Kinetics of the Deprotonation of Methylnitroacetate by Amines: Unusually High Intrinsic Rate Constants for a Nitroalkane

A kinetic study of the reversible deprotonation of methylnitroacetate (4H) by primary aliphatic amines, secondary alicyclic amines, hydroxide ion, and water in water at 25 °C and in 50% DMSO/50% water (v/v) at 20 °C is reported. Intrinsic rate constants, k₀, determined by extrapolation or interpolation of Brønsted plots have been determined. In comparison to proton transfers involving other nitroalkanes, the intrinsic rate constants for 4H are exceptionally high; for example, log k₀ for the reaction of 4H with secondary alicyclic amines in water (1.22) is 1.81 log units higher than log k₀ for nitromethane (−0.59), while in 50% DMSO/50% water, log k₀ for 4H (2.44) is 1.71 log units higher than that for nitromethane (0.73). A general discussion of the factors affecting intrinsic rate constants of proton transfer from nitroalkanes is presented; it provides the context for an understanding as to why k₀ is so high for the proton transfers from 4H. The correlation between intrinsic rate constants for the addition of nucleophiles to alkenes of the type R‘R‘ ‘CCXY and the intrinsic rate constants of proton transfers from carbon acids of the type H₂CXY is also discussed as a general proposition as well as with specific reference to the Ph(SMe)CC(NO₂)CO₂Me/H₂C(NO₂)CO₂Me pair.