Kinetic Isotope Effect in the Chromic Acid Oxidation of Secondary Alcohols

The kinetic isotope effect is an invaluable tool in studying certain organic reaction mechanisms. Two activities involving the technique that are suitable for introductory organic laboratory students are described. A simple competition experiment utilizing the benzhydrol–benzhydrol-d₁ system and chromic acid oxidation is used to demonstrate qualitatively that there is a kinetic isotope effect involved in this process. GC-MS operating in the "specific ion monitoring" mode is used for analysis. As a second experiment, a procedure for the quantitative determination of k_H/k_D in this system is described. The rates of oxidation of the nondeuterated and the deuterated alcohol are followed in separate reactions. Concentrations are determined by GC using an internal standard. Examples of student results for both activities are presented. Both procedures are general and may be used with other alcohols that exhibit strong molecular-ion peaks in their mass spectra. These experiments are ideal for small groups of students and are suited to being performed along with work already being done in the laboratory program.