Chemometric analysis of disubstituent effects on the 13 C chemical shifts of the carboxyl carbons (δ CO ) of benzoic acids. A comparative study of the substituent effects on the strength of benzoic acids in apolar aprotic media

The results of measurements of substituent induced chemical shifts of carboxyl carbons (δ CO ) of dichloro‐ and difluorobenzoic acids, including the monosubstituted ones with substituents at meta‐ and/or ortho‐ positions, in chloroform‐d and strengths of these acids (log K) in chlorobenzene show an anomalous reverse trend between δ CO and log K, while the electron density at carboxyl carbons should influence similarly both δ CO and log K. A detailed chemometric analysis of comparison of disubstituent effects between δ CO 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 δ CO . Further, steric factors play a significant role in determining δ CO 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, δ CO 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.