Practical 5: preparation of cyclohexene from cyclohexanol

• Practical 5: preparation of cyclohexene from cyclohexanol

  1. Pour 20 cm3​ ​of cyclohexanol into a 50 cm3​ ​pear-shaped flask that has been weighed. Reweigh the flask and record the mass of cyclohexanol. This is weighing by difference
• Practical 5: preparation of cyclohexene from cyclohexanol

  2. Using a plastic graduated dropping pipette, carefully add approximately 8.0 cm3​​ of concentrated phosphoric acid to the flask.
• Practical 5: preparation of cyclohexene from cyclohexanol

  3. Add a few anti-bumping granules to the flask. Heat the flask gently, distilling over any liquid which boils below 100 °C. As the cyclohexene has a lower boiling point
• Practical 5: preparation of cyclohexene from cyclohexanol

  4. Pour the distillate into a separating funnel and add 50 cm of saturated sodium chloride solution. Shake the mixture and allow the two layers to separate.
• Practical 5: preparation of cyclohexene from cyclohexanol

  5. Carefully run off the lower layer into a beaker (for later disposal) and then transfer the upper layer, which contains the crude cyclohexene, into a small conical flask.
• Practical 5: preparation of cyclohexene from cyclohexanol

  6. Add a few lumps of anhydrous calcium chloride or to the crude cyclohexene to remove water. Stopper the flask, shake the contents and allow this to stand until the liquid becomes clear.
• Practical 5: preparation of cyclohexene from cyclohexanol

  7. Decant the liquid into a clean, dry sample container that has been weighed.

  8. Reweigh the container, calculate the mass of dry cyclohexene produced and determine the percentage yield of your product. Assume that all of the dry distillate is cyclohexene
• Practical 5: preparation of cyclohexene from cyclohexanol

  9. Test the distillate using Bromine water, to confirm that it contains an alkene. (Goes colourless if present).
• Why is saturated sodium carbonate solution added to the mixture in the separating funnel?

  To neutralise all the phosphoric acid
• Why does the tap of the separating funnel need to be opened frequently when shaking the mixture with the saturated sodium carbonate solution?

  To release pressure building up due to the gas. If the pressure of the gas did not escape, the stopper would blow out and the mixture would splash out
• State the strongest intermolecular force present between the molecules in cyclohexanol, phosphoric acid and cyclohexene. Which one would have the lowest boiling and explain your answer.

  > Cyclohexanol has hydrogen bonding between the molecules

  > Phosphoric acid has hydrogen bonding between the molecules

  > Cyclohexene has VdW forces between the molecules

  > Cyclohexene has the lowest boiling point as it has the weakest intermolecular force between its molecules which requires the least amount of energy to overcome
• The density of cyclohexene is 0.81 g/cm3. In which layer would you expect to find cyclohexene and why?

  The upper organic layer as it is less dense than water (1.00 g/cm3)
• Why are a few lumps of anhydrous calcium chloride or anhydrous sodium sulfate added to the crude cyclohexene?

  The cyclohexene is wet. The anhydrous calcium chloride absorbs water
• When excess anhydrous calcium chloride is added to the crude cyclohexene, an observation is seen. The mixture turns from cloudy to?

  Clear
• Give two properties of a drying agent such as anhydrous calcium chloride.

  > Absorb water

  > Insoluble
• How is the drying agent removed from the mixture?

  Filtration
• The dry cyclohexene can be further purified. What practical technique could be used to purify the dry cyclohexene?

  Distillation
• State the test for cyclohexene and the observation.

  Add bromine water and it changes from orange to colourless