Dehydration of Cyclohexanol

Introduction:

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 :

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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 crystal structure. 


Purpose
:

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.


Procedure
:

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 STOPPERED CONTAINER.

  1. Weigh a 250mL two necked round bottom flask to three decimal places.
  2. 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..
  3. Add 50 mL of 65% sulfuric acid and a magnetic stir bar to the round bottom flask.
  4. 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.
  5. 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.
  6. 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.
  7. The cyclohexene will distill over as a water azeotrope somewhere around 65-67 C.   Collect the distillate in the 50mL receiver flask.
  8. 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!
  9. You should notice that the azeotrope forms two layers upon cooling.  Which one is your product?
  10. Transfer the contents of the receiver to a separatory funnel and remove the aqueous layer.
  11. 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.
  12. 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.
  13. 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.
  14. 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.
  15. Transfer your product to a pre-weighed capped vial and determine the weight.  Determine the purity by gas chromotagraphy (GC)..
  16. 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 provided.
  17. 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.
  18. Turn in your product in a labeled, tightly closed container. 

 

Conclusions: As part of your conclusion, address the following:


(Created by C. Snelling

(Updated 06/14/10 by J. Neilan)