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

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

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

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

转载自savemyexams