Kinetic Study of Proton-Transfer Reactions of Phenylnitromethanes. Implication for the Origin of Nitroalkane Anomaly

Measurements of rate constants and substituent effects for three important elementary steps of proton-transfer reactions of phenylnitromethane were reported. The Hammett ρ values for the deprotonation of ArCH2NO2 with OH−, protonation of ArCH═NO2− with H2O, and protonation of ArCH═NO2− with HCl were determined in aqueous MeOH at 25 °C. Comparison of these experimentally observed ρ values with those calculated at B3LYP/6-31G* revealed that aci-nitro species (ArCH═NO2H), which is formed on the O-protonation of ArCH═NO2−, does not lie on the main route of the proton-transfer reaction. Analysis of the Brønsted plot implies that the proton-transfer reaction of most XC6H4CH2NO2 exhibits nitroalkane anomaly, but not for p-NO2C6H4CH2NO2, and that the transition state charge imbalance is an origin of anomaly.