Kinetics of Ionization of Nitromethane and Phenylnitromethane by Amines and Carboxylate Ions in Me2SO-Water Mixtures. Evidence of Ammonium Ion-Nitronate Ion Hydrogen Bonded Complex Formation in Me2SO-Rich Solvent Mixtures

Article abstract of DOI:10.1021/jo00249a040

Rate constants of ionization of phenylnitromethane in seven amine buffers and six carboxylate buffers and of nitromethane in five amine buffers were determined in 90percent Me2SO-10percent water.Similar experiments with a smaller selection of buffers were performed in water, 50percent Me2SO-50percent water, and 70percent Me2SO-30percent water.Buffer plots with amine buffers in 70percent and 90percent Me2SO, but not with carboxylic acid buffers, showed downward curvature, which was attributed to a hydrogen-bonded association complex between the nitronate ion and the protonated amine.Asocciation equilibrium constants for these complexes were determined, and τ values for hydrogen bond ing were calculated on the basis of the Hine equation.These τ values ranging from 0.024 to 0.039 are much higher then those for the association between protonated amines and phenoxide ions in water (τ = 0.013, Stahl and Jencks, J.Am.Chem.Soc. 1986, 108, 4196), presumably because of reduced hydrogen-bonding stabilization of the nitronate ions by the solvent.Absence of downward curvature in the buffer plots with carboxylic acids is believed to be a consequence of somewhat lower association constans and, more importantly, of competing nitronic acid formation at the low pH values required to study the carboxylic acids.The possibility that the association complex might represent Bordwell's intermediate in the deprotonation of nitroalkanes (BH⁺...<*>H(R)NO2) is discussed and rejected.The intrinsic rate constants for proton transfer (k₀ = k₁ ^B/q = k_-1 ^BH/p at ΔpK + log p/q = 0) increase strongly with increasing Me2SO content of the solvent, but more so when the ionizing base is a carboxylate ion than when it is an amine.This increase is mainly due to a transition state in which solvation of the developing nitronate ion lags behind proton transfer.When the ionizing base is a carboxylate ion, early desolvation of the base adds to the solvent effect on k₀.The Broensted β values show the familiar increase with increasing Me2SO content of the solvent, irrespective of buffer type.