Influence of Substituents on Kinetic Isotope Effects

The E2 reactions of methyl- and fluoro-substituted ethyl chlorides with CH 3 -, NH 2 -, OH -, F -, SH -, and Cl - have been investigated at the MP2/6-31+G(d) level of theory for probing whether kinetic isotope effects are related to the geometry of the transition stucture. The primary kinetic isotope effects only display the expected maximum for the most symmetric transition structure when corrections for equilibrium isotope effects are taken into account. Such corrections are in general not possible from experimental data alone. Secondary kinetic isotope effects at the CH 2 Cl end of ethyl chloride correlate qualitatively with the carbon hybridization, although a quantitative relationship is lacking, even for closely related systems. The corresponding isotope effects at the carbon where a hydrogen is abstracted show little correlation with the geometry of the transition state.