THE ACI-NITRO EQUILIBRIUM OF PICRYLACETONE - A KINETIC AND THERMODYNAMIC STUDY IN 50 50 AND 30/70 (V/V) H2O-ME2SO MIXTURES/

Rates of deprotonation of picrylacetone (3a) by a variety of bases B ( phenoxide and carboxylate ions, primary amines, OH-, H2O) and of repro tonation of the resulting carbanion (C3a) by the conjugate acids BH ha ve been measured in 50:.50 and 30:70 (v/v) H2O-Me2SO mixtures at 25 de grees C. In contrast with the situation in phenol and amine buffers, w here only one relaxation time corresponding to a simple equilibrium ap proach according to 3a + B half arrow right over half arrow left 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 car boxylic acid buffers and HCl solutions. The resulting short-lived nitr onic acid species C-3aH has been characterized by UV-Visible stopped-f low spectrophotometry and all data (pK(a)(NO2H), PKaCH, pK(T)) pertain ing to the aci-nitro equilibrium of 3a in the two solvent mixtures stu died could be determined. A notable solvent effect shown to be consist ent with the preferential stabilization of the large polarizable carba nion C3a 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 Bro nsted plots for the various types of catalysts in the two solvent mixt ures, are also found to agree with C-3a being an sp(2)-hybridized carb anion whose negative charge is largely dispersed through the picryl ri ng.