1,3-Dinitrobenzene – hydroxide ion interactions. Equilibrium constants for σ-complex formation in the dimethylformamide–water solvent system

Abstract

The results of a uv–visible spectroscopic study of the interaction of 1,3-dinitrobenzene (DNB) with hydroxide ion in aqueous dimethylformamide (DMF) media are reported. Formation of several spectral species has been discerned, depending on factors such as the relative concentrations of the reactants, the time scale of the experiments, and the medium composition. The principal interaction is formation of the 1-hydroxy-2,4-dinitrocyclohexadienate anion. The molar absorptivities of this σ-complex, and the equilibrium constants (Ke) for its formation, have been determined as a function of medium composition. Values of Ke increase from 7.5 M−1 to 3 × 105M−1 as the DMF content of the medium is changed from 57.4 to 95.8 mol% DMF. A solvent isotope effect, Ke(D2O)/Ke(H2O), of ca. 0.4 has been observed. Structural and medium effects in this and related systems are evaluated. Correlations between log Ke and H− are examined using the stoichiometric as well as the calculated free hydroxide ion concentrations. Attention is drawn to an important condition for equilibrium constant measurement in systems where one of the interacting species contributes significantly to the overall basicity, or to another property of the medium.