We examined cyclohexanol dehydration in pure water at temperatures of 250, 275, 300, 350, and 380 °C with water densities ranging from 0.08 to 0.81 g/cm3. Under these conditions, cyclohexanol dehydrates readily in the absence of added catalysts to form cyclohexene as the major product. The most abundant minor products are 1- and 3-methyl cyclopentenes. The reaction rate and product distribution at 380 °C show a remarkable sensitivity to the water density. At low densities, the reaction is slow, and cyclohexene is the only product. At high densities, the reaction is nearly complete, and methyl cyclopentenes appear along with cyclohexene. The experimental results implied a reaction mechanism that comprises two pathways: (1) reversible cyclohexanol dehydration to form cyclohexene through an E2 mechanism, and (2) subsequent cyclohexene protonation to form the cyclohexyl cation, which rapidly rearranges to form methyl cyclopentyl cations, which then lose a proton to form methyl cyclopentenes. A kinetics model based on the proposed mechanism was able to predict the striking effect of the water density on the product yields at 380 °C and, thereby, to demonstrate that the proposed mechanism captures the trends in the experimental data. An analysis of mechanistic issues regarding cyclohexanol dehydration in high-temperature water (HTW) revealed three roles for water. Water participates in elementary reaction steps as a reactant and as a product, water is the source of the acid catalyst (H3O+), and water also drives the mechanism toward E2 by favoring, through solvation, the oxonium ion rather than the carbocation as the reaction intermediate. This study provides further evidence that acid-catalyzed reactions can be accomplished readily in HTW in the absence of added acid and that HTW has potential applications in environmentally benign industrial chemistry.
All Science Journal Classification (ASJC) codes
- Chemical Engineering(all)
- Industrial and Manufacturing Engineering