In the Laboratory
the microscale laboratory
Microscale Dehydration of Cyclohexanol Using a Macroreticular Cation Exchange Resin as Catalyst Harriet P. Moeur, Sharon A. Swatik, and Robert P. Pinnell* W. M. Keck Science Center, Claremont McKenna, Pitzer, and Scripps Colleges, Claremont, CA 91711 The acid-catalyzed dehydration of cyclohexanol is a frequently used experiment in the organic chemistry laboratory (1–3). The acid catalyst is usually concentrated sulfuric acid or a mixture of phosphoric and sulfuric acids. As an alternative, a solid-phase anhydrous macroreticular ion-exchange resin (Amberlyst 15) can be substituted for the liquid acid catalyst. The resin is a highly cross-linked polystyrene matrix substituted with strongly acidic sulfonic acid groups. The experiment is particularly informative if the procedure is carried out by pairs of students, one student using the liquid acid catalyst and the other partner using the resin. Procedure Preheat a sand bath to a temperature of approximately 190–200 °C while the reaction apparatus is being set up. Place 2.0 g of cyclohexanol in each of two 5-mL conical vials. One partner should add 0.20 g of the ion exchange resin (Amberlyst 15) to one vial1 and the other should add a mixture of 0.25 mL of 85% phosphoric acid and 2 drops of concentrated sulfuric acid. Equip each vial with a spin vane or boiling stone and a Hickman distillation head fitted with a water-cooled condenser. Prepare a sampling pipet by heating and carefully bending the bottom 3/4 inch of a 9-in. disposable pipet to such an angle that it will fit through the center column of the condenser into the collecting well of the Hickman head. Gradually heat the mixture until the product begins to distill. Regulate heating so that the distillation takes 30–45 minutes. As crude product distills from the reaction mixture, use the bent pipet to transfer distillate from the head to a stoppered vial. When the distillation is finished, rinse the inside of the head with 1 mL of saturated NaCl solution and add this to the product mixture. Separate the lower aqueous layer, then *Corresponding author.
carefully decant the organic layer into a dry test tube and add ca. 0.2 g of anhydrous Na2SO4 . While the product is drying, clean and dry the Hickman head and spin vane; then reassemble the apparatus with a thermometer suspended through the condenser into the bottom of the head. Pipet the clear organic liquid into a dry 3-mL conical vial and add a boiling stone or spin vane. Redistill the product, collecting the fraction boiling from 82 to 85 °C. Results Comparable yields have been obtained with the resin and the liquid acid mixture. The reaction mixture with the resin catalyst tends to “bump” if heated too strongly; however, very little tar is formed, making the apparatus easier to clean. This dehydration has also been carried out with hydrated sulfonic acid resins (including a student-prepared resin made by sulfonating a cross-linked polystyrene [4]). The yields of alkene obtained were respectable but lower than those with Amberlyst 15. Note 1. Ensure that none of the resin beads are stuck to the inner ground joint of the vial (they may stress the joint upon assembly and lead to breakage).
Literature Cited 1. Hart, H.; Craine, L. E. Laboratory Manual, Organic Chemistry, A Short Course, 8th ed.; Houghton Mifflin: Boston, 1991; p 79. 2. Pavia, D. L.; Lampman, G. M.; Kriz, G. S.; Engel, R. G. Introduction to Organic Laboratory Techniques, A Microscale Approach; Saunders: Fort Worth, 1990; p 152. 3. Williams, K. L. Macroscale and Microscale Organic Experiments, 2nd ed.; Heath: Lexington, MA, 1994; p 278. 4. Pinnell, R. P.; Holboke, A. E. J. Chem. Educ. 1989, 66, 613– 614.
Vol. 74 No. 7 July 1997 • Journal of Chemical Education
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