Organic Techniques - Preparation (Edexcel International A Level Chemistry)

Revision Note

Richard

Author

Richard

Last updated

Reflux & Types of Distillation

Simple distillation

  • Simple distillation is a common practical completed in organic chemistry
  • It 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
  • Simple distillation allows you to separate compounds by their boiling point
    • Chemicals with the lowest boiling point will distill first
  • One of the most common simple distillation practicals is the oxidation of primary and secondary alcohol to aldehydes and ketones

The simple 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

Steam distillation

  • Steam distillation is used to separate an insoluble liquid from an aqueous solution
    • Steam is bubbled through a reaction mixture containing the aqueous solution and the insoluble liquid that forms a separate layer
    • As the steam bubbles through the reaction mixture it mixes the layers so they form part of the evaporating liquid
    • The resulting distillate can be cloudy if the desired compound is not miscible in water
  •  Advantages of steam distillation are:
    • The insoluble liquid distils at a temperature below its usual boiling point
    • It reduces the chances of thermal decomposition of the insoluble liquid 

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

Examiner 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

You've read 0 of your 10 free revision notes

Unlock more, it's free!

Join the 100,000+ Students that ❤️ Save My Exams

the (exam) results speak for themselves:

Did this page help you?

Richard

Author: Richard

Expertise: Chemistry

Richard has taught Chemistry for over 15 years as well as working as a science tutor, examiner, content creator and author. He wasn’t the greatest at exams and only discovered how to revise in his final year at university. That knowledge made him want to help students learn how to revise, challenge them to think about what they actually know and hopefully succeed; so here he is, happily, at SME.