Techniques (OCR A Level Chemistry)

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

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 +/- 2^oC 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

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

• There are different methods to purify organic liquids, including:
  • Use of a separating funnel
  • Use of drying agents 
  • Redistillation

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

• 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

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 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

Redistillation

• Sometimes when an organic liquid is obtained via distillation there can be traces of other liquids
  • This is particularly the case if any liquids in the reaction vessel or other by-products have a similar boiling point to the desired organic liquid
• In this situation, the distillation apparatus is cleaned, dried and set up for a second distillation
• The aim of the second distillation is to obtain just the desired organic liquid
• This can be done by aiming for a narrower boiling point range in the distillate
  • The narrower the boiling point range, the more pure the organic liquid is