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Elucidation of Reaction Mechanisms - Kinetic Isotope Effects

Kinetic Isotope Effects - Introduction

Kinetic isotope effect
It is often the case that the exchange of an atom in the starting product of a reaction for one of its heavier isotopes leads to a reduction of the reaction rate.
This phenomenon is called kinetic isotope effect.

Usually, the larger the relative mass difference between the isotopes of the respective element is, the larger the kinetic isotope effect is, as well. The largest kinetic isotope effects are therefore found when a hydrogen atom H is exchanged for deuterium D (about twice the mass of H) or even for tritium T (about triple the mass of H). Therefore, the kinetic isotope effects induced by H/D exchanges are the most common isotope effects employed in the investigation of reaction mechanisms, as the radioactive tritium, in comparison to the non-radioactive deuterium, is much more difficult to handle. That explains why only these kinetic isotope effects are discussed below. However, the respective principles are also applicable to other chemical elements.

There are different types of kinetic isotope effects:

  1. The numerical quantity of a kinetic isotope effect is expressed by the ratio of the rate constants kH/kD. The value of kH/kD can amount up to seven (reaction with H is seven times faster than with D) if a C-H/C-D bond is cleaved during the reaction (primary kinetic isotope effect).
  2. If the C-H/C-D bond (or X-H-/X-D bond with X = O, N, S, etc., respectively) is not cleaved during the reaction (secondary kinetic isotope effect), the effect is usually smaller (kH/kD < 2).
  3. In the case of secondary kinetic isotope effects, kH/kD can also become smaller than one. The reaction rate is then lower with H than it is with D (inverse secondary kinetic isotope effect).
  4. Last but not least, kinetic solvent isotope effects may also be found. Such an effect occurs, for instance, when the solvent H2O is exchanged for D2O. In the case of solvent isotope effects, kH/kD can display all values that are obtained with primary, secondary, and inverse secondary kinetic isotope effects.
Use of investigation of kinetic isotope effects
The results of the investigation of kinetic isotope effects serve to ...
  • ... identify the rate-determining step (elementary reaction) of multistep reaction mechanisms and ...
  • ... to get more information about the structure of the transition state in the rate-determining step.
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