Use of kinetic isotope effects in mechanism studies. Anomalous Arrhenius parameters in the study of elimination reactions

A kinetic study of elimination reactions using the three hydrogen isotopes was performed to determine if an internal reaction mechanism was the cause of the low primary kinetic isotope effect, K/sup H//K/sup D/ = 3.00 at 0/sup 0/. The expected Arrhenius preexponential factor of A/sup H//A/sup D/ = 1.0 +- 0.4/sup 3/ was not observed for these reactions and the anomalous Arrhenius parameters were reported for various dehydrohalogenations. The Swain-Schaad equation and Streitwieser extention were found to be inapplicable in interpreting this data because the isotope effects arose from entropy rather than enthalpy terms. Possible reaction mechanisms were discussed. (DDA)