Abstract
AbstractThe use of kinetic isotope effects (KIEs) for the short‐lived radionuclides 11C and 18F in the study of reaction mechanisms is described using some examples. Leaving group fluorine KIEs (k18/k19) have been utilized to determine the rate‐limiting step for nucleophilic aromatic substitution reactions (SNAr). The fluorine KIE was also used to probe the effect of changing solvent and nucleophile steric hindrance on rate‐limiting step. The mechanism for a base promoted elimination reaction was determined to be stepwise (E1cB) by a multiple KIE study including the leaving group fluorine KIE. The transition state structures for aliphatic nucleophilic substitution reactions (SN2) have been investigated by multiple KIE studies for cases where the substrate substitution, leaving group or solvent has been varied. Carbon KIEs for labelled α‐carbon atom in the substrate are large, k11/k14 = 1.189–1.220. For labelled nucleophile cyanide ion, k11/k14 = 0.99951–1.0119. Copyright © 2007 John Wiley & Sons, Ltd.
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