Proton-transfer reactions between nitroalkanes and hydroxide ion under non-steady-state conditions. Apparent and real kinetic isotope effects

The kinetics of the proton-transfer reactions between 1-nitro-1-(4-nitrophe nyl)ethane (NNPEH(D)) and hydroxide ion in water/acetonitrile (50/50 vol %) were studied at temperatures ranging from 289 to 319 K. The equilibrium co nstants for the reactions are large under these conditions, ensuring that t he back reaction is not significant. The extent of reaction/time profiles d uring; the first half-lives are compared with theoretical data for the simp le single-step mechanism and a 2-step mechanism involving initial donor/acc eptor complex formation followed by unimolecular proton transfer and dissoc iation of ions. In all cases, the profiles for the reactions of both NNPEH and NNPED deviate significantly from those expected for the simple single-s tep mechanism. Excellent fits of experimental data with theoretical data fo r the complex mechanism, in the presteady-state time period,were observed i n all cases. At all base concentrations (0.5 to 5.0 mM) and at all temperat ures the apparent kinetic isotope effects (KIEapp) were observed to increas e with increasing extent of reaction. Resolution of the kinetics into micro scopic rate constants at 298 K resulted in a real kinetic isotope effect (K IEreal) for the proton-transfer step equal to 22. Significant proton tunnel ing was further indicated by the temperature dependence of the rate constan ts for proton and deuteron transfers: KIEreal ranging from 17 to 26, E-a(D) - E-a(H) equal 2.8 kcal/mol, and A(D)/A(H) equal to 4.95.