Dehydration of Cyclohexanol
There are four basic types of chemical reactions in organic chemistry:
combination, elimination, substitution, and rearrangement. Today, you
will conduct your first organic synthesis: you will produce cyclohexene
through the acid catalyzed elimination of water from cyclohexanol
(dehydration). The overall reaction and mechanism is shown below :
Unfortunately, substitution reactions always compete with
elimination reactions. However, elimination reactions are favored by high
temperatures and acids whose conjugate bases are poor nucleophiles, whereas
substitution reactions are favored by lower temperatures and acid whose
conjugate bases are good nucleophiles, such as HCl.
Therefore, we will be conducting this experiment at relatively high
temperatures and we will also be using sulfuric acid as the catalyst, since HSO4-
is a poor nucleophile. Also, we will be
removing both the cyclohexene and the water from the reaction (in the form of an
azeotrope) as they are formed to force the
elimination reaction to completion (remember Le Chatelier
and his principle).
Finally, even after you have collected all of the cyclohexene, it is not
pure because it distilled as an azeotrope with
water. Azeotropes are particularly troublesome
because they are mixtures with a constant boiling point and so cannot be
separated into their components by distillation. We are lucky that one of
the components of our azeotrope is water, since it is
not appreciably soluble in cyclohexene .
Moreover, the trace amount of water in the cyclohexene can be removed by the
use of a chemical 'drying agent' such as calcium chloride, sodium sulfate,
magnesium sulfate, or zeolites to remove the
water. These drying agents form hydrates that trap the water in their
The purpose of this lab is produce cyclohexene through the
acid catalyzed elimination of water from cyclohexanol (dehydration). You
will see how the experimental conditions can be controlled to favor the desired
elimination product over other undesirable substitution products. The
product will be removed from the starting material by simple
distillation. Finally, you will use GC and several chemical tests to
determine the purity of your product.
Before you come into lab, make sure you have filled in your table of reagents
and products. You will need these values (particularly the molecular
formula and molecular weight) to determine the identity of your products and to
calculate your final yield. You will also need to come to lab with IRs of
your starting materials and expected product(s) already in your notebooks (Spectral
Database). Finally, it is important that you know exactly what
you are going to be doing so you can work more efficiently:
CYCLOHEXENE HAS A PARTICULARLY OBJECTIONABLE ODOR. IT IS IMPORTANT TO CARRY OUT ALL OPERATIONS UNDER THE SNORKLE HOODS OR IN THE
BENCH HOODS. THIS INCLUDES THE
SEPARATION STEPS USING THE SEPARATORY FUNNEL.
WEIGHING OPERATIONS CONTAINING CYCLOHEXENE SHOULD BE DONE IN A CAPPED OR
- Weigh a 250mL two necked
round bottom flask to three decimal places.
- Place 25mL of cyclohexanol in
the flask and re-weigh the flask.
Determine how many grams of cyclohexanol have been added. Clamp the flask securely to your support
rod at your lab station at a sufficient height that a lab jack, magnetic
stirrer and a heating mantle will fit under the flask..
- Add 50 mL of 65% sulfuric
acid and a magnetic stir bar to the round bottom flask.
- Fit the flask for a simple
distillation, using a distillation take off adapter, condenser and 50mL
round bottom flask to serve as the distillate receiver. Clamp the apparatus securely and place a
heating mantle and magnetic stirrer under the reaction flask. As for any distillation set up, make
sure there is a pressure vent somewhere in the system. Make sure the vent is under the snorkel hood.
- Place a thermometer in the
distillation adapter to monitor the vapor temperature and a second
thermometer in the thermometer well of the flask to monitor the liquid
temperature. Remember to lubricate
all ground glass joint connections with a small amount of silicon
grease. Secure the joints with the
blue clips. Have your apparatus
checked by your lab instructor before continuing.
- Begin the reaction by heating
with about 60-75% power on the Variac. You
may need to raise or lower this to maintain a steady distillation. Turn on the magnetic stirrer to maintain
a steady rate of stirring.
- The cyclohexene will distill
over as a water azeotrope somewhere around 65-67
°C. Collect the distillate in the 50mL receiver flask.
- Continue to distill until the
pot temperature reaches 110 °C. Do not let the temperature get above
110 °C, if it goes any higher, you may distill some of the cyclohexanol
over. Higher temperatures may also cause considerable darkening of
the reaction. DO NOT allow the pot to run dry!
- You should notice that the azeotrope forms two layers upon cooling. Which
one is your product?
- Transfer the contents of the
receiver to a separatory funnel and remove the aqueous layer.
- CAREFULLY wash the organic
layer remaining in the separatory funnel with 25mL of 5% sodium
bicarbonate. Be careful since any acid neutralized with sodium
bicarbonate could generate high gas pressures by formation of CO2
on reacting with acid.
- Remove the aqueous layer and
place it in the aqueous waste in the hood.
Transfer the crude product to a centrifuge tube that can be fitted
with a cap.
- Dry the cyclohexene by adding
a small amount of anhydrous magnesium sulfate. Remember that you
want to see the drying agent 'swirl' like a snow globe, not clump.
Although magnesium sulfate is not as fast as some drying agents, it has
more capacity. Continue to add the
MgSO4 in small portions until the solid remains suspended when
swirled. Let the sample stand with occasional
swirling for 5 minutes to complete the drying.
- Place the tube in a
centrifuge ( make sure it is balanced with a tube
of approximately equal weight) and centrifuge for 1 minute to force the
magnesium sulfate to the bottom and allow the cyclohexene product to be
drawn off with a pipette.
- Transfer your product to a
pre-weighed capped vial and determine the weight. Determine the
purity by gas chromotagraphy (GC)..
- Conduct a bromine test of
your product. Add 3 drops of 4 M bromine (in acetic acid) to a clean
small test tube and then add 2-3 drops of your product. Swirl and
note any changes. Repeat this test with a sample of pure cyclohexene
- Conduct a permanganate test
of your product. Add 3 drops of 2% potassium permanganate to a clean
small test tube that contains approximately 1 mL of ethanol. Now add
2-3 drops of your product, swirl, and note any changes. Repeat this
test with pure cyclohexene.
- Turn in your product in a
labeled, tightly closed container.
As part of your conclusion, address the following:
- Calculate the theoretical,
actual, and percent yields for cyclohexene.
- What steps are likely to
produce significant loss of material resulting in reduced yields?
- Propose any modifications
that may improve product recovery.
- Use your GC data to determine
the purity of your cyclohexene.
- What were the results of your
chemical tests and what do they say about the purity of your product?
- How could you use an IR
spectrum look for residual cyclohexanol?
Is there evidence of cyclohexanol in your product?
(Created by C. Snelling)
(Updated 06/14/10 by J. Neilan)