Solvent sensitivity of ortho substituent effect on ¹³C NMR chemical shift of the carboxyl carbon (δ_co) in benzoic acid

Abstract

A reverse ortho effect is observed for the (13)C NMR chemical shifts of the carboxyl carbon (δ(co)) in benzoic acids measured in aprotic solvents of varying polarity. The ortho effect on δ(co) is best described by a combination of the reverse field and steric accelerating effects of the substituent in an 80: 20 pattern in apolar aprotic solvents and a 60: 40 pattern in dipolar aprotic ones. Interestingly, no good enough correlation was found between δ(co) and log k(1) of the acids measured in similar solvents. A critical analysis of the results clearly indicates the use of an apolar aprotic solvent and not a dipolar aprotic one as the solvent of choice for investigating intrinsic substituent effects on δ(c) in an aromatic system.

The unexpected roles of σ and π orbitals in electron donor and acceptor group effects on the 13C NMR chemical shifts in substituted benzenes

“…ortho -Positions show a more complex behavior than the meta and para analogs due to the proximity of the substituent. 17 – 19 For example, the nitro group causes an increased shielding of ortho -carbons instead of the expected de-shielding similar to that taking place at the para position. 1 , 11 …”

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 …”

“…The study has demonstrated further that the electronic effect of an m ‐substituent in benzoic acid on the chemical shift at more than two‐bond distant atoms is relayed by a combination of several non‐classical modes, through space‐localized (direct) π‐polarization and through bond extension of the combination being governed by the site of the atom in the molecule . For interpreting the transmission of the o ‐substituent effect in benzoic acid on the chemical shift of the carboxyl carbon, an additional classical mode, the steric effect mode, is taken into account . 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.…”

“…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.…”