CH 227
What Did You Do?
What Did You Observe?
What Does It Mean?
Synthesis of Cyclohexene from Cyclohexanol (Adapted from Laboratory Manual to
Accompany Organic Chemistry: A Short Course, H. Hart, L. E. Craine, D. J. Hart, and T.K. Vinod
12th ed. Houghton-Mifflin, Boston, 2007.)
Introduction
One general synthetic method used to prepare alkenes involves dehydration of an alcohol.
Reaction:
OH H
C
C
+
C C
H OH
A strong and high-boiling mineral acid, such as sulfuric acid or phosphoric acid, is the catalyst for the
reaction. The acid protonates the alcohol. Subsequently, a molecule of water and a proton are
eliminated.
Mechanism:
H
OH
C
C
H
H
E2
H
O
C
-H
-H
C
-H2O
H
E1
C C
-H
-H2O
C
C
H
If the alcohol is tertiary or has other structural features that stabilize the corresponding carbocation,
the elimination usually proceeds stepwise (E1 mechanism). Alternatively, if the alcohol is primary,
the loss of H2O and H+ usually occurs in one step (E2 mechanism). If the alcohol is secondary, either
(or both) mechanism(s) is (are) possible. If the alkyl groups attached to the hydroxyl-bearing carbon
atom are different from one another it is possible to obtain two or more different alkenes. In this
experiment, the symmetrical alcohol cyclohexanol produces one alkene.
In this experiment, the secondary alcohol cyclohexanol will be dehydrated to cyclohexene:
H
OH
H
+
H2O
heat
cyclohexanol
cyclohexene
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Experimentally, advantage is taken of the fact that alkenes boil at much lower temperatures than the
alcohols from which they are prepared. The alcohol is heated (with acid) to a temperature above the
boiling point of the alkene but below that of the alcohol. The alkene and water distill from the
reaction flask as they are formed, whereas the unchanged alcohol remains behind to be further acted
on by the acid. In the present case, the dehydration is carried out at 130-140 oC, which is above the
boiling point of cyclohexene (83 oC) but below that of cyclohexanol (161 oC).
Caution: Sulfuric acid is a corrosive liquid. Avoid contact of this acid with your skin or clothing.
Wear disposable gloves. If any should splash on you, wash the affected area with running water for at
least five minutes and notify your instructor. Watch carefully for drips and runs on the outside
surfaces of reagent bottles and graduated cylinders when you pick them up. Apply sodium
bicarbonate (NaHCO3) solution to the area. This can be found in a squirt bottle in the hood with the
other squirt bottles. Wipe this up with your sponge before you leave lab. This prevents those
unsightly white residues on the bench tops.
Rinse all ground-glass apparatus with acetone before you return it to the stockroom.
Procedure
Add 8 mL (8 g, 0.08 mol) of cyclohexanol (specific gravity – 0.96 g/mL) and 2 mL of concentrated
sulfuric acid to a 25 mL round-bottomed flask. Add a small magnetic stir bar to the flask to the flask.
Arrange a simple distillation apparatus. (SEE BELOW) After the apparatus is assembled begin
stirring the mixture to create a homogeneous solution.
This will show you how to set up your apparatus and what it should look like. Use a minimal amount
of sand in the sand bath. You only want enough so that the sand barely touches the distillation flask.
To take advantage of the lab jack, raise it up several inches before assembling your distillation
apparatus.
DISTILLATION APPARATUS
Heat the distillation flask and observe the temperature of the distilling vapor. Make sure the
temperature does not exceed 100 oC. Continue the distillation until only a few milliliters of high-
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boiling residue remain in the flask. If white fumes appear near the end of the distillation, stop heating
at once by lowering the heat source; these fumes are oxides of sulfur.
The distillate in the receiving flask will contain some water, even if you can’t see it. Add
anhydrous calcium chloride to the cyclohexene and allow it to stand for 10 – 15 min, swirling it
occasionally. The calcium chloride is used to dry (that is, remove traces of water from) the
cyclohexene. The product should be clear, not cloudy. Decant or filter the cyclohexene from the
calcium chloride into a labeled, tared vial. Weigh the cyclohexene in the vial and calculate the
percentage yield. Save your product for the next experiment.
Put the calcium chloride in the “SOLID ORGANIC WASTE” container on the side shelf.
Cyclohexene has “an intense aversive characteristic sharp smell reminiscent of an oil refinery.”
(Wikipedia) Therefore, do not subject yourself or you fellow students to the odor of cyclohexene.
Keep it in the hood at all times, and keep it tightly sealed when you must move it from the hood; for
example, when weighing it in the balance room.
Clean Up
If you have a black residue in your round-bottomed distillation flask, add water and pour it into the
bottle in the hood labeled “ACID WASTE.” Take care not to pour the magnetic stir bar into the acid
waste bottle while you are doing this. Then rinse the flask with acetone, swirl and pour that into the
organic liquid waste bottle. Finally, add dichloromethane (DCM), swirl, and pour that into the
halogenated organic waste bottle. The DCM procedure may need to be repeated multiple times in
order to clean the black residue from the flask.
References:
Lab Web Page: http://chemed.ces.clemson.edu/SOL/SuperOrganicLab/
“The Organic Laboratory Survival Manual” 8th Ed., p. 164.
“Laboratory Manual: Organic Chemistry: A Short Course”, H. Hart, L. E. Craine, D. J. Hart,
and T.K. Vinod 12th ed. Houghton-Mifflin, Boston, 2007; p 107.
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