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Often, the desired product of a chemical reaction is part of a more complex reaction mixture, which can be composed of the solvent, starting materials, and impurities. Learning how to purify organic compounds properly is a valuable technique in organic chemistry. Recrystallization harnesses the differences in solubility of the desired compound and the impurity in the solvent to purify the desired product as a solid. There are three standard methods of purification: distillation, extraction, and recrystallization.
The solubility of a substance is the maximum amount that dissolves in a fixed volume of a given solvent at a given temperature. Different solutes have different solubilities and dissolve in different solvents. Solutes may have defining characteristics that lend themselves to be exploited for recrystallization. Compounds exhibit one of the following behaviors in a solvent. First, the compound can be insoluble or have very low solubility in the solvent at all temperatures. Second, the compound can be soluble in the solvent at higher temperatures. Third, the compound can be soluble in the solvent at all temperatures.
A major factor in determining whether a solute dissolves in a solvent and forms a solution is the strength and type of intermolecular forces between the solute and solvent. The general rule of thumb is “like dissolves like,” meaning that substances with similar types of intermolecular forces dissolve in each other. For example, polar substances such as table salt (NaCl), dissolve well in polar water.
Another key factor that improves solubility is temperature. For many substances, solubility greatly increases at higher temperatures. This is due to the fact that the increased kinetic energy at higher temperatures breaks the solute intermolecular forces that keep molecules together. This is seen in everyday life. For example, we know that table salt (NaCl) dissolves well in water; however, more dissolves at higher temperatures than at lower temperatures.
Qualitatively, a solution is considered unsaturated if the maximum amount of dissolved solute has not yet been reached. When the maximum possible solute has dissolved, the solution is saturated. A supersaturated solution contains more dissolved solute than the maximum possible amount under typical conditions.
Recrystallization takes advantage of the differences in solubility between the desired product and the contaminants at high temperatures. The first step of recrystallization is to dissolve the product mixture in a minimal volume of heated solvent that still results in a saturated — but not supersaturated — solution. Then, the solution is cooled to room temperature, decreasing the solubility of both the desired compound and the impurity.
As the solution cools, crystallization of the pure component begins, while the still soluble impurities do not. This occurs when the component of interest is in a significantly higher concentration than the impurity. First, in the nucleation phase, the solvent initiates the random agglomeration of the solute molecules, forming the first crystal called a seed or nucleus. Next, in the particle growth or crystallization phase, more molecules are added to the seed, forming a crystal. The crystal contains the pure compound, while the impurity remains in the solvent.
Nucleation proceeds faster than particle growth in a supersaturated solution. With more seeds, each crystal is smaller. Thus, if the solution is saturated, rather than supersaturated, fewer seeds form, resulting in larger crystals. Heating the solution to a higher temperature before cooling to room temperature enables the dissolution of a higher concentration of solute, decreasing supersaturation. Additionally, rapid cooling results in quick nucleation, forming many small crystals and trapping the impurity inside of the crystals. Slow cooling is preferred to achieve fewer, larger crystals.
Once the solution has cooled to room temperature, and the crystals have formed, the solution is filtered using vacuum filtration. Then, the crystals are allowed to dry. The percent recovery is calculated by dividing the mass of the recovered product by the mass of the crude product.
The recovery is rarely 100%, as the solubility of the compound at low temperatures governs how much of the compound is crystallized.
For crystallization to be effective, the optimal solvent must be used. The desired product should have low solubility in the selected solvent at room temperature but high solubility in the solvent at a higher temperature. Ideally, the impurities should be soluble in the solvent at all temperatures. Thus, when the mixture is added to the solvent at a high temperature, the desired product and impurities dissolve readily.
As the solution is cooled, the solubility of the desired product decreases and crystallization begins to occur, forming purified product. Occasionally, impurities may remain insoluble in the solvent of choice, even at high temperatures. Hot gravity filtration of the solution that contains dissolved product can remove the solid impurities. The product can then be recrystallized by cooling the sample.
Ideally, the solvent to be used should be able to boil at a temperature well below the melting point of the desired product. The solvent should also be inert and not react with the desired purified product.