Carbon-13 Nuclear Magnetic Resonance Studies of Substituted Benzoic Acids

Abstract

C-NMR spectra were measured for eighteen mono-substituted carboxylic acids in 10 mol% DMSO solutions and total electron densities for m- and p-substituted compounds were calculated by means of CNDO/2 MO method.
Values of plCa, of m- and p-substituted benzoic acids are respectively in parallel with the total electron densities at oxygen atom which hydrogen atom bonded to. As seen in Fig.2, there exist good linear relationships of negative slope between the chemical shifts of carboxyl carbons and pIC-values in DMSO for m- and p-substituted compounds, respectively. '3C chemical shifts at the ring carbon to which the carboxyl group attaches in p-substituted compounds show a good linear relationship of positive slope with piCa-values (Fig.3) The chemical shifts of carboxyl carbons in p-substituted compounds show a roughly linear relationship of positive slope with 7r-electron densities at the carbons, while in m-substituted compounds the relationship of negative slope is found (Fig.4).
These results suggested that in p-substituted carboxylic acids the changes of the physical values are influenced by the resonance effect between the substituent and the benzene ring, while in m-substituted compounds the inductive effect of the substituent seem predominant.