1. Dissolving: The impure compound is dissolved in a hot solvent. This is usually done using a solvent in which the desired compound is very soluble at high temperatures but much less soluble at low temperatures. Impurities are often either very soluble in the solvent at all temperatures or are insoluble.
2. Hot Filtration: The hot solution is filtered to remove any insoluble impurities. This leaves the desired compound and any soluble impurities dissolved in the solvent.
3. Cooling and Crystallization: As the solution cools, the solubility of the desired compound decreases. It begins to crystallize out of the solution, leaving behind most of the soluble impurities in the remaining solution.
4. Isolation and Washing: The crystals are isolated by filtration, separating them from the solvent containing the soluble impurities. The crystals are then washed with fresh solvent to remove any remaining impurities that might have adhered to the crystal surface.
Why does this work?
* Differential solubility: The desired compound and impurities have different solubilities in the chosen solvent. The compound we want is more soluble at high temperatures and less soluble at low temperatures. Impurities may be either very soluble or insoluble at all temperatures.
* Crystallization: As the solution cools, the desired compound crystallizes out because its solubility drops. The soluble impurities remain dissolved in the remaining solvent.
* Purity: The crystals formed during cooling are relatively pure, containing fewer soluble impurities.
Important considerations:
* Solvent choice: The solvent must be chosen carefully. It should dissolve the desired compound at high temperatures but not at low temperatures. It should also not react with the compound and should be easily evaporated later.
* Rate of cooling: A slow, controlled cooling rate allows for the formation of larger, more pure crystals.
* Multiple recrystallizations: If necessary, the process can be repeated multiple times to further purify the compound.
In summary, recrystallization works by exploiting the different solubilities of the desired compound and impurities to selectively crystallize the pure compound, leaving the impurities dissolved in the solvent.
