Contribution of Substituent Groups to the Extractability of Phenols and Benzoic Acids

The contribution of substituent groups to the extractability of phenols and benzoic acids was examined by using a substituent constant, π, derived from the distribution coefficients of nonionic substances. This constant is defined as π(X)=log K(D(X))-l8o9g K(D(H)), where K(D(X)) is the distribution coefficient of a derivative and log K(D(H)) is that of the parent compound. The substituent constants of the groups at the ortho-position of phenol were found to vary with a variation of the extraction solvent, possibly due to competition between the hydration and solvation of an organic solvent. From the K(D) values of the phenols and benzoic acids determined between chloroform and water, the substituent constants of halogeno, cyano, trifluoromethyl and nitro groups were calculated. In phenols, the π values of the ortho-substituting group (π(o)) were greater than those of the meta- and para-substituting group, π(m) and π(p), respectively, whereas in benzoic acids π(o) are smaller than π(m) and π(p). Such results can be explained by a masking effect and the overlapping effect: the masking effect increases the π(o) values, whereas the overlapping effect decreases the π(o) values. A method for predicting the total π values of di- and tri-substituted phenols and benzoic acids is proposed.