17O NMR study of substituent effects in 4-substituted N-chlorobenzamides and comparison with 4-substituted benzamides: sensitivity to ring substituents (ρ) reflects electronic and steric effects

Abstract

Natural abundance ¹⁷O NMR spectra of 4-substituted N-chlorobenzamides were obtained in acetonitrile at 75 °C and compared with those for similarly substituted benzamides. An excellent correlation (δ = 5.43σ ⁺ + 350.1, r = 0.998) was obtained when the ¹⁷O chemical shifts of N-chlorobenzamides were plotted against σ⁺ and using σ^- for the 4-nitro derivative. Based on the greater electronegativity of the N-chloro group the value of ρ observed for N-chlorobenzamides (5.4) would have been expected to be greater compared to that of benzamides (7.3). It is concluded that the ¹⁷O chemical shift in N-chlorobenzamides is very sensitive to torsion angle and that is why a low value of ρ is obtained. Sensitivity (ρ) of the chemical shift to ring substituents cannot be used as an unambiguous measure of electronic effects in systems where the torsion angle is greater than zero.