IMPROVED YUKAWA-TSUNO EQUATION AND THE SUBSTITUENT EFFECT ON PYRIDINEBASICITY

An improved Yukawa-Tsuno equation was obtained by inserting the resona nce demand parameter r in the tetralinear extension of the Hammett equ ation constrained by a hyperbolic relation between meta and para subst ituent effects. The new equation with five adjustable parameters and o ther Yukawa-Tsuno models were applied to selected literature data for the ionization equilibria of 21 meta- and 20 para-substituted pyridini um ions in water at 25 degrees C. The data are best described by the n ew equation using either the sigma-benzoic acid scale with r = 1.35 +/ - 0.16 or a truncated sigma-plus scale (in which sigma-zero constants are assigned to 4-alkyl substituents) with r = 0.29 +/- 0.04. Standard errors were estimated by Monte Carlo simulation. The results for the pyridinium ion demonstrate that the hyperconjugative effect by alkyl g roups is inoperative and that through-resonance effects by electron-do nating substituents are considerably hindered. The origin of this hind rance is ascribed to the particularities of nitrogen orbitals in heter oaromatic compounds. It is concluded that the pyridine reactivity lies between those of benzoic acid and benzylic cation, being much closer to the former. (C) 1998 John Wiley & Sons, Ltd.