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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
Author(s) -
Sen Gupta Susanta K.,
Shrivastava Ruchi
Publication year - 2007
Publication title -
magnetic resonance in chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.483
H-Index - 72
eISSN - 1097-458X
pISSN - 0749-1581
DOI - 10.1002/mrc.2098
Subject(s) - chemistry , substituent , steric effects , chemical shift , chlorobenzene , hammett equation , chloroform , electronic effect , computational chemistry , density functional theory , lipophilicity , stereochemistry , medicinal chemistry , organic chemistry , reaction rate constant , physics , quantum mechanics , kinetics , catalysis
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.