Distillation of a Product from a Reaction (AQA A Level Chemistry) : Revision Note

Distillation of an Organic Product

REQUIRED PRACTICAL 5

Distillation of an organic product:

• Distillation is a common practical completed in organic chemistry

• For this course, the most common practical in which you will come across distillation and reflux apparatus is the oxidation of primary and secondary alcohol

• This required practical gives you the opportunity to show that:

  • You can use an electric heating mantle rather than a Bunsen burner for heating

  • Use laboratory apparatus for a variety of experimental techniques and can set up glassware successfully

  • Safely and carefully handle different liquids, including those which are corrosive, irritant, flammable and toxic

The Distillation Process:

• To  produce an aldehyde from a primary alcohol the reaction mixture must be heated

• The aldehyde product has a lower boiling point than the alcohol ( since it has lost the H-bonding) so it can be distilled off as soon as it forms

Heating under Distillation Apparatus

• The reaction mixture needs to be heated until it boils using a heating mantle

  • Electric heating mantles are used for this because the temperature can be controlled, and because you are using chemicals which are flammable

• Quickfit apparatus is then set up, including a pear shaped flask, a still head and a condenser

• A Quickfit thermometer can be used, with the thermometer bulb sitting exactly where the vapours will pass into the condenser

• A steady and constant stream of water passes through the condenser in a 'water jacket' - it enters at the bottom of the condenser and the drainage pipe removes the water from the top of the condenser

• The distillate which forms in the condenser drips directly into a receiving vessel

  • The distillate which should be collected, is that which is given off at +/- 2 ^oC of the boiling point of the desired product

  • Some distillate may be given off below this temperature - this needs to be discarded and a clean vessel used to collect the desired product

  • Stop collecting the distillate if the temperature rises above +/- 2^oC of the boiling point of the desired product

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