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 :

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 can not 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.  However, there is no free lunch.  While these drying agents are very efficient at removing small amount of water, their use does add another separation step.  Traditionally, this has been accomplished using gravity or vacuum filtration with yet another loss of product.  However, we will use centrifugation, which is faster and minimizes loss of product.


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 (General Chemistry Links).  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.

  1. Accurately measure to 0.1mL approximately 50 mL of cyclohexanol into a clean, dry 250 mL round bottom flask.  Use the density to determine the mass of the cyclohexanol
  2. Add 100 mL of 65% sulfuric acid, a couple of boiling chips, and a magnetic stir bar to the round bottom flask.
  3. Fit the flask with two thermometers.  One will be used to monitor the temperature of the solution, and the other will be used to monitor the temperature of the vapor above the solution. 
  4. Attach the flask directly to a distilling adapter (position the thermometer so the bulb is completely below the take off) and a condenser.  Remember to lubricate all ground glass joints with a minimal amount of silicon grease.  Make sure that all joints are secured (blue clips) and that the whole apparatus is properly clamped.  Remember:  heating mantels are NEVER plugged directly into a 120 V outlet, only into a Variac.  Have your apparatus checked by your instructor before continuing.
  5. 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.
  6. The cyclohexene will distill over as a water azeotrope somewhere around 65-67 °C.   Collect the distillate in a clean 50 mL round bottomed flask.
  7. 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!
  8. You should notice that the azeotrope forms two layers upon cooling.  Which one is your product?
  9. Use your separatory funnel to remove the aqueous layer.
  10. CAREFULLY wash the organic layer with 50 mL 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.
  11. Remove the aqueous layer and transfer the crude product to a clean, dry Erlenmeyer flask.
  12. 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.
  13. Transfer the dried product and magnesium sulfate to a clean, dry centrifuge tube and centrifuge for a minute.  This will force the drying agent to the bottom and make it easier to quantitatively remove the cyclohexene.
  14. Transfer your product to a pre-weighed tube or other container and determine the weight.  Determine the purity by gas chromotagraphy (GC).
  15. 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 pure cyclohexene.
  16. 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.
  17. Turn in your product in a labeled, tightly closed container.  You will be using this product next semester to synthesis adipic acid and then nylon-6,6.

Your Name:

Class/Section:

Date:

Compound:

B.P.:

R.I.:

Actual Yield (g):

Theoretical Yield (g):

Percent Yield:

Purity (FTIR):

Purity (RI):

 

 

Conclusions:

  • Calculate the theoretical, actual, and percent yields for cyclohexene.
  • 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?


 (Updated 11/27/05 by J. Neilan)