Transition state imbalance in the ionization of nitroaromatic hydrocarbons: 2,2′,4,4′-tetranitrodiphenylmethane and 2,4,4′-trinitrodiphenylmethane in aqueous dimethyl sulphoxide solutions

Abstract

Rates of reversible proton abstraction from 2,2′,4,4′-tetranitrodiphenylmethane and 2,4,4′-trinitrodiphenylmethane by a variety of bases in 50% water–50% dimethyl sulphoxide have been measured at 25 °C. Brönsted α_CH and β_B values have been determined and show a charge imbalance in the transition states for these reactions, in accord with findings for similar carbon acids involving extensive charge delocalization on ionization. Intrinsic rate constants (in the Marcus sense), k_o′ have been obtained for these systems, and are shown to vary considerably with choice of catalyst type. As expected, high intrinsic energy barriers for these ionizations are predicted, arising from the extensive molecular and solvent reorganization required to produce planar, delocalized carbanions, Data for the former compound have also been obtained in water, which allow a direct estimation of its pK_a^H₂O value to be made; pK_a^H₂O= 13.16.