Conjugation and proton exchange equilibria. Heteroconjugation constants in substituted phenol–piperidine systems in acetonitrile

Abstract

It has been shown by mathematical transformations that the final equations and expressions required for determining equilibrium concentrations of major species in HA + B systems are analogous to those used previously for HA + A⁻₁ systems. Heteroconjugation constants _AHB = [AHB]/([HA][B]) for eight substituted phenol (HA)–piperidine (B) systems in acetonitrile (AN) were determined from emf measurements. A fairly linear dependence between log _AHB and ΔpK_a^AN = pK^AN_BH⁺ − pK_HA^AN was observed with a slope of 0.52. The [K²_AHB/(K_AHA⁻K_BHB⁺)]_{at ΔpKa = 0} quotient appeared to be much greater than [K²_AHA1⁻/(K_AHA⁻K_A1HA1⁻)]_{at ΔpKa = 0} calculated from results obtained previously for HA + A⁻₁ type systems. From this fact it has been concluded that the formation of AHB type complexes (relative to the AHA⁻ and BHB⁺ type complexes) is likely to be favoured by their overall neutrality ensuring weaker peripheral interactions.