Chemometric analysis of disubstituent effects on the 13C 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

✍ Scribed by Susanta K. Sen Gupta; Ruchi Shrivastava

• Publisher: John Wiley and Sons
• Year: 2007
• Tongue: English
• Weight: 109 KB
• Volume: 45
• Category: Article
• ISSN: 0749-1581
• DOI: 10.1002/mrc.2098

✦ Synopsis

Abstract

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.