Chemometric analysis of disubstituent effects on the ¹³C 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

Abstract

The results of measurements of substituent induced chemical shifts of carboxyl carbons (deltaCO) 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 deltaCO and log K, while the electron density at carboxyl carbons should influence similarly both deltaCO and log K. A detailed chemometric analysis of comparison of disubstituent effects between deltaCO 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 pi-polarization mechanism relative to simple electrostatic effects upon deltaCO. Further, steric factors play a significant role in determining deltaCO 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, deltaCO is a very sensitive probe for determining changes in electron density at the carboxyl carbon of the unionized acid.

Effect of ortho substituents on carbonyl carbon 13C NMR chemical shifts in substituted phenyl benzoates

“…The influence of the ortho effect on the 13 C NMR chemical shift of the side‐chain functional group was studied for substituted methyl benzoates,17, 20, 21 ethyl benzoates,26 benzoic acids,21, 23–25 thiocyanatobenzenes,27 arylacetamides,28–30 acetonitriles,31 acetophenones, and benzaldehydes 32–36. Guy27 obtained excellent correlations for the 13 C NMR chemical shift in ortho ‐substituted thiocyanatobenzenes with the inductive, resonance, and the steric substituent parameters.…”

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

“…Meta-Substituted Benzoic Acids in Apolar Aprotic Solvents d co in benzoic acids is a measure of p z electron population on their carboxyl carbon whereas the acids' reactivity/acidity, which shows normal substituent effects, depends on the total electron population at their carboxyl carbon [8,12,18,21,25,28]. To probe how similar is the influence of substituent parameters on d co and acid's reactivity, d co was correlated to a reactivity parameter of the acids.…”

“…A polarization mechanism has been put forward to explain the anomalous effect with good qualitative success [8,12,18,19,22,25,27,28]. To develop insights into the origin of the anomalous effect of a substituent on d co , occupancies of natural atomic orbitals at the carboxyl and ring carbons of a set of 10 meta-substituted benzoic acids have been calculated at the density functional theory level using the B3LYP function with split valance 6-311G ??…”

“…An interesting finding is the anomalous (reverse) substituent effect on the chemical shift at the a-carbon ðd C a Þ of an unsaturated side chain (containing a C=C, C:C, C=N, C:N, or C=O group) of an aromatic compound: an electron-withdrawing substituent causes an upfield shift at the a-carbon [8,10,12,[16][17][18][19][20][21][22][23][24][25] in contradiction to the expected decrease in the electron population there.…”

“…Considerable progress has been made towards understanding the physical basis of these shifts [8][9][10][11][12][13][14][15][16], their dependences on substituent effects [8,10,12,[16][17][18][19][20][21][22][23][24][25][26][27][28], and their correlations with the substrate reactivity [22,[25][26][27][28]. An interesting finding is the anomalous (reverse) substituent effect on the chemical shift at the a-carbon ðd C a Þ of an unsaturated side chain (containing a C=C, C:C, C=N, C:N, or C=O group) of an aromatic compound: an electron-withdrawing substituent causes an upfield shift at the a-carbon [8,10,12,[16][17][18][19][20][21][22][23][24][25] in contradiction to the expected decrease in the electron population there.…”