Recrystallization is a widely used laboratory technique in organic chemistry designed to purify solid compounds. The process separates a desired solid from trace amounts of soluble or insoluble contaminants. It relies on the inherent differences in the physical properties of the target compound and the impurities present in the mixture. The goal is to obtain a highly pure substance by encouraging the formation of an ordered solid structure.
The Foundational Principle of Solubility
The entire purification process hinges on the principle of differential solubility based on temperature. An appropriate solvent must be chosen where the compound being purified is highly soluble at the solvent’s boiling point but has minimal solubility when the solvent is cold. This temperature-dependent solubility is often described as having a high temperature coefficient for the desired compound.
Conversely, the impurities should behave in one of two ways to be successfully removed. Soluble impurities must remain dissolved in the solvent even at low temperatures, staying in the liquid phase (the mother liquor). Insoluble impurities, such as dust or side products, should not dissolve in the solvent at all, even when heated to boiling.
Choosing the correct solvent is crucial because the solvent must not chemically react with the compound during the heating or cooling stages. An ideal solvent should have a boiling point lower than the compound’s melting point to prevent the compound from melting into an unpurifiable oil. The solvent should also be volatile enough to be easily removed from the final product once the purification is complete.
The Step-by-Step Purification Process
The process begins by dissolving the impure solid in the chosen solvent, using the minimum possible amount heated to its boiling point. Using the smallest volume of hot solvent ensures the solution becomes saturated quickly when it cools, maximizing the eventual yield. If any insoluble impurities are observed in the hot solution, they are typically removed immediately via a hot gravity filtration step.
Once the compound is fully dissolved and any insoluble material has been filtered out, the solution is allowed to cool slowly and undisturbed. Slow cooling promotes the formation of large, well-formed crystals rather than fine powder. As the temperature drops, the solution becomes supersaturated, meaning the solvent holds more dissolved compound than it can thermodynamically support at that lower temperature.
Supersaturation drives the dissolved molecules of the target compound to organize and precipitate out of the solution. After the solution has cooled to room temperature, it is often placed in an ice bath to further reduce the temperature and maximize the amount of solid that crystallizes. The final step is isolating the purified product from the mother liquor, typically achieved through vacuum filtration. The crystals are then briefly washed with a small amount of fresh, ice-cold solvent to remove residual mother liquor before being dried.
How Impurities Are Separated
The fundamental mechanism for separation lies in the precise, repeating, and ordered structure of a crystalline lattice. As the target compound precipitates from the solution, its molecules align themselves into a specific, three-dimensional arrangement. This self-assembly process is highly selective, meaning there is a strong tendency to exclude foreign molecules that do not fit neatly into the established crystalline pattern.
Impurities, which are structurally different from the main compound, are physically blocked from inclusion in this organized structure. They are instead forced to remain dissolved in the surrounding liquid solvent, which is a disordered medium. The purification is a physical consequence of the difference between the ordered arrangement of the solid phase and the random, liquid environment.
While the exclusion process is highly efficient, some impurities may occasionally become trapped. This can include surface deposition on the crystal face or, rarely, forming a solid solution if the impurity is structurally similar to the main compound. However, the physical exclusion of most impurities into the mother liquor is the primary driver of purity improvement during the crystallization step.
Troubleshooting Common Issues
A common problem is the failure of the compound to form crystals, even after sufficient cooling. This often occurs if too much solvent was used initially, resulting in a solution that is not sufficiently saturated to drive the precipitation of the solid. If this happens, the solvent volume can be reduced by gentle heating and re-cooling the more concentrated solution.
In cases where the solution is saturated but no crystals appear, the process may be lacking a nucleation site to start crystal growth. This can be remedied by gently scratching the inner surface of the flask beneath the liquid with a glass rod, providing microscopic surfaces for organization. A few grains of pure product, known as seed crystals, can also be added to initiate crystallization.
Another frequent issue is “oiling out,” which occurs when the compound separates as a liquid oil instead of a solid. This happens because the separation temperature is higher than the compound’s melting point, often due to impurities. To fix this, the solution must be reheated to redissolve the oil, and a small amount of extra solvent can be added to change the solubility dynamics before attempting a much slower cooling process.