¹³C nuclear magnetic resonance study of π-polarization in 3- and 4-substituted benzamides and N-chlorobenzamides

The substituent effect on the carbonyl carbon [C(α)] ¹³C chemical shift in 3- and 4-substituted benzamides and N-chlorobenzamides has been studied in (CD₃)₂SO. Chlorine shields the carbonyl carbon. A cross correlation was carried out for C(α) and the carbonyl carbon in N-chlorobenzamides is less sensitive to substituents than in the benzamides. The dual substituent parameter method indicated a significant diminution in the π-polarization effect (ρ₁) in N-chlorobenzamides compared to benzamides. This is in contrast to other carbonyl systems where ρ₁ has been found to be essentially independent of the substituent attached to the carbonyl carbon. It is concluded that π-polarization in benzamide and its derivatives is more sensitive to the substituent attached to the carbonyl carbon than in the other classes of aromatic carbonyl compounds studied to date.