Influence of Aprotic Solvents on the Transmission of Anomalous Substituent Effects on 13C NMR Chemical Shifts at the Carboxyl Carbon (δco) in Meta-Substituted Benzoic Acids: A Strong Evidence for π-Polarization Mechanism

Abstract

Anomalous (reverse) substituent-induced 13 C nuclear magnetic resonance chemical shifts at the carboxyl carbon (d co ) in meta-substituted benzoic acids have been studied for 11 substituents having varying electronic effects in 4 aprotic (nonhydroxylic) solvents of varying polarity by employing different dual substituent parameter models. The regression results for apolar aprotic solvents provide a strong evidence for through space p-polarization mode of transmission of reverse metasubstituent effects on the carboxyl carbon in benzoic acids. The results for dipolar aprotic solvents indicate significant specific solvation of p-polarized forms of the acids. The study showed further that an apolar aprotic solvent has a distinct preference over a dipolar aprotic one for investigating intrinsic substituent effects on chemical shifts in aromatic molecules.

Proton transfer reactions in apolar aprotic solvents

“…The results of solvent effects on δ co of substituted benzoic acids provide a strong evidence for the through‐space π‐polarization mode of transmission of reverse substituent effects on the carboxyl carbon in benzoic acids. The results also show that an apolar aprotic solvent is a distinct preference over a dipolar aprotic one for investigating intrinsic effects of substituents on 13 C chemical shifts at the side chain or ring carbon centers (δ c ) in benzoic acids and other aromatic molecules . The study has shown, in particular, that the transmission of the anomalous (reverse) effect of an m ‐substituent on δ co in benzoic acid molecule can be successfully interpreted by a 5.5:−2.5:1 combination of the localized, extended, and resonance‐induced π‐polarization mechanisms …”

“…This reversal in the trend of substituent effects, particularly electronic ones, on δ co and acidity parameters (log K /log k +1 ) of benzoic acids reflects significant differences in the transmission modes of substituents' electronic effects. These authors obtained the following statistically well‐significant correlations for substituent effects on δ co in benzoic acids: for m ‐methyl‐substituents, methoxy‐substituents, phenoxy‐substituents, fluoro‐substituents, chloro‐substituents, bromo‐substituents, iodo‐substituents, benzoyl‐substituents, trifluoromethyl‐substituents, nitro‐substituents, and hydrogen substituents following Taft's dual‐substituent parameter model with substituent coefficients from Charton's compilation; …”

“…Sen Gupta et al . also included DMSO‐ d 6 and acetone‐ d 6 in their study and found that the reverse effect in these solvents is significantly less pronounced as compared with benzene‐ d 6 or chloroform‐ d . This reversal in the trend of substituent effects, particularly electronic ones, on δ co and acidity parameters (log K /log k +1 ) of benzoic acids reflects significant differences in the transmission modes of substituents' electronic effects.…”

“…An apolar aprotic solvent is naturally the solvent of choice for investigating intrinsic substituent effects on both acidity constants (log K /log k 1 ) and δ co of benzoic acid, the standard model for studying substituent effects. Sen Gupta et al . probed in detail substituent effects on δ co of benzoic acid in benzene‐ d 6 and chloroform‐ d , employing selected o ‐substituted, m ‐substituted, and di‐substituted benzoic acids, which would offer minimum complications from intramolecular effects at a low enough concentration to avoid intermolecular effects, and noted an anomalously reverse trend in substituent effects on δ co (benzene/chloroform) and log k +1 (toluene)/log K (chlorobenzene) for each class of acids.…”