Organic Chemistry

Exp’t 72

Synthesis and Reactions of Alkenes: Cyclohexene from Cyclohexanol

from K. L. Williamson, Macroscale and Microscale Organic Experiments, 2nd Ed. 1994, Houghton Mifflin, Boston d. p268; revised 10/10/98

Prelab Exercise: Prepare a detailed flow sheet for the preparation of cyclohexene, indicating at each step which layer contains the desired product.

Introduction

OH H2SO4 + H2O

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. The procedure selected for this experiment involves catalysis by sulfuric acid. When a mixture of cyclohexanol and sulfuric acid is heated in a flask equipped with a fractionating column, the formation of water is soon evident. On further heating, the water and the cyclohexene formed distill together by the principle of steam distillation, and any high-boiling cyclohexanol that may volatilize is returned to the flask. However, after dehydration is complete and the bulk of the product has distilled, the column remains saturated with water-cyclohexene that merely refluxes and does not distill. Hence, for recovery of otherwise lost reaction product, a “chaser” solvent is added, and distillation is continued. A suitable chaser solvent is the water-immiscible, aromatic solvent toluene, boiling point 110°C; as it steam-distills, it carries over the more volatile cyclohexene. When the total water-insoluble layer is separated, dried, and redistilled through the dried column, the chaser again drives the cyclohexene from the column; the difference in boiling points is such that a sharp separation is possible. The column holdup (retention of distillate) in the metal sponge-packed column is so great that if a chaser solvent is not used in the procedure, the yield will be much lower. The mechanism of this reaction involves initial rapid protonation of the hydroxyl group by the sulfuric...