Conjugation and proton exchange equilibria. Heteroconjugation constants for the 4-nitrobenzoic acid-substituted phenolate systems in acetonitrile

Abstract

The heteroconjugation constants _AHA1⁻  = ([AHA₁⁻]y_AHA1⁻ /[HA][A₁⁻]y_A1⁻) for 4-nitrobenzoic acid (HA)-substituted phenolate (A₁⁻) systems in acetonitrile (AN) were determined by an electrometric method neglecting the formation of heteroconjugates with the solvent molecules. The constants referred to as were compared to those calculated directly from the relationship between homo-, heteroconjugation and proton exchange constants reported previously. Both lg and lg show a fairly linear dependence on ΔpK_a^AN values referred to as pK_HA1^AN−pK_HA^AN. For both lg and lg the deviations from this general dependence (caused by nonequal steric effects during hydrogen bond formation) observed for the same systems are similar in direction and extent. This finding strongly suggests that the relationship between homo-, heteroconjugation and proton exchange constants is also a reflection of certain specific properties of the heteroconjugate and may therefore successfully be used as a ‘standardizing equation ’ for all methods of determining heteroconjugation constants. Some practical comments about the applicability of the electrometric method in its current shape are given.