Purifying Organic Compounds
Recrystallisation
If the product of an organic reaction is a solid the simplest way of purifying it is recrystallisation.
1. Add a minimum volume of hot solvent to the impure solid until it dissolves.
The solvent should dissolve the solid when hot but not cold.
A minimum volume of hot solvent generates a saturated solution which helps
crystallisation upon cooling.
2. This should give a saturated solution of impure product with the dissolved solid. Insoluble
impurities would sink to the bottom.
3. Hot filtration using the Buchner Apparatus (under pressure) removes any insoluble
impurities. The heat prevents the crystals from reforming.
4. Leave the solution to cool down slowly. Crystals of the product form as it cools.
5. The impurities stay in the solution and are present in much smaller amounts and are not
saturated so take longer to crystallise.
6. Remove the crystals by filtration and wash them with ice cold solvent.
7. The crystals then need to be dried leaving crystals of the product much purer than the
original solid.
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Recrystallisation
If the product of an organic reaction is a solid the simplest way of purifying it is recrystallisation.
1. Add a minimum volume of hot solvent to the impure solid until it dissolves.
The solvent should dissolve the solid when hot but not cold.
A minimum volume of hot solvent generates a saturated solution which helps
crystallisation upon cooling.
2. This should give a saturated solution of impure product with the dissolved solid. Insoluble
impurities would sink to the bottom.
3. Hot filtration using the Buchner Apparatus (under pressure) removes any insoluble
impurities. The heat prevents the crystals from reforming.
4. Leave the solution to cool down slowly. Crystals of the product form as it cools.
5. The impurities stay in the solution and are present in much smaller amounts and are not
saturated so take longer to crystallise.
6. Remove the crystals by filtration and wash them with ice cold solvent.
7. The crystals then need to be dried leaving crystals of the product much purer than the
original solid.
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