Organic techniques

In a nutshell

There are a number of techniques available to separate out and obtain or check for pure organic compounds. To name a few, there is reflux, distillation, the use of drying agents and checking for boiling point.

Reflux

A reaction mixture is refluxed when organic molecules used are flammable and volatile. The flask is attached to a Liebig condenser and electrically heated to prevent the mixture from catching fire. The Liebig condenser allows the mixture to continuously boil, evaporate and condense back in to the flask.

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Distillation

A reaction mixture is gently heated using a distillation apparatus. The mixture and if any impurities are present, are separated out via evaporation in order of increasing boiling point. 

As a thermometer is attached to the equipment, the temperature can be monitored and you can see the boiling point of any substance separating out. 

A product with a lower boiling point than the starting materials can be evaporated out and collected from the flask. A pure product from an impure product can also be obtained using this method.

Distillation can be used to prevent further reactions from happening.

Example

Primary alcohols can be oxidised to aldehydes which may further oxidise to a carboxylic acid. In order to obtain an aldehyde only, primary alcohols can be distilled, as aldehydes have a lower boiling point than alcohols and will evaporate out through the condenser before it can react further. 

Separation

Separation can be used where an organic product is immiscible in water but contains water soluble impurities. 

procedure

Drying agents

Purifying a product via separation will leave trace amounts of water in the organic layer, which needs to be removed. To remove this, you can use drying agents such as anhydrous magnesium sulfate. The anhydrous salt binds with any water molecules present, becoming hydrated. You will know if you have added enough drying agent to the organic layer if the mixture is not lumpy and you are able to swirl the mixture around. 

The drying agent can be removed via filtration.

Determining purity

Pure chemical substances have fixed melting and boiling points. Substances can be identified by finding out the boiling or melting point and comparing it to reference values. If impurities are present it will change the melting and boiling point or you may find that the sample evaporates over a range of temperatures, so you can easily identify whether impurities are present or not.

Note: Some organic compounds may have very similar boiling and/or melting points so it is important to use other analytical techniques to determine purity.

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