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.