Among the free-radical fluorination, chlorination and bromination of an alkane, the kinetic isotope effect (k H /k D ) for replacing H by D is smallest for :

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Fluorination is very exothermic, so its transition state comes very early and the C–H bond has hardly started to break. Little of the C–H versus C–D zero-point energy difference is lost, so k H /k D is closest to 1. The later the transition state (Cl, then Br), the larger the isotope effect.

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