Download Dehydration of Cyclohexanol

Dehydration of Cyclohexanol
Introduction:
The purpose of this experiment is to dehydrate cyclohexanol to form cyclohexene
and to characterize the product using simple chemical tests.
OH
85% H3PO4
cat. H2SO4
Δ
cyclohexanol
cyclohexene
Secondary alcohols can easily be converted to alkenes by acid-catalyzed
dehydration, an E1 reaction. In this experiment, a mixture of concentrated phosphoric
acid and concentrated sulfuric acid is used to prepare cyclohexene from cyclohexanol via
an E1 reaction.
The mechanism of this E1 reaction involves three steps. First, is the rapid (and
reversible) protonation of the alcohol, followed by the much slower rate-determining
step, the loss of water to give a secondary carbocation. In the final step, a base (water or
the conjugate base of one of the acids) removes a β-hydrogen to form the alkene.
Characterization of the alkene product takes advantage of the fact that the
electrons of the π-bond can act as nucleophiles. Addition of bromine (Br2) across the
double bond gives a vicinal dibromide as the product. The bromine solution is red, and
the product is colorless. The disappearance of the red color of the bromine solution is
therefore considered a positive test result for the presence of an alkene. (see section 8.8 in
your textbook for more details). The reaction of an alkene with a dilute solution of
permanganate (KMnO4) results in the formation of a 1,2-diol. The permanganate solution
is purple, and when the solution is added to an alkene, the purple color disappears and
solution turns a murky, opaque brown color (from the MnO2 biproduct). The color
change from purple to brown is considered a positive test result for the presence of an
alkene.
Procedure:
Part A: Synthesis of cyclohexene
To a 5 mL conical vial add a boiling chip, 2.0 mL of cyclohexanol, 0.5 mL of 85%
H3PO4, and eight drops of conc. H2SO4. Fasten the Hickmann still to the conical vial and
secure it to the ring stand using the clamp. Below the conical vial place the sand bath on
the hot plate and begin heating. Cover the bottom with sand and push sand up on the
sides of the conical vial to achieve uniform heating. Place the air condenser on the top of
the Hickmann still to contain the product vapors.
Continue heating until the mixture begins to boil. The cyclohexene product will be the
first component to distill and will collect in the reservoir of the Hickmann still. As the
product fills the reservoir, remove the air condenser and pipette the product into a vial
with a tight fitting cap. Replace the air condenser and continue distilling until no more
product forms.
After distillation of the product is completed remove the apparatus from the heat. Using
0.5 mL portions of saturated NaCl solution rinse the inside walls of the air condenser and
the Hickmann still. Pipette this rinse solution out of the Hickmann still reservoir and
combine it with the product in the vial. Repeat the rinse two more times.
Cap the vial containing the product and the saturated NaCl solution and shake the
mixture. Let the mixture separate and remove the aqueous layer using a pipet (Hint: The
density of cyclohexene is 0.811 g/mL, and the density of water is 1.00 g/mL). Dry the
remaining cyclohexene layer using anhydrous sodium sulfate. Pipet the liquid into a preweighed vial with a cap, and obtain a mass. (Pre-weigh the vial with the cap on so that
you can weigh your product with the cap on. Cyclohexene is volatile, and it smells bad!!)
Part B: Qualitative Chemical Tests for Reactivity
Label four small test tubes A-D. To test tubes A and B add four drops of
cyclohexanol and to test tubes C and D add ten drops of the cyclohexene product. To test
tubes A and C add two drops of the KMnO4 solution and record your observations. Do
one test tube at a time to effectively observe the results. To test tubes B and D add two
drops of Br2/CH2Cl2 solution and record your observations.
Safety Considerations:
- Take care when using the concentrated acid to not get it on your skin or clothing.
- Use the bromine solution in the fumehood, and wear gloves!
- There will be a waste container for organic liquid waste
- Dispose of used glass pipets in the broken glass container
Data and Calculations:
Obtain the mass of the cyclohexene product and calculate
theoretical and % yields. Record the observations of the simple chemical tests and draw
conclusions that illustrate the success or failure of the E1 reaction. (ie.- Did you
successfully form the alkene or not?). Draw a complete mechanism for the dehydration
reaction. Represent the cyclohexanol as a chair structure for your mechanism.
Example of a Hickmann still distillation apparatus