Montmorillonite-Catalyzed Dehydration of Terpene Alcohols: A Greener Alternative for Organic Chemistry Lab

Montmorillonite was used to catalyze the dehydration of the terpene alcohols menthol, isopulegol, and terpineol. Montmorillonite is a natural clay that serves two roles- as a heterogeneous acid catalyst and as an absorbent of water formed in the reaction. These terpene alcohols are also derived from nature, as these can be extracted from mint and cardamom. These agents are commonly found in fragrances and produce many different isomers (and scents) when done reacting.

The dehydration reactions were carried out using conventional heating with reflux columns and distillation apparatus as well as microwave heating. The reaction proceeds to full conversion when heated in the MW synthesizer at 200 °C for 20 minutes. The identity of the products obtained was established by separation and analysis of the reaction mixture using GCMS. The formation of multiple products can be attributed to the reversibility of the hydration-dehydration reaction and carbocation rearrangements. A mechanism to explain the formation of thermodynamic and kinetic products can be proposed using the relative stability of products and intermediates. These energy values were calculated using the molecular modeling software Spartan.

Currently in the organic chemistry lab, this experiment is done with cyclohexanol and phosphoric acid to produce cyclohexene. This reaction requires the use of concentrated phosphoric acid and the cyclohexene formed has a pungent smell. Therefore, using montmorillonite clay and terpene alcohols, which are derived from nature, provides a greener approach to the laboratory experience. In addition, these reactions require a thorough analysis of the product mixtures using analytical techniques such as FTIR, GCMS, and ¹H-NMR. Students also have the opportunity to compare the relative stability of the carbocation intermediates and subsequent products.