The Isomerization of (-)Menthone to (+)Isomenthone Catalyzed by an Ion-Exchange Resin: A "Green" Organic Laboratory Experiment
Authors:Nick Baca, Aurora Ginzburg
Mentor:Philip Hampton, Professor of Chemistry, California State University Channel Islands
As part of our research into introducing green chemistry experiments in our chemistry laboratories, we have developed a modification of a classic organic chemistry laboratory experiment, the isomerization of (-)menthone to (+)isomenthone. We have discovered that the mineral acid catalysts (glacial acetic acid and 1 M HCl) traditionally used in this experiment can be replaced with an ion exchange resin, Amberlyst 15. In addition to reducing dramatically the quantity of chemicals used and the waste generated in the experiment, the use of the solid phase Amberlyst 15 catalyst has the additional advantage of allowing samples of the reaction to be analyzed during the course of the isomerization. In the traditional experiment, only the final position of the equilibrium is analyzed by polarimetry and gas chromatography after work-up of the reaction. From graphs of the ratio of peak areas vs. time, both an equilibrium constant and a rate of isomerization can be determined. Since the reaction proceeds across a range of temperatures (60 – 110 °C), one can determine not only the free energy change in the reaction but also the enthalpy and entropy changes. With different catalyst amounts, the experiment allows students to observe the difference between kinetics and thermodynamics since the quantity of catalyst only affects the rate that equilibrium is attained, not the equilibrium constant. These changes to the traditional experiment provide not only a more environmentally-friendly experiment but also a more pedagogically effective experience for the students.