The aci-nitro equilibrium of picrylacetone: a kinetic and thermodynamic study in 50∶50 and 30∶70 (v/v) H2O–Me2SO mixtures†

Abstract

Rates of deprotonation of picrylacetone (3a) by a variety of bases B (phenoxide and carboxylate ions, primary amines, OH^–, H₂O) and of reprotonation of the resulting carbanion (C-3a) by the conjugate acids BH have been measured in 50∶50 and 30∶70 (v/v) H₂O–Me₂SO mixtures at 25 °C. In contrast with the situation in phenol and amine buffers, where only one relaxation time corresponding to a simple equilibrium approach according to 3a + B ⇄ C-3a + BH is observed, a fast equilibrium protonation of the carbanion C-3a is found to precede its conversion to 3a at low pH in a number of carboxylic acid buffers and HCl solutions. The resulting short-lived nitronic acid species C-3aH has been characterized by UV–Visible stopped-flow spectrophotometry and all data (pK _a^NO₂H, pK _a^CH, pK_T) pertaining to the aci-nitro equilibrium of 3a in the two solvent mixtures studied could be determined. A notable solvent effect shown to be consistent with the preferential stabilization of the large polarizable carbanion C-3a by Me₂SO is observed on pK _a^NO₂H and pK _a^CH, but not on pK_T, on going from 50 to 70% Me₂SO. The solvent dependence and the magnitude of the intrinsic reactivity of 3a, as determined from the Brønsted plots for the various types of catalysts in the two solvent mixtures, are also found to agree with C-3a being an sp²-hybridized carbanion whose negative charge is largely dispersed through the picryl ring.