Kinetic isotope effects in the reactions of 4-nitrophenylnitromethane with alkylamidine bases in toluene

Abstract

Kinetic isotope effects have been determined for the proton-transfer and deuteron-transfer reactions of 4-nitrophenylnitromethane (4-NPNM; NO₂C₆H₄CH₂NO₂, NO₂C₆H₄CD₂NO₂) with two alkylamidine bases [HN = C(NEt₂)R; R = n-butyl, n-nonyl] in toluene solution. The observed values of the isotopic rate ratio k^H_f/k^D_f at 25°C are 11.7 ± 1.0 and 8.0 ± 0.3, respectively. The former value indicates an appreciable tunnel correction; this is confirmed by a value of (ΔH^{[graphic omitted]D}-ΔH^{[graphic omitted]H})= 2.5 ± 0.8 kcal mol^–1(10 ± 3 kJ mol^–1). The isotopic differences are not precise enough to allow calculations of the dimensions of the energy barrier.