Substituent Effects on the Decarboxylation of Dinitrobenzoate Ions, Representative Aromatic SE1 Reactions

Article abstract of DOI:10.1021/jo00207a027

Kinetics of decarboxylation, in water solution, of several 4-substituted 2,6-dinitrobenzoic and 2-substituted 4,6-dinitrobenzoic acids were determined, at several temperatures.The reactions are first order at the low concentrations employed, at which the acids are fully dissociated.Decarboxylation is accelerated by electron-attracting substituents; the Hammett ρ for the effects of 4-substituents is about +2.6.All the decarboxylations involve large positive enthalpies and entropies of activation.The solvent kinetic isotope effect (D2O/H2O) is not significantly different from unity.The data support a mechanism where in the substituted benzoate ion loses CO2 in the rate-determining step, forming a substituted aryl anion, which is then rapidly hydronated by the solvent.In the case of 2-methoxy-4,6-dinitrobenzoate ion, reaction occured with strikingly different activation parameters.