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

Abstract

Spectrophotometric measurements of the equilibrium constant (Kapp) for formation of a Meisenheimer adduct from 2,4-dinitroanisole and ethanolic sodium and potassium ethoxide have been used to define an acidity function JE for alkoxide solutions in ethanol (analogous to the functions JM 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 Kapp 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 Kapp 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.