related parameters, it is appropriately labeled. The Hammett equation represents relationships between the logarithms of rate or equilibrium constants and substituent constants. The linearity of many of these relationships led to their designation as linear free energy relationships. The Hansch approach represents an extension of the Hammett equation from physical organic systems to a biological milieu. It should be noted that the simplicity... 

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