Kinetic isotope effect in reaction between 4-nitrobenzyl-cyanide and ethoxide ion in ethanol + ether

Abstract

The rate of the deuteron-transfer reaction between ethoxide ion and 4-nitrobenzylcyanide-α, α-d₂ has been determined at six temperatures over the temperature range –40 to –90°. At the highest temperatures, the Arrhenius plot shows a negative deviation from linearity, and approaches that for the proton-transfer; this behaviour confirms the previous interpretation in terms of an unstable intermediate complex. At lower temperatures (–60 to –90°), the plot for the deuteron-transfer reaction is linear; the energy of activation is 1.8₅± 0.2 kcal mole^–1 higher than for the proton-transfer, and the A-factor is about 5 times larger. These values can be interpreted in terms of tunnelling. On fitting the equations for a parabolic energy-barrier, with ΔH°= 0, we find for the height 10.3 kcal mole^–1 and for the width 1.63 Å. These are in good agreement with the values found independently from the non-linear Arrhenius plot for the proton-transfer reaction in the same medium (10.7 kcal mole^–1 and 1.62 Å).