ISSN:
0894-3230
Keywords:
substituent effect
;
Yukawa-Tsuno equation
;
resonance demand
;
pyridine basicity
;
Chemistry
;
Theoretical, Physical and Computational Chemistry
Source:
Wiley InterScience Backfile Collection 1832-2000
Topics:
Chemistry and Pharmacology
,
Physics
Notes:
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 ± 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 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. © 1998 John Wiley & Sons, Ltd.
Additional Material:
3 Ill.
Type of Medium:
Electronic Resource
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