Basicity of the carbonyl group. Part 11. A re-examination of the electrophilic substituent constants for amino-substituents from thermodynamic measurements

Hammett–Brown correlations between σp+ and pKBH+ values or enthalpies of complexation, ΔH°, with boron trifluoride for three series of carbonyl compounds, 3-substituted 5,5-dimethylcyclohex-2-enones, 4-substituted acetophenones, and benzaldehydes, have enabled a re-evaluation of electrophilic substituent constants for various amino-groups to be made. A relationship is obtained between σp+(NR1R2) and Σσ*(R). The electron-releasing ability of the amino-groups is found to be larger, in a given series, than those expected from the Brown–Okamoto classical values and other more recent estimates. The discrepancies are attributed to charge delocalisation on the probe, steric inhibition of resonance, and to specific solvent associations with the amino-substituents. Owing to their large σp+ values, we suggest deletion of amino-group data when using a non-linear treatment of substituent effects.