How to measure both primary and secondary hydrogen isotope effects in hydrogen abstraction from a substrate having two or more equivalent reactive hydrogen atoms

Abstract

For a reaction of the type RH/sub n/ + B ..-->.. RH/sub n-1/ + BH, the substrate may be tracer labeled with tritium to give a mixture of RH/sub n/ and RH/sub n-1/T. Determination of the molar activity of BH after a low (< 5%) extent of reaction and of recovered RH/sub n/ after a known extent of reaction permits the precise determination of both the primary and secondary intermolecular kinetic isotope effects. The method is applied to the E2 reaction of PhCHTCH/sub 2/NMe/sub 3//sup +/ with ethoxide ion in ethanol at 40/sup 0/C to give a primary isotope effect of 4.067 +- 0.127 and a secondary isotope effect of 1.311 +- 0.014.