Applications
Reaction mechanisms are routinely elucidated using the kinetic isotope effect, e.g. the halogenation of toluene:
In this particular "intramolecular KIE" study the radical substitution of hydrogen by bromine is examined in the reaction involving N-bromosuccinimide as the brominating agent. With mono-deuterated toluene (obtained by organic reduction of benzyl chloride with zinc and deuterated acetic acid), it was found that hydrogen is replaced by bromine faster than deuterium, the reaction product gets enriched in deuterium. A KIE of 4.86 was determined. This finding is in accordance with the general accepted view of a radical substitution in which a hydrogen atom is removed by a bromine free radical species in the rate-determining step.
A large KIE of 5.56 is also reported for reaction of ketones with bromine and sodium hydroxide forming a haloketone with the α-carbonyl positions deuterated.
In this reaction the rate-limiting step is enolate formation by proton (deuterium) abstraction from the ketone by base. In this study the KIE is calculated from the reaction rate constants for regular 2,4-dimethyl-3-pentanone and its deuterated isomer by optical density measurements.
Read more about this topic: Kinetic Isotope Effect