Preparation of Cyclohexene (OCR A Level Chemistry A): Revision Note

PAG 5.2: Preparation of cyclohexene

• In this experiment, cyclohexene is prepared by dehydratng of cyclohexanol using an acid catalyst such as phosphoric acid

C₆H₁₁OH  C₆H₁₀ + H₂O

• This is one of the most common methods of preparing alkenes

• The crude product is contaminated with:

  • Water

  • Unreacted alcohol

  • Phosphoric acid

  • Some side products

• Wash the crude product with saturated sodium hydrogen carbonate to remove acid

• Then, wash with water to remove any remaining carbonate

• Add solid calcium chloride to remove residual water

• The elimination mechanism is shown below

  • You do not need to learn it, but you may be asked to draw in charges or lone pairs

Preparation of cyclohexene

1. Measure approximately 20 cm³ of cyclohexanol into a weighed 50 cm³ pear-shaped flask

2. Use a pipette to slowly add 8 cm³ of concentrated phosphoric acid

3. Add a few anti-bumping granules to the flask

4. Assemble the distillation apparatus so you can distil the contents of the flask

5. Heat the flask gently using an electric heater or water bath

6. Collect the distillate

Purification of cyclohexene

1. Pour the distillate into a separating funnel

2. Add 10 cm³ of sodium hydrogen carbonate solution

   • This is to remove unreacted acid and acidic impurities

3. Shake the funnel and allow the layers to separate, releasing pressure occasionally

4. Carefully run off the lower aqueous layer

5. Transfer the upper organic layer (crude cyclohexene) into a conical flask

6. Add solid anhydrous calcium chloride to remove water

7. Stopper the flask, shake it gently, and allow it to stand

8. Once the liquid is clear, transfer it to a clean, dry beaker

9. Test a small portion of the distillate with bromine water to confirm the presence of an alkene

   • A positive result is a colour change from orange-brown to colourless

Practical skills reminder

• This practical demonstrates the purification of a liquid product using:

  • A separating funnel to remove aqueous impurities

  • Washing steps with sodium hydrogen carbonate and water

  • A drying agent (anhydrous calcium chloride) to remove water

  • Final distillation to isolate and test the purified alkene