The stabilities of Meisenheimer complexes. Part I. Adducts from sodium methoxide and highly activated anisoles in methanol

Abstract

The thermodynamic equilibrium constants (K₁) for the formation of Meisenheimer complexes by methoxide addition to several substituted anisoles have been determined spectrophotometrically in methanol. The values are for 2-methoxycarbonyl-4,6-dinitroanisole, K₁= 10 l mol^–1 and for 4-methoxycarbonyl-2,6-dinitroanisole K₁= 6 l mol^–1. Complex formation for these compounds and for 2-chloro-4,6-dinitroanisole is sufficiently slow to allow individual rate coefficients, k₁ and k_–1, for the equilibrium process to be determined. The values of the equilibrium constants increase markedly with sodium methoxide concentration owing mainly to apparent decreases in k_–1 values. It is suggested that this may be due to stabilisation of the adducts by ion association.