Synthesis of Cyclohexene from Cyclohexanol by Acid Catalyzed ( E1 ) Elimination

 

Goal

          This experiment is designed to demonstrate a simple method for forming an alkene from a secondary alcohol by means of acid-catalyzed dehydration.  Specifically, cyclohexanol is heated in the presence of concentrated phosphoric acid to cause an E1 elimination reaction. 

 

 

 

Background

 

Elimination reactions.  In this experiment, cyclohexanol will be dehydrated (loss of H2O) to form cyclohexene under acidic conditions (see Figure 1). 

 

 

Figure 1. The overall reaction.

 

The substrate or starting material is cyclohexanol (R-OH).  Phosphoric acid is present as a catalyst which promotes the reaction but is not consumed in it.  The hydroxyl group in R-OH is a poor-leaving group because it would have to leave as a hydroxide ion (HO-).  Therefore, an acid is used to protonate the alcohol (step 1) and form R-OH2+ (see Figure 2).  Thus, water (a much better leaving group) is the leaving group in this reaction (step 2) and the product is a secondary carbocation.  In the following step (step 3), a molecule of water deprotonates the carbocation at either of the adjacent carbons.  The remaining electrons flow towards the positive charge producing a p–bond between the carbons and forming a double bond. 

 

 

 

 

 

 


Figure 2.  The reaction mechanism for this experiment.

 

 

Procedure

          Measure out the cyclohexanol (30 mL) and determine its mass.  Place it into a round bottomed flask no less than 100mL volume.   Add 8 mL of concentrated H3PO4  and a boiling chip. Cork the flask and gently swirl the mixture. Use a collection flask or beaker that is no less than 50 mL in volume.  Gently heat the mixture to no more than 103 oC degrees and continue heating.  Distill the mixture until about 5mL of liquid remains in the round bottom flask. 

            The distillate will be a mixture of water and cyclohexene.  To this mixture add NaCl until saturation (no more will dissolve).   Next add enough of a 10% solution of NaHCO3 to make the solution basic to pH paper test.  Transfer the mixture to a separatory funnel and remove the aqueous phase.  Dry the organic phase using anhydrous sodium sulfate.  Transfer the contents of the organic phase to a dry round bottom flask.  You may need to add a toluene chaser if your volume is small.  Next, redistill by simple distillation. Be sure that the temperature never exceeds 85 degrees.   Record the boiling point, and the mass and volume of the pure product.  From this you will determine the density of the distillate.  how do the physical properties of the distillate compare to the accepted values for cyclohexene? 

Transfer approximately 2ml of your product to a dry test tube.  Into a second test tube transfer approximately 2ml of cyclohexanol.  From the dropping bottle provided, transfer 5 drops of 5% Bromine in 1,2dichlorocyclohexane.  Note the color of each solution after addition of the bromine solution.  A positive result for the formation of cyclohexene is indicated by a clear colorless solution upon addition of bromine to your product.