The Derivation of Inductive Substituent Constants from pKa Values of 4‐Substituted Quinuclidines. Polar effects. Part I

Abstract Thermodynamic pKa‐values have been determined for 38 4‐substituted quinuclidinium perchlorates. They are remarkably sensitive to the polar effect of the substituent and cover a range of 3.63 pKa units. Furthermore, they vary linearly and almost equally with temperature since the contribution of the TΔS° term to the free energy of ionization is relatively small and constant. The magnitude of the polar effect of the 4‐cyano group varies with the solvent and appears to depend on its ability to form hydrogen bonds to the substituent rather than its dielectric constant. New inductive substituent constants σIq are derived from the pKa values. Their correlation with known inductive constants is only fair or unsatisfactory, especially as regards the relative order of hydrogen and the alkyl groups. The discrepancies can be ascribed mainly to the different models used to derive the substituent constants.