Proton-Transfer Reactions of Nitroalkanes: The Role of aci-Nitro Species

Proton-transfer reactions of two systems, ionization of a series of small carbon acids in water (the Pearson system) and reactions of substituted phenylnitromethanes, were examined in detail computationally. Comparison of experimental reactivity and pK_a with calculated relative activation barrier and reaction energy for the Pearson system suggested that the origin of the well-know nitroalkane anomaly does not reside in the reactivity but in the equilibrium. For the reactions of substituted phenylnitromethanes, proton transfers among three species, PhCH₂NO₂, PhCHNO₂⁻, and PhCHNO₂H, were examined, and the role of the aci-nitro species (PhCHNO₂H) was evaluated on the basis of its stability and reactivity. Protonation of PhCHNO₂⁻ by H₂O was suggested to occur kinetically on the oxygen site, but due to its instability PhCHNO₂H does not contribute to the overall reaction energetics. The protonation of PhCHNO₂⁻ under acidic conditions occurs on the oxygen site to give PhCHNO₂H both kinetically and thermodynamically. The aci-nitro species thus formed appears to give PhCH₂NO₂ via intramolecular H₂O-mediated proton transfer, but a possibility of the route through PhCHNO₂⁻−C-protonation would not be fully eliminated.