Meisenheimer adduct formation between 2,4-dinitrophenetole and ethanolic alkali ethoxide solutions: the course of the acidity function JE and thermodynamic solvent isotope effects

Abstract

Spectrophotometric measurements of the equilibrium constant (K_app) for formation of a Meisenheimer adduct from 2,4-dinitroanisole and ethanolic sodium and potassium ethoxide have been used to define an acidity function J_E for alkoxide solutions in ethanol (analogous to the functions J_M for methanol). Sodium and potassium ethoxide behave identically up to a concentration of ca. 0.3 mol dm^–3; above this value solutions of sodium ethoxide are slightly less basic. The dependence of K_app on base concentration is interpreted in terms of ion-pairing effects. The formation of a 1 : 2 adduct becomes significant at high base concentrations, and appears to involve cation-specific ion-pairing effects. The limiting value of log K_app at low concentrations is -3.1. In EtOD solutions of the same alkoxide concentration, the equilibrium constant is greater than that in EtOH solutions by a factor of 2.5. This result and measurements in mixtures of EtOH and EtOD are consistent with the formulation of the ethoxide ion as an entity containing three hydrogen-bonded solvent molecules, and a deuterium fractionation factor of ca. 0.72–0.73 for the hydrogen-bonded positions.