Download Cyclohexene.pdf and more Slides Organic Chemistry in PDF only on Docsity!
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:
C
OH C
H 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:
C
OH C H
H -H
C
O C
H
H H
E1 (^) -H 2 O
E
-H
-H 2 O
C C
C C
H
-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 H 2 O 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 heat
cyclohexanol cyclohexene
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 (NaHCO 3 ) 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-
