Here is a walkthrough. I wrote this for a lab final.
Your goal is the purify molecule A. This may be tainted with molecule B. The solubilities of these compounds in a few solvents are given below. How would you purify your product and verify its purity.
Solute A (hot) Solute A (cold) Solute B (hot) Solute B (cold)
Dichloromethane 5 g/L 0.35 g/L 4.1 g/L 0.25 g/L
Toluene 4.6 g/L 1 g/L 2 g/L 1 g/L
Hexanes 5.3 g/L 0.14 g/L 4.8 g/L 4.5 g/L
The recrystallization process is very systematic. First, a recrystallization solvent with high solubility for A when hot and limited solubility for A when cold must be chosen. This property will allow for effective crystallization of the desired product. In addition, in order to eliminate the impurity B, the recrystallization solvent should have high solubility for B when cold and preferably low solubility or insolubility for B when hot. The solvent should also have a boiling point range of 50-100°C lower than the melting point of the either A or B. Additionally, the solvent must not chemically react with the product being purified, so as to recover the purified material without any chemical changes. The purified form of the sample can be recovered through recrystallization by dissolving the sample in the smallest amount of boiling solvent, filtering the hot solution to dispose of insoluble impurities, cooling the solution to stimulate recrystallization, and separating the solid from the solution through vacuum filtration.
The sample should be dissolved in hexanes, as this will allow for the best separation of A from B according to theory of recrystallization. B has a high solubility in the cold solution and A has a low solubility in the cold solution. Therefore, most of A will precipitate and most of B will stay in solution.
Although recrystallization is an effective method of purification, it has some limitations. For example, during cold filtration, if the amount of impurity in the sample is greater than the saturation point of the solvent at the given temperature, the impurities will not dissolve and be filtered out but will remain in the collected crystals. In addition, if a greater purity is desired, the process could be repeated. However, this will result in a lower yield of the desired organic substance as material is not only lost in each step but is also left dissolved in the cold mother liquor.
