C-13 NUCLEAR-MAGNETIC-RESONANCE STUDY OF PI-POLARIZATION IN 3-SUBSTITUTED AND 4-SUBSTITUTED BENZAMIDES AND N-CHLOROBENZAMIDES

The substituent effect on the carbonyl carbon [C(alpha)] C-13 chemical shift in 3- and 4-substituted benzamides and N-chlorobenzamides has b een studied in (CD3)2SO. Chlorine shields the carbonyl carbon. A Cross correlation was carried out for C(alpha) and the carbonyl carbon in N -chlorobenzamides is less sensitive to substituents than in the benzam ides. The dual substituent parameter method indicated a significant di minution in the pi-polarization effect (rho(I)) in N-chlorobenzamides compared to benzamides. This is in contrast to other carbonyl systems where rho(I) has been found to be essentially independent of the subst ituent attached to the carbonyl carbon. It is concluded that pi-polari zation in benzamide and its derivatives is more sensitive to the subst ituent attached to the carbonyl carbon than in the other classes of ar omatic carbonyl compounds studied to date.