Practical techniques
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Summary
Practical techniques
In a nutshell
There are a number of techniques available to obtain, separate and obtain pure organic compounds. To name a few, there is reflux, distillation, filtration and recrystallisation.
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. This ensures the reagents are not lost and enables the mixture to react.
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. It is crucial to control the temperature and keep an eye on it to ensure the starting material remains in the flask.
1. Flame; 2. Mixture; 3.Flask; 4. Thermometer; 5. Cooling water going out;
6. Condenser; 7. Cooling water going in; 8. Pure compound.
A pure product from an impure product can also be obtained using this method if they have different boiling points. In this instance the flask will contain an impure product and not a reaction mixture.
When there is a change in temperature, place a different beaker or flask at the end of the condenser as a different liquid will be condensing out.
Distillation can also 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.
Steam distillation
Organic liquids with high boiling points or those that decompose readily upon heating cannot be purified using simple distillation. Instead, steam distillation can be used if the product is immiscible in water. This is because the presence of steam decreases the boiling point of the product. This enables the product to distil below its boiling point and before decomposition.
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PROCEDURE
| 1. | Heat water in a conical flask until it evaporates. |
| 2. | Allow steam to pass through the flask containing the impure product. |
| 3. | Boiling points of the components in the mixture is reduced by steam. This leads to their evaporation at a lower temperature. |
| 4. | If the volatility of the organic product is lower than the other components in the mixture, it will evaporate out from the impure mixture, with the steam. |
| 5. | The organic product and steam can be condensed out into another flask or vial. |
| 6. | Separate the product from the water using filtration and/or solvent extraction. |
Washing
Washing is the addition of another solvent to the mixture and shaking the new mixture to remove contaminants. The contaminants may be unused reagents or other products that may have formed during the reaction.
Successful washing requires the contaminants to be more soluble in the solvent added and the desired product must be immiscible in the solvent added.
Example
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An organic solvent contains the organic product and inorganic impurities. Pour the impure product into a separating funnel, then add distilled water. The inorganic impurities are more soluble in water than the organic solvent, so will readily move into the aqueous layer.
Solvent extraction
Solvent extraction is a type of separation method which can be used where an organic product is immiscible in water but contains water soluble impurities.
1. Separating funnel.
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. |
The method can be reversed whereby the mixture is in an aqueous solvent or the organic product is slightly miscible in water. In this instance, pour the mixture into the separating funnel, then add an organic solvent. Proceed with the rest of the technique as described in the procedure.
Adding salt such as sodium chloride to the mixture will lead to the organic product to move into the organic layer. This is because the organic product will be far less soluble in the highly polar aqueous layer.
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.
Gravity filtration
Gravity filtration is used when the desired product is in the filtrate.
PROCEDURE
| 1. | Place a funnel into a conical flask. |
| 2. | Place a fluted filter paper into the funnel. |
| 3. | Pour the mixture in to the filter paper. The solution will pass through the filter paper and the solid will be left behind (residue). |
| 4. | Rinse the residue with the solvent of the filtrate. This ensures all the soluble product passes through the filter paper and collected in the conical flask. |
Vacuum filtration
Vacuum filtration is filtration done under reduced pressure. It is used when you wish to keep the solid and discard the filtrate.
PROCEDURE
| 1. | Place a Büchner funnel and bung onto a side-arm flask. |
| 2. | Place a circular filter paper in the Büchner funnel. Ensure the diameter is slightly smaller than the base of the funnel but covers all the holes. |
| 3. | Wet the filter paper with a bit of solvent (same solvent as the mixture) so it is stuck to the base of the funnel. This ensures the filter paper does not move when the mixture is being poured and product is not lost. |
| 4. | Turn the vacuum on. |
| 5. | Pour the mixture into the funnel. The liquid will be sucked in to the conical flask, leaving the solid behind on the filter paper. |
| 6. | Rinse the solid with some solvent (same solvent as the mixture) to wash away any traces of soluble impurities. This will give a purer solid. |
| 7. | Turn the vacuum off and dissemble apparatus if no longer needed. |
| 8. | Leave the solid on the filter paper to air dry completely. |
Recrystallisation
Recrystallisation can be used to purify a solid organic product.
For this process, a suitable solvent for a given substance/mixture must be used. The substance must be very soluble in the boiling solvent and insoluble when the same solvent is ice-cold.
If the substance is not soluble in the boiling solvent, the substance will not dissolve at all. If the substance is too soluble in the cold/ice-cold solvent, most of the substance will remain in solution after cooling. Upon filtration most of the product will be lost in the filtrate, giving rise to a low percentage yield.
PROCEDURE
| 1. | Add a boiling solvent to the impure product until it just dissolves. This will give a saturated solution of the impure product. Important: Do not add too much solvent. Only add enough to dissolve the solid and obtain a saturated solution. |
| 2. | Filter the solution whilst hot via gravity filtration to remove insoluble impurities. |
| 3. | Leave the solution to cool down. You will find crystals forming as the solution is cooling. Note: Impurities will remain in the solution as they are present in smaller amounts and will take longer to crystallise out. |
| 4. | Separate the crystals formed using vacuum filtration. |
| 5. | Wash the crystals with ice-cold solvent (same solvent as the solution). |
| 6. | Leave crystals to dry. Organic product will be purer than before recrystallisation. |
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 (also referred to as data tables and data books). If impurities are present it will change the melting and boiling point or you may find that the sample evaporates (or melts) over a range of temperatures, so you can easily identify whether impurities are present or not.
Important: Some organic compounds may have very similar boiling and/or melting points so it is important to use other analytical techniques to determine purity.
Boiling point
A distillation apparatus can be used to measure the boiling point of a liquid product. Gently heat the liquid until it evaporates. Read the temperature at the point of evaporation using the thermometer attached to the apparatus.
Melting point
Pure organic solids have a fixed melting point. Measuring the melting point is an excellent indicator with respect to the purity of a product.
PROCEDURE
| 1. | Place a sample of the solid into a glass capillary tube. |
| 2. | Place the glass capillary tube inside the heating element. |
| 3. | Increase the temperature until the sample turns to liquid. |
| 4. | Record the temperature at which the sample starts to melt and when the sample has melted completely. This is called the melting range. |
| 5. | Compare the melting point or range to references, to identify whether you have a pure product or not. |
FAQs - Frequently Asked Questions
When do you use gravity filtration and vacuum filtration?
Gravity filtration is used when the desired product is in the filtrate. Vacuum filtration is used when you wish to keep the solid and discard the filtrate.
When is steam distillation used?
Steam distillation is used when organic liquids have a high boiling point or decompose readily upon heating.
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.