Alkenes from Alcohols: Cyclohexene from Cyclohexanol

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Alkenes from Alcohols: Cyclohexene from Cyclohexanol

By | April 2011
Page 1 of 4
I. Objectives
* Be able to prepare cyclohexene from the dehydration of cyclohexanol * Understand the mechanisms of the dehydration reaction (acid-catalyzed dehydration). * Know how to use the necessary equipment for this reaction, such as the fractioning column. * Obtain positive results in unsaturation tests for the presence of carbon-carbon double bond (cyclohexene).

II. Background
Cyclohexanol, the reagent of this experiment, is used in the production of nylon, paints, plastics, detergents, textiles and pesticides. The dehydration of cyclohexanol to cyclohexene can be accomplished by pyrolysis of the cyclic secondary alcohol with an acid catalyst at a moderate temperature or by distillation over alumina or silica gel. We will be performing catalysis by phosphoric acid. The mixture of cyclohexanol with phosphoric acid is heated in a flask equipped with a fractionating column, forming water. Further heating promotes the distillation of water and cyclohexene by the principle of stem distillation. A chaser solvent is added to recover of the reaction product that may get lost in the fractionating column. Cyclohexene itself is used in manufacturing of adipic acid, hexahydrobenzoic acid, maleic acid, cyclohexanol and cyclohexeneoxide. Further, it is used as a solvent.

III. Equations

H3PO4

+ H2O

IV. Mechanisms
Step 1: Protonation of the hydroxyl group (fast equilibrium)

Step 2: Ionization to a carbocation (slow, rate-limiting)

Step 3: Deprotonation of the carbocation to give the alkene

V. References
Macroscale and microscale: Organic experiments, 5th ed. (2007), K.J. Williamson, R. Minard, and K.M Masters. Houghton Mifflin Company. P.p. 351-352. Organic Chemistry, 7th ed. (2010), L.G. Wade, Jr. Prentice Hall. P.p. 308-311 www.wikipedia.org

VI. Table
Name| Structure| Boiling point (°C)| Melting point (°C)| MW(g/mol) | Density (g/ml)| Amount| Cyclohexanol| | 160.84| 25.93| 100.16|...

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