Rates of proton abstraction from (4-nitrophenyl)nitromethane (1) by a variety of bases B (phenoxide and carboxylate ions, primary amines) and of reprotonation of the resulting carbanion (C-1) by the conjugate acids BH have been measured in a 50 : 50 (v/v) H 2 O-Me 2 SO mixture at 25 ЊC. In contrast with the situation at pH у 4.2 where only one relaxation time corresponding to a simple equilibrium approach according to 1 ϩ B C-1 ϩ BH is observed, the interconversion of 1 and C-1 proceeds in two steps in acidic media. Kinetic studies as well as data obtained by 1 H and 13 C NMR or UV-VIS spectroscopy indicate that the first step corresponds to the protonation of the exocyclic nitro group of C-1, giving rise to the corresponding nitronic acid C-1a,H. All rate and equilibrium parameters obtained for the aci-nitro behaviour of 1, including the log k 0 values measuring its intrinsic reactivity towards the various types of bases studied, emphasize a nitroalkane behaviour rather than a 4-nitrotoluene behaviour of this carbon acid in 50 : 50 (v/v) H 2 O-Me 2 SO. Although they clearly reveal some contribution of the nitrophenyl ring to the stabilization of C-1 in Me 2 SO, the NMR data remain consistent with a major role of the exocyclic nitro group in governing the ionization behaviour of 1 in this dipolar aprotic solvent.
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