Note: The attractive forces that hold solute molecules to other solute molecules usually Van der Waals interactions are overcome during the recrystallization process. Recrystallization does not involve breaking any chemical bonds. The following is an outline of the recrystallization process. Pick the solvent. In the introductory organic lab course, the solvent for recrystallization is usually determined for you. The criteria used to choose an appropriate recrystallization solvent includes: a.
The solvent must not dissolve the compound at low temperatures that includes room temperature , but must dissolve the compound at high temperatures. The solute must dissolve in order to rid its lattice of impurities, but must not remain dissolved at room temperature after all, recovery of the solid is essential!
If the solvent dissolves the impurities readily even at room temperature then the impurities will not become trapped in the developing crystal lattice, but will remain dissolved in the solvent. If the impurities do not dissolve even at elevated temperatures then they can be easily removed by gravity filtration. As mentioned earlier, recrystallization does not chemically alter a molecule. No chemical bonds must be broken in the solute molecule. The crystal lattice is dissolved at elevated temperatures, but this only involves overcoming the intermolecular attractive forces.
Solvents with low boiling points i. Dissolve the solute. Remember that the solute should dissolve only when the solvent is heated. Therefore, the solvent is heated to its boiling point remember to use boiling stones! If too much solvent is added, the solution will not be saturated upon cooling and no crystals will form. Dissolving the solute generally involves adding a small volume of hot solvent, swirling the flask or stirring the solution , and watching to see if the solute dissolves.
It is actually a very challenging process to get completely right. The goal, is to obtain a compound in high purity as uniform crystals. Recrystallization is therefore a purification technique.
The key features necessary for a successful recrystallization process, are a very controlled temperature decrease and sufficient time. Because most solids have a better solubility at higher temperatures, we can sat- urate or almost saturate a solution at high temperature usually the boiling temperature of the solvent , and then slowly allow the solution to reach room temperature.
As the temperature starts to decrease, so does the solubility of the compound. As the solubility decreases, the solution at some point becomes supersaturated and crystals will start to form. Before we move on, let me address the main problem associated with crystallization: the formation of precipitate, versus crystals. A precipitate is simply a mixture of compounds in the solution that crash out.
A precipitate may not pure, because it can contain several compounds. Crystals, however, are often composed solely of one compound. It is very easy to get a precipitate, but very difficult to get crystals. We already mentioned temperature and time being key factors for successful recrystallization. Dissolve the solute in the solvent: Add boiling solvent to a beaker containing the impure compound.
Heat the beaker and keep adding solvent until the solute is completely dissolved. See Figure 1. Cool the Solution: The solution is cooled in open air first, and then cooled in an ice bath. Slow cooling often leads to purer crystals. Crystals should form on the bottom of the beaker. The process of "seeding" can be used to aid the formation of crystals- this means adding a pure crystal of the compound. The pure crystal forms a surface for the solute to crystallize upon.
See Figure 2. Obtain the crystals from the solute : The purer crystals precipitated from the solute are the desirable part of the mixture, and so they must be removed from the solvent.
The process used for isolating the crystals that remain in the beaker still is called vacuum filtration. Suction is created using an aspirator, and whatever remains in the beaker is poured though a Buchner funnel. If for some reason there are no crystals visible, a gravity filtration can be performed. Activated carbon is added to the solution, the mixture is boiled, and a funnel system is used to transfer the new mixture to a new beaker of boiling solvent.
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