Edexcel International A Level Chemistry

Revision Notes

2.10.1 Organic Chemistry Techniques

Reflux, Distillation & Boiling Point Determination

Heating under reflux

  • Organic reactions often occur slowly at room temperature 
  • Therefore, organic reactions can be completed by heating under reflux to produce an organic liquid
  • This allows the mixture to react as fully as possible without the loss of any reactants, products or solvent
    • In distillation, you are trying to separate a chemical or product from a mixture
    • When heating under reflux, you aim to keep all the chemicals inside the reaction vessel

The Heating under Reflux Process:

  • Example reactions where heating under reflux could be used include:
    • The production of a carboxylic acid from a primary alcohol using acidified potassium dichromate
    • The production of an ester from an alcohol and acid in the presence of an acid catalyst
  • The reaction mixture is placed into a pear-shaped or round bottomed flask
  • Anti-bumping granules are, again, added to promote smooth boiling
  • The flask is placed in a heating mantle or it can be immersed in a water bath for heating
  • Quickfit apparatus is then set up with the condenser clamped vertically in place 
    • The joints of the Quickfit apparatus are commonly greased as with distillation
  • 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 water is heated and the reaction mixture allowed to boil 
  • The heated is stopped and the mixture allowed to cool back to room temperature

Preparation of ethyl ethanoate, downloadable AS & A Level Chemistry revision notes

The preparation of ethyl ethanoate involves heating under reflux for about 15 minutes

Distillation

  • Distillation is a common practical completed in organic chemistry
  • Distillation is used as there are times that a reaction does not go to completion or there are other chemicals produced as well as the desired product
  • Distillation allows you to separate compounds by their boiling point
    • Chemicals with the lowest boiling point will distill first
  • One of the most common distillation practicals is the oxidation of primary and secondary alcohol to aldehydes and ketones

The Distillation Process:

  • To produce an aldehyde from a primary alcohol, a reaction mixture containing the primary alcohol and acidified potassium dichromate solution is placed into a pear-shaped or round bottomed flask
  • Anti-bumping granules are added to promote smooth boiling
  • Quickfit apparatus is then set up, including a still head and condenser connected to the side
    • The joints of the Quickfit apparatus are often have a thin layer of silicon grease smeared over them to give a better seal as well as to make it easier to disassemble the equipment afterwards
  • A Quickfit thermometer can be used, with the thermometer bulb sitting 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

Carbonyl Compounds Synthesis of Aldehydes, downloadable AS & A Level Chemistry revision notes

Heating under Distillation Apparatus

  • The reaction mixture is 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
  • 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 +/- 2oC of the boiling point of the desired product

  • 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

Exam Tip

  • These practicals give you the opportunity to discuss:
    • The use of an electric heating mantles and water baths rather than a Bunsen burner 
    • The choice and setup of laboratory apparatus 
    • Health and safety considerations including the careful handling of different liquids, including those which are corrosive, irritant, flammable and toxic

Boiling point determination

  • The boiling point of a liquid is indicative of its purity and identity
  • Boiling point is determined by distillation
  • The sample is gently heated until it boils and this temperature is recorded
    • This boiling point can then be compared against literature / database values
  • If the sample contains impurities:
    • The boiling point may appear higher than the literature / database values
    • The sample may boil over a range of temperatures instead of at a single temperature

Solvent Extraction & Drying

  • There are different methods to purify organic compounds, including:
    • Solvent extraction / use of a separating funnel
    • Use of drying agents 

Solvent extraction / use of a Separating Funnel

  • When organic liquids are being prepared, water can often be obtained along with the organic product
  • The water will usually form an aqueous layer with the product in the organic layer
    • It can sometimes be hard to identify which layer is the organic layer - this can be achieved by simply adding water and seeing which layer increases in volume
  • Other organic reactions may need to be neutralised before being purified, this can be achieved by adding sodium carbonate solution to the reaction vessel or separating funnel
    • This can also be used to remove other impurities by washing

  • The contents of the reaction vessel are transferred to a separating funnel and a stopper added
  • The separating funnel is inverted and the stopcock opened to release the pressure - this is repeated 15-20 times
    • If neutralisation has occurred then the stopcock is opened slowly to avoid losing any product
  • The two layers are allowed to separate 
    • In the following example, the aqueous layer is the bottom layer inside the separating funnel
  • The stop cock is opened so that the aqueous layer drains away and the organic layer can be drained into a clean beaker

Separation of ethyl ethanoate, downloadable AS & A Level Chemistry revision notes

A separating funnel allows the product to be cleaned and isolated

Use of drying agents

  • Drying agents can be used to remove traces of water from an organic product
  • Drying agents are usually anhydrous inorganic salts that readily hydrate in the presence of water
    • Anhydrous calcium chloride is commonly used to dry hydrocarbons
    • Anhydrous calcium or magnesium sulfate are used as more general purpose drying agents
  • A spatula of drying agent is added into the organic product and swirled
    • If the organic product has a low boiling point, a lid / stopped can be added to reduce the potential evaporation of any product
  • If the drying agent clumps together, then there is still water in the organic liquid
  • More drying agent is added until some remains dispersed in the organic liquid as a fine powder
  • The dry organic liquid can then be decanted or filtered
    • If the organic liquid is dry then it should appear clear

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