Using Incoming Group 11C/14C Kinetic Isotope Effects To Model the Transition States for the SN2 Reactions between Para-Substituted Benzyl Chlorides and Labeled Cyanide Ion

A large incoming group 11 C/ 14 C kinetic isotope effect of 1.0104 ± 0.0023 has been measured for the S N 2 reaction between benzyl chloride and labeled cyanide ion in 20% aqueous DMSO at 30.00 °C. These large incoming group kinetic isotope effects have also been measured for the S N 2 reactions of other para -substituted benzyl chlorides with cyanide ion. A combination of the incoming group 11 C/ 14 C and chlorine (Hill J. W.; Fry, A., J. Am. Chem. Soc . 1962, 84, 2763) leaving group kinetic isotope effects have been used to model the S N 2 transition states for these reactions. Adding a more electron-withdrawing para substituent to the substrate does not affect the stronger N⋮C- - -C α reacting bond significantly but shortens the weaker C α - - -Cl reacting bond markedly.