On the nature of resonance interactions in substituted benzenes. Part 3. A 13C nuclear magnetic resonance study of substituent effects in 4-substituted benzamides and methyl benzoates in dimethyl sulphoxide

Abstract

The substituent effect on the carbonyl-carbon chemical shift in 4-X-benzamides 2 and in 2,6-dimethyl-4-X-benzamides 2′ has been studied in (CD₃)₂SO. Comparison of the results with those obtained from the corresponding methyl 4-X-benzoates 1 and 1′ in the same solvent and their analysis by both the dual-substituent parameter (DSP) and Exner-Budesinsky′s methods suggest that in the less hindered benzamides 2 the carbamoyl-carbon chemical shift is not significantly affected by through-conjugation with the para-substituent, π-polarization appearing to be the prevalent resonance effect.