Analysis of the acidities of 3- and 4-substituted pyridinium and anilinium ions

Abstract

The title aldehydes 1 and 3 exist in equilibrium with cyclic hemiacetal forms 2 and 4, respectively, with equilibrium constants in water of 6.7 (1 -2) and 20 (3 -4). These equilibria are displaced toward the hemiacetals in strongly basic solutions because of their ionization. Acidity constants have been determined to be 12.29 (2) and 12.38 (4). The kinetics of the equilibration have been studied in the pH range 1-8 in carboxylic acid and alkylphosphonic acid buffers. The base forms (RCOO" and RP031 2') produce a common Bronsted line, while the acid forms produce

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

“…As noted before, 11 our hyperbolic model is consistent with errorfree data for which r 3n and r 3s are different, this difference being related to 3s [see Eqn (41) The behaviour of 3s substituents in the pyridine reactivity has been qualified as unexpected 28 and its origin ascribed 28,29 to the field-induced resonance effect. 30 This effect, which cannot be accounted for separately from pure field effects, should therefore explain the increased reaction constant for 3s substituents relative to 3n substituents, a feature precluded by the bilinear and unilinear models.…”

“…In the meta series, our constrained tetralinear approach accommodates well the increased effect by electron-donating substituents, in relation to the other substituents, 27 originated in the fieldinduced resonance effect. [28][29][30] This effect is considered to be responsible for an observed ! value below theoretical calculations.…”

“…Later, analysis by the dual substituent parameter treatment 32 assigned ' R constants to the pyridinium ion system, and from a remark by Ellam and Johnson 33 a long controversy ensued. 13,29,[34][35][36][37][38] Basically, Johnson and co-workers 33,36,38 consider the pyridine reactivity to be closer to the benzoic acid reactivity rather than to the sigma-plus reactivities. Their contention was then supported only by Sawada et al 13 More recently, Reynolds et al 39 suggested that ' R BA and ' R scales could be replaced by a new ' R scale such that ' R BA values would be generated by linear interpolation between ' R and the new ' R values.…”

“…value can be ascribed to indirect meta resonance or field-induced resonance effects (FIRE). [28][29][30] Since 11 ! = !…”

Computational study of the substituent effects on the gas-phase stabilities of phenylboranylmethyl anions

“…1 have been seen to date [3,4]. However, the establishment of LFER for anions has been slow because of experimental difficulties [5,6]. In recent years, the development of computational chemistry has allowed us to compute highly accurate structures and energies of chemical species [7].…”