Dehydration of Cyclohexanol to Produce Cyclohexene

Dehydration of Cyclohexanol to Produce
Cyclohexene
Oxidation of Cyclohexene to Produce
Adipic Acid
Chemical Concepts
Dehydration of alcohols to produce alkenes; multistep syntheses;
liquid-liquid extractions; drying agents; simple and fractional
distillation; boiling point determination; infrared (IR) spectroscopy
Oxidative cleavage of an alkene carbon-carbon double bond;
phase transfer catalysis; recrystallization; melting point
determination
Green Lessons
Less hazardous chemical syntheses; solventless reactions; "green"
redox agents
Estimated Lab Time: 6 hours
Introduction and Reaction Mechanism
This is a two part experiment. In Part I we prepare an alkene by dehydrating an
alcohol. In Part II we subject the alkene to vigorous oxidation conditions to form a
dicarboxylic acid.
The elimination reaction makes use of phosphoric acid to catalyze the reaction
rather than the traditional strong acid, sulfuric acid. Sulfuric acid is highly corrosive, and it
has a tendency to react with organic compounds producing dark colored residues
(“road tar” in common lab conversation). Phosphoric acid is less corrosive than sulfuric
acid, and it is less likely to react with your starting material. The chemistry of this
elimination reaction is made green by the use of a less toxic acid and by the lack of a
solvent. The reactants are all liquids that mix together.
H
H
H
H
H
H+
:O
..
+
+
OH
OH2
H
OH2
H
H
H
H
H
-H2O
+
H
H
H
H
H
-H+
+ H3O+
Dehydration of Cyclohexanol and Oxidation of Cyclohexene
Notice that the hydroxide group is converted into a better leaving group (H 2O) by the
addition of an acid. The H+ is regenerated at the end as a catalyst should be. All the
reactions are reversible.
A carbon-carbon double bond is a site of relatively high electron density. As
such, it is susceptible to oxidation. Vigorous conditions will result in cleavage of the
double bond forming ketones and aldehydes (which are further oxidized to acids)
depending on the substitution on the alkene.
O
O
HO
OH
cyclohexene
1,6-hexanedioic acid or adipic acid
In the laboratory, hot basic potassium permanganate solution is often used to
accomplish this oxidation. The KMnO4 must be used in stoichiometric amounts. Large
quantities of MnO2 waste are generated. In industry nitric acid is used to produce adipic
acid. This method presents many chemical safety and environmental risks. Nitric acid
can react violently with organic compounds, and serious accidents have been reported.
Also the industrial process produces N2O emissions (a suspected greenhouse gas).
We will use sodium tungstate, Na2WO4, as a catalyst for the oxidation of
cyclohexene with hydrogen peroxide. Only a catalytic amount of sodium tungstate is
required, and water is generated as a product. This avoids the decidedly “ungreen”
strongly basic potassium permanganate reaction with the MnO 2 waste. The net reaction
may be written as follows"
O
O
4 H2O2 +
HO
+ 4 H2O
OH
cyclohexene
1,6-hexanedioic acid or adipic acid
Sodium tungstate is water-soluble, but cyclohexene is not. To overcome the problem of
a reaction mixture with two immiscible liquids we will use a “phase transfer catalyst”.
Ammonium salts bearing hydrophobic groups are ionic, but they are much less polar
than water. The diagram on the next page shows how this phase transfer catalyst works.
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Dehydration of Cyclohexanol and Oxidation of Cyclohexene
adipic acid
2 R4N+X-
(R4N)2WO4
(R4N)2[reduced W]
2 R4N+Xorganic phase
aqueous phase
Na2WO4
2 NaX
2 NaX
Na2[reduced W]
H2O2
Phase transfer catalysis
K. Sato, M. Aoki, and R. Noyori, “A ‘Green’ Route to Adipic Acid: Direct Oxidation of
Cyclohexenes with 30 Percent Hydroxide Peroxide,” Science 1998, 281, 1646
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Dehydration of Cyclohexanol and Oxidation of Cyclohexene
Dehydration of Cyclohexanol
Experimental Procedure
Pre-Lab Preparation
Carry out pre-lab preparations as called for by your instructor.
Safety considerations
Wear suitable protective clothing, gloves, and eye/face protection!
You should read the online MSDS for:
cyclohexanol
phosphoric acid
sodium sulfate
cyclohexene
Reaction
1. To a 25-mL round-bottom flask containing a magnetic stir bar (or boiling stone), add
0.074 moles of cyclohexanol and 1.75 mL of 85% H3PO4. Use gentle swirling to mix the
two layers.
2. Fit the flask with a fractionating column, a distillation adapter, a thermometer, a
condenser, and a vacuum adapter as for fractional distillation (see illustration). A rubber
septum should be used to provide a seal between the thermocouple or thermometer
and the glassware. Be sure that the seal is good — if it is not, cyclohexene will escape
from your glassware, causing your experiment to fail, and your classmates who find the
odor of cyclohexene objectionable will complain loudly! A drying tube, as shown in the
illustration on the next page, can help to control the disagreeable odor of cyclohexene.
3. Heat the reaction mixture first at a gentle reflux for about 5 minutes. Then heat the
flask more strongly in order to distill the mixture into the collection flask. Keep distilling
until the volume remaining in the distillation flask has been reduced to approximately 1
mL.
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Dehydration of Cyclohexanol and Oxidation of Cyclohexene
Apparatus for dehydration of cvclohexanol
Workup
4. Transfer the distillate to a separatory funnel and wash with approximately 5 mL of
water. Carefully separate the layers and transfer the organic layer into a small, dry
Erlenmeyer flask. If any water droplets are visible, remove them before adding the drying
agent (sodium sulfate). Add a small amount (~ 1 g/25 mL liquid) of anhydrous sodium
sulfate to the flask. Let the mixture stand for 5 minutes, occasionally swirling it gently. If the
drying agent completely clumps together, its capacity to remove water has been
exceeded and a little more sodium sulfate should be added. If you have successfully
removed the water, the liquid should be clear, and at least a little of the drying agent
should remain free flowing.
5. Decant or pipette the organic liquid away from the drying agent and place it in a
clean, dry round-bottom flask. This will be the distillation flask for the next step. The
appropriate size depends upon your yield. The flask should be about half full at the
beginning of the distillation.
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Dehydration of Cyclohexanol and Oxidation of Cyclohexene
Distillation
6. Fit the flask with a distillation adapter and condenser in preparation for a simple
distillation. The apparatus will look the same as that used for fractional distillation, except
that there will be no Vigreux or other fractionating column.
7. Be sure that your thermometer is properly positioned in order to measure the
temperature of the distilling liquid accurately (see the illustration above). Carefully distill
the organic material, collecting the material that distills in the range of 80 — 90°C.
Typically there will he very little material remaining in the distillation flask. Be sure to
record the boiling range that you observe.
Characterization
8. Transfer the distilled cyclohexene to a clean, dry, pre-weighed sample vial and
determine the mass of the product. If time permits, record an infrared spectrum of the
distilled product.
Storage
9. You will need the cyclohexene for the synthesis of adipic acid. Keep it in a well-sealed
and suitably labeled sample vial until then.
Post-Lab Questions and Exercises
1. Describe the color and state of your purified product. Report the mass and percent of
theoretical yield of the purified product.
2. What boiling point range did you observe during your (a) initial distillation and (b) your
final distillation? How do you explain the difference between these, if there was one?
3. What does the phosphoric acid do in the reaction?
4. Interpret your IR spectrum. Assign vibrations for the major peaks in the spectrum.
5. One of the objectives of green chemistry is to prevent waste. Hence, synthetic
methods should attempt to incorporate all the starting reagents into the final product. If
all starting reagents are incorporated into the final product, the atom economy is said to
be 100%. Atom economy can be calculated using the following equation:
% atom economy 
FormulaM ass of Product
 100
 (FormulaM asses of All ReactantsUsed)
Calculate the atom economy for the reaction.
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Dehydration of Cyclohexanol and Oxidation of Cyclohexene
Synthesis of Adipic Acid
Experimental Procedure
Pre-Lab Preparation
Carry out pre-lab preparations as called for by your instructor.
Safety considerations
Wear suitable protective clothing, gloves, and eye/face protection!
You should read the online MSDS for:
adipic acid
cyclohexene
potassium bisulfate
Aliquat 336
hydrogen peroxide
sodium tungstate dihydrate
Note: You need at least 2 g of cyclohexene to perform this experiment. If you did not
obtain this quantity of cyclohexene from the prior experiment, obtain the necessary
amount of cyclohexene from your instructor.
Reaction
I. Place 0.50 g of sodium tungstate dihydrate (Na2WO42H2O) in a 50-mL round-bottom
flask containing a stir bar and fitted with a water-cooled condenser.
Notes: The efficient stirring important for the success of this reaction is more easily
achieved in a round-bottom flask than in a pear-shaped flask. An efficient water-cooled
condenser is required to avoid loss of cvclohexene during the reaction.
2. Add 0.5 g of Aliquat 336 — this is a very viscous liquid that is hard to transfer, so weigh it
directly into your reaction flask. It is not necessary to obtain exactly 0.50 g. Next, add
11.98 g of 30% hydrogen peroxide and 0.37 g of KHSO4 to the reaction mixture. Stir, then
add 2.00 g of cyclohexene.
Note: The order of addition of the reagents is important.
3. Heat the mixture to reflux on a sand bath, then continue to heat at reflux for I hour
while stirring vigorously. About halfway through the reflux period, rinse down any
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Dehydration of Cyclohexanol and Oxidation of Cyclohexene
cyclohexene trapped in the condenser with a few mL of water, added via pipette. Phase
transfer catalysis depends upon very efficient mixing of the organic and aqueous layers,
so it is important to stir as fast as possible throughout the reaction. (Generally, when using
a magnetic stirrer, the closer your flask is to the surface of the stirrer, the easier it is to
maintain rapid stirring.) The reaction will not proceed if it is heated at below reflux, but it is
also very important that you do not heat the mixture too strongly. If you do, cyclohexene
may be lost through the top of the condenser. Watch the condenser closely
If you see liquid condensing near the top, you need to reduce the heat. You may need
to remove the flask from the heat source temporarily in order to bring the reflux back
under control. Stop the stirring occasionally to see if there are still two layers present. The
reaction is complete when it no longer separates into two layers.
Workup
4. Use a Pasteur pipette to transfer the hot reaction mixture into a small beaker. Leave
behind any of the phase transfer catalyst that may have separated. (If the catalyst
separates — and it does not always do so — it will stick to the walls of the flask or form a
separate oily layer at the bottom of the flask. Careful execution of this step is the key to a
successful purification. It is better to leave a little of the aqueous solution behind than to
risk contamination of your solution with the phase transfer catalyst.)
5. Cool the beaker containing the reaction mixture rapidly in an ice bath. A precipitate
should form within 20 minutes. Collect the crude product by vacuum filtration using a
Buchner funnel.
6. After the crude material has air-dried, weigh it and determine its melting point.
Purification and Characterization
7. Recrystallize the crude product from the minimum required amount of hot water
(solubility: 160 g/100 mL boiling water). Determine the mass and melting point of the
recrystallized product and (if time permits) obtain its infrared spectrum.
Post-Lab Questions and Exercises
1. Describe the color and melting point range of your crude product.
2. Describe the color and melting point range of your recrystallized product. Report
the mass and percent of theoretical yield of the recrystallized product.
3. Attach the IR spectrum and identify the major peaks of the spectrum.
4. Calculate the atom economy for the reaction.
Modified March 18, 2015 by Sharmaine S. Robinson, East Stroudsburg University
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