Edexcel A Level Chemistry:复习笔记7.6.4 Organic Techniques - Preparation

Distillation & Refluxing

 

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

3.5-Carbonyl-Compounds-Synthesis-of-Aldehydes

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
  •  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

 

8.3.2-Preparation-of-ethyl-ethanoate

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

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

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