Intermediates in nucleophilic aromatic substitution. Part XV. Thermodynamic stabilities of hydroxy and methoxy Meisenheimer complexes of substituted arenes

Abstract

Considering available thermodynamic data on the interaction of hydroxide and methoxide ions with substituted arenes, relative free energies of Meisenheimer complex stabilization have been calculated for given substituents at different positions on the benzene and naphthalene ring as well as those for benzo-fusion. Using these values stabilities of Meisenheimer complexes can be predicted. Excellent agreement has been obtained between the predicted and experimentally determined stabilities for 116 pairs of Meisenheimer complexes. Satisfactory correlations have also been obtained between rate and equilibrium constants for the formation and decomposition of σ-complexes and the sums of appropriate substituent constants. Additionally, stabilization of Meisenheimer complexes by dipolar aprotic DMSO and DMF have been shown to be independent of the number and type of substituents on the aromatic nucleus, on the attacking nucleophile, or on the co-solvent. The significance of these results is that they provide a coherent framework for the mechanistic discussion of nucleophilic aromatic substitution.