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Improved Yukawa–Tsuno equation and the substituent effect on pyridine basicity

✍ Scribed by João Carlos R. Reis; Manuel A. P. Segurado; Jaime D. Gomes de Oliveira

Publisher
John Wiley and Sons
Year
1998
Tongue
English
Weight
151 KB
Volume
11
Category
Article
ISSN
0894-3230

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✦ Synopsis

An improved Yukawa-Tsuno equation was obtained by inserting the resonance demand parameter r in the tetralinear extension of the Hammett equation constrained by a hyperbolic relation between meta and para substituent effects. The new equation with five adjustable parameters and other Yukawa-Tsuno models were applied to selected literature data for the ionization equilibria of 21 meta-and 20 para-substituted pyridinium ions in water at 25 °C. The data are best described by the new equation using either the sigma-benzoic acid scale with r = 1.35 AE 0.16 or a truncated sigma-plus scale (in which sigma-zero constants are assigned to 4-alkyl substituents) with r = 0.29 AE 0.04. Standard errors were estimated by Monte Carlo simulation. The results for the pyridinium ion demonstrate that the hyperconjugative effect by alkyl groups is inoperative and that through-resonance effects by electron-donating substituents are considerably hindered. The origin of this hindrance is ascribed to the particularities of nitrogen orbitals in heteroaromatic compounds. It is concluded that the pyridine reactivity lies between those of benzoic acid and benzylic cation, being much closer to the former.

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