Preparation of Cyclohexene (OCR AS Chemistry)

Preparation and Purification of Cyclohexene

• In this experiment an alkene, cyclohexene, will be prepared by dehydration of an alcohol, 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 and some side products
• Treatment with a saturated solution of sodium hydrogen carbonate removes traces of acid and a final wash with water removes any remaining carbonate
• Addition of solid calcium chloride will remove remaining water
• The mechanism for the reaction is shown below - you do not need to know this, but you could be asked in a questions to draw in the relevant charges or lone pairs

Elimination mechanism for cyclohexanol

Preparation

• Pour approximately 20 cm³ of cyclohexanol into a weighed 50 cm³ pear shaped flask
• Slowly add 8 cm³ of concentrated phosphoric acid into the flask using a pipette 
• Add a few anti-bumping granules to the flask and assemble the distillation apparatus, so that the contents of the flask may be distilled
• Heat the flask gently using an electric heater or water bath
• Collect the distillate 

Purification

• Pour the distillate into a separating funnel and add 10 cm³ of sodium hydrogen carbonate solution, this will remove unreacted acid and acidic impurities in the organic liquid  
• Shake the funnel and allow the mixture to separate, release the pressure built up in the funnel
• Carefully run off the lower aqueous layer and then transfer the upper layer which contains the crude cyclohexene into a conical flask 
• Add solid anhydrous calcium chloride to the crude cyclohexene to remove water
• Stopper the flask, shake the contents and allow to stand 
• Once clear, transfer the liquid to a clean, dry beaker
• A small portion of the distillate can be tested with bromine water to confirm if the product is an alkene
  • The colour change will be from orange brown to colourless

A separating funnel allows the product to be cleaned and isolated