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

Abstract

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)—log K_D(H), where K_D(X) is the distribution coefficient of a derivative and log K_D(X) 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(X) 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.