Illustrated Glossary of Organic Chemistry

Kinetic isotope effect (KIE): The ratio of reaction rates for two reactions that differ only by one or more isotopic substitutions (for example hydrogen versus deuterium) among atoms in the reaction. The magnitude of the ratio can be used to confirm or disprove a mechanism, as well as determine the mechanism's rate-determining step.


This deprotonation reaction displays a kinetic isotope effect: The ratio of reaction rates for X = H (KH) versus X = D (KD) is KH/KD = 10. This indicates that the C-X bond is broken during the rate-determining step for this reaction.