The nitroalkane behaviour of (4-nitrophenyl)nitromethane: a kinetic and structural study in H2O–Me2SO mixtures

Abstract

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) H2O–Me2SO mixture at 25 °C. In contrast with the situation at pH 4.2 where only one relaxation time corresponding to a simple equilibrium approach according to 1 + B ⇌ C-1 + 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 1H and 13C 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 k0 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) H2O–Me2SO. Although they clearly reveal some contribution of the nitrophenyl ring to the stabilization of C-1 in Me2SO, 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.