Using Secondary α Deuterium Kinetic Isotope Effects To Determine the Symmetry of SN2 Transition States

The secondary α deuterium and heavy atom kinetic isotope effects found for two different S N 2 reactions suggest that the magnitude of secondary α deuterium kinetic isotope effects can be determined by the length of only the shorter (stronger) reacting bond in an unsymmetrical S N 2 transition state rather than by the usual nucleophile−leaving group distance. Although this means the interpretation of these isotope effects is more complex than has been recognized, the results suggest that they can be used to determine whether an S N 2 transition state is symmetrical or unsymmetrical.