The kinetic isotope effect (KIE) is a variation in the reaction rate of a chemical reaction when an atom in one of the reactants is replaced by one of its isotopes
An isotopic substitution will greatly modify the reaction rate when the isotopic replacement is in a chemical bond that is broken or formed. In such a case, the rate change is termed a primary isotope effect. When the substitution is not involved in the bond that is breaking or forming, one may still observe a smaller rate change, termed a secondary isotope effect. Thus, the magnitude of the kinetic isotope effect can be used to elucidate the reaction mechanism. Isotope effects are most easily observed when they occur in the rate-determining step of a reaction.
The kinetic isotope effect is applied in reaction mechanism elucidation, for instance in the halogenation of toluene
In this particular intramolecular KIE study the radical substitution of hydrogen by bromine is examined with mono-deuterated toluene (obtained by organic reduction of benzyl chloride with zinc and deuterated acetic acid) and N-bromosuccinimide. As hydrogen is replaced by bromine faster than deuterium, the reaction product gets enriched in deuterium.
In this reaction the rate-limiting step is enolate formation by proton (deuterium) abstraction from the ketone by base.
Moderation Team
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