Using kinetic isotope effects to determine the structure of the transition states of SN2 reactions

Abstract

Publisher Summary This chapter focuses on the use of kinetic isotope effects (KIEs) as a method of determining the mechanism and transition state structures of SN2 reactions. A primary KIE is found when the bond to the isotopically labeled atom is breaking or forming in the transition state of the slow step of the reaction. Three basic relationships indicate how the magnitude of a primary KIE varies with transition state structure in SN2 reactions. Because the smallest α-carbon KIE reported for an SN2 reaction is 80% of the largest-observed KIE, the qualitative conclusion would be that all these SN2 reactions have symmetric or almost symmetric transition states. A secondary KIE is observed when the bond to the isotopically substituted atom is not being broken or formed in the transition state of the rate-determining step of the reaction. KIEs still remain one of the most convincing probes of transition state structure and reaction mechanism, especially when applied at several positions in a reaction.