Carbon-13 NMR Spectra of Benzene Mono- and Di-Carboxylic Acids and Their Analytical Applications

Carbon-13 NMR spectra of benzoic, and phthalic acids were compared both in D2O with an excess of sodium hydroxide and in a mixture of DMSO-d6 and H2O (4:1). Tight ion pair forms of benzoate anion were considered. A simple additive rule for isomeric phthalic acids was employed to assign the spectral peaks. These carboxylic acids gave characteristic peaks and so can be distinguished from others in mixtures. Quantitative analyses of these carboxylic acids with calibration curves are also possible by a simple preparation of basic D2O solutions. An example of the use of these data is the Henkel process (cf. Fig. 1) for the synthesis of terephthalic acid.