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Elucidation of Reaction Mechanisms - Kinetic Isotope Effects

Kinetic Solvent Isotope Effects

In many cases, the reaction rate is influenced by an isotopic exchange even if it is an atom of the solvent, rather than that of a reactant that has been exchanged for one of its isotopes. Such an effect is called kinetic solvent isotope effect. This effect can occur, for instance, when the solvent H2O is exchanged for D2O, or when C2H5OH is exchanged for C2H5OD.

Principally speaking, kinetic solvent isotope effects depict all known values from primary and secondary kinetic isotope effects (kH/kD about 0.7 to 8). This range of kH/kD values is the result of the fact that kinetic solvent isotope effects, in reality, are "hidden" primary or secondary kinetic isotope effects.

Take a look at the acid-catalysed cleavage of ethyl vinyl ether, for instance.

Acid-catalysed cleavage of ethyl vinyl ether.

Essentially, there are three factors that lead to kinetic solvent isotope effects:

  1. Solvent molecules participate as reactants in the reaction. Depending on the other preconditions, actual primary or secondary kinetic isotope effects then appear. In the ethyl vinyl ether cleavage, for example, the solvent molecule H3O+ serves as a reactant. In connection with H1, a primary kinetic isotope effect occurs, while a secondary kinetic isotope effect results with H2 and H3.
  2. The transition state of the reaction can be stabilized by interactions between the reactants and solvent molecules. The extent of these interactions in isotope-substituted solvents can differ from that in the original solvent. As a result, the transition state's energy and thus, the activation energy Ea of the respective reaction step (elementary reaction) are altered. If this particular reaction step is the rate-determining step, the result is a kinetic isotope effect. The positively charged transition state of the ethyl vinyl ether cleavage, for example, interacts with the polar water molecules to a considerable degree.
  3. Especially in acidic, deuterated solvents (e.g. D2O or C2H5OD), an H/D exchange or a D transfer between the solvent molecules and the reactants can occur. Consequently, an actual primary or secondary kinetic isotope effect is obtained. After it has been transferred to ethyl vinyl ether, D (H2) could cause a secondary kinetic isotope effect if the rate-determining step still follows.

Data about the the rate-determining step and the structure of the transition state, as well as the participation of solvent molecules in the reaction by either solvating the transition state or acting as reactants can be inferred from kinetic solvent isotope effects and not only from pure and obvious primary or secondary kinetic isotope effects. It should be taken into consideration that the formerly mentioned causes of kinetic isotope effects can occur simultaneously and can enhance each other, as is the case in the ethyl vinyl ether cleavage.

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