Isotope effects for fluorine-18 and carbon-11 in the study of reaction mechanisms

摘要

The use of kinetic Isotope effects (KIEs) for the short-lived radionuclides ~(11)C and ~(18)F In the study of reaction mechanisms is described using some examples. Leaving group fluorine KIEs (k~(18)/k~(19)) have been utilized to determine the rate-limiting step for nucleophilic aromatic substitution reactions (SnAt). 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 (ElcB) by a multiple KIE study including the leaving group fluorine KIE. The transition state structures for aliphatic nucleophilic substitution reactions (S_N2) have been investigated by multiple KIE studies for cases where the substrate substitution, leaving group or solvent has been varied. Carbon KIEs for labelled a-carbon atom-in the substrate are large, k~(11)/k~(14) = 1.189-1.220. For labelled nucleophile cyanide ion, k~(ll)/k~(14) = 0.99951-1.0119.