Kinetic analysis of deuteron-transfer reactions of alkylamidines with [2H2]-4-nitrophenylnitromethane. Implications of isotopic scrambling for the determination of kinetic isotope effects

Abstract

The kinetics of the deuteron transfer reactions of [2H2]-4-nitrophenylnitromethane with N,N-diethyl-n-butylamidine in toluene has been studied. The reaction is, in contrast to the corresponding proton transfer, kinetically complex, making it impossible to obtain accurate values for the deuteron transfer rate constant and hence for the isotope effect. The kinetic behaviour can be accounted for by a reaction mechanism involving H–D exchange between the acid and the base. This latter mechanism approaches equilibrium slower than the one-step process of the corresponding proton transfer. Theoretical runs calculated from the H–D exchange scheme fit the experimental data quantitatively if an isotopic rate ratio of about 14 is assumed.