Organic techniques
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Summary
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.
1. Anti-bumping granules; 2.Round-bottom flask; 3. Water going in; 4. Liebig condenser; 5. Water going out
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.
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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
| 1. | Pour the mixture containing the product in to a separating funnel. |
| 2. | Add distilled water to the separating funnel. |
| 3. | Ensure the tap and lid are closed, then shake the funnel. |
| 4 | Allow contents in the separating funnel to settle. |
| 5. | The organic layer and aqueous layer will not mix and you see two very distinct layers. |
| 6. | Open the tap and run each layer into separate beakers. |
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.
FAQs - Frequently Asked Questions
What is an example of a drying agent?
An example of a drying agent is anhydrous magnesium sulfate.
Why is the mixture electrically heated during reflux?
A reaction mixture is refluxed when organic molecules used are flammable and volatile. The flask is electrically heated to prevent the mixture from catching fire.
How can the drying agent be removed from the product once it is used?
The drying agent can be removed via filtration.