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

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-l) by the conjugate acids BH have been measured in a 50:50 (v/v) H2O-Me2SO mixture at 25 degrees C, In contrast with the situation at pH greater than or equal to 4.2 where only one relaxation time corresponding to a simple equilibrium approach according to 1 + B reversible arrow C-1 + BH is observed, the interconversion of 1 and C-l proceeds in two steps in acidic media, Kinetic studies as well as data obtained by H-1 and C-13 NMR or UV-VIS spectroscopy indicate that the first step corresponds to the protonation of the exocyclic nitro group of C-l, giving rise to the corresponding nitronic acid C-la,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) H2O-Me2SO. Although they clearly reveal some contribution of the nitrophenyl ring to the stabilization of C-l 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.