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.