Conformational analysis by NMR. 13C nuclear magnetic resonance spectra of saturated and unsaturated carboxylic acids and their corresponding esters and anhydrides

Abstract

The 13C NMR spectra of a large number of closely related saturated and unsaturated dicarboxylic acids and their anhydrides and derivatives have been analyzed to determine the structural and electronic effects which govern their chemical shifts. In monocarboxylic acids the conversion of an acid to its methyl ester causes an upfield shift of −5.9 ± 0.5 ppm for the carboxyl carbon while in diacids the effect is only −1.6 ± 0.6 ppm. The conversion of an aliphatic monocarboxylic acid (or its methyl ester) to the corresponding anhydride causes an upfield shift of −3.2 ± 0.6 ppm in the carboxyl carbon resonance. The closure of saturated diacids to their corresponding anhydrides has a variety of effects which can, in general, be rationalized in terms of the stereochemistry and structure of the diacids. The closure of an α,β-unsaturated diacid to its corresponding anhydride causes an upfield shift of −2.6 ± 1.1 ppm in the carboxyl carbon resonance and a downfield shift of 6.4 ± 1.2 ppm in the olefinic carbon resonance. These and other additivity effects and regularities in the 13C chemical shifts of these diacids and anhydrides are discussed in terms of electronic and steric effects.