THERMOCHEMICAL ASPECTS OF PARTITION - A STUDY ON THE NOVEL HYDROPHOBIC PARAMETERS OF MULTIPLY-SUBSTITUTED BENZOIC-ACIDS

The thermochemical aspects of partition have been measured for multipl y-substituted benzoic acids with the intention of disclosing the mode of change of the proposed novel hydrophobic parameters on multiple sub stitution in the phenyl ring. The substituent hydrophobic parameter (p i) has been separated into two terms of enthalpic and entropic nature (pi(H) and pi(S)). It is necessary to take into account the effect of solvation around the hydrogen-bond-participating groups, such as OH, O CH3, and NO2, besides the benzoic CO2H in interpreting the substituent dependency of pi, pi(H) and pi(S). Some Hammett sigma parameters, suc h as sigma(o)(-->OH) and sigma(o)(-->OCH3), which mean the ortho subst ituent effect to the functional OH and OCH3 groups, respectively, act in a compensating manner to pi(H) and pi(S), resulting in a vanishing effect on pi. These trends are in good conformity with the enthalpy-en tropy compensation rule in chemical thermodynamics.