Chemometric analysis of disubstituent effects on the13C chemical shifts of the carboxyl carbons (δco) of benzoic acids. A comparative study of the substituent effects on the strength of benzoic acids in apolar aprotic media

Abstract

The results of measurements of substituent induced chemical shifts of carboxyl carbons (δco) of dichloroand difluorobenzoic acids, including the monosubstituted ones with substituents at meta- and/or orthopositions, in chloroform-d and strengths of these acids (log K) in chlorobenzene show an anomalous reverse trend between δco and log K, while the electron density at carboxyl carbons should influence similarly both δco and log K. A detailed chemometric analysis of comparison of disubstituent effects between δco and log K on the basis of Fujita-Nishioka's multiparameter approach and assumption of additivity of substituent effects shows a dominance of the localized π polarization mechanism relative to simple electrostatic effects upon ico- Further, steric factors play a significant role in determining δco whereas with respect to log K they were insignificant. The overall anomaly has been rationalized keeping in mind that, while log K is a gross measure of energy differences between the ionized and unionized forms of the acids, δco is a very sensitive probe for determining changes in electron density at the carboxyl carbon of the unionized acid. Copyright © 2007 John Wiley & Sons, Ltd.