Recrystallization is a method to separate and purify solid organic compounds. Solid organic compounds separated from organic reactions are usually impure, which are often mixed with some reaction by-products, unreacted raw materials and catalysts. The method of purification is usually recrystallization with suitable solvents.
Basic Principle of Recrystallization
The solubility of solid organic compounds in solvents is closely related to temperature. Generally, as the temperature increases, the solubility increases. If the solid is dissolved in the hot solvent to reach saturation, the bad solvent is added when cooling, and the solution becomes supersaturated solution and precipitates crystals due to the decrease of solubility. During this process, impurities mixed with solid organic matter, which are either insoluble or have low solubility in this solvent, can be removed by filtration; Alternatively, the solubility in this solvent is high and remains in the crystalline mother liquor to achieve purification.
Selection of Solvents
In recrystallization, the choice of solvent is very important, which is directly related to the quality of the product, the difficulty of operation, as well as the economic benefits. The following conditions must be considered when selecting solvents:
1. There is no chemical reaction with the purified substance.
2. The solubility of the purified substance in the solvent varies greatly at different temperatures, and the solubility increases with the increase of temperature, while the solubility is very small with the decrease of temperature.
3. The solubility of impurities is very large and remains in the mother liquor after crystallization, or it is so small that it can be filtered and removed.
4. Easy to volatilize and easy to be removed by crystallization separation.
5. A better crystal shape can be given.
6. Non-toxic or low toxicity, easy to operate and low price.
Operation Steps
Add 2g crude benzoic acid, proper amount of water and a few grains of zeolite to the 250mL conical bottle and heat until boiling to make the solid dissolve. If it is not completely dissolved in boiling state, 3~5mL hot water can be added every time until it is completely dissolved. However, we should pay special attention to whether the crude products contain insoluble impurities, so as not to add too much solvent. After all the solids are dissolved, add 20% more water, remove the heat source, add a small amount of activated carbon after cooling, and continue to heat and boil 5~10min.
Solubility of benzoic acid in water
| t/℃ | 18 | 30 | 60 | 70 | 75 |
| Solubility/[g●(100 mL)-1] | 0.27 | 0.42 | 1.2 | 1.78 | 2.2 |
While heating and dissolving crude benzoic acid, prepare Brinell funnel and suction bottle for thermal filtration, make them fully preheated, cut the filter paper and set aside. After the boiling solution of crude benzoic acid is ready, the filter device is installed and connected quickly (in 1~2min), and the filter paper is soaked and tightened with hot water, and the dissolved hot solution of crude benzoic acid is poured into the funnel as soon as possible. Quickly transfer the filtrate to another clean and preheated 100mL cone bottle or beaker, cover and place it to cool and crystallize. If you want to get a crystal with larger particles, you can reheat the filtrate to dissolve, and then cool slowly at room temperature.
After the crystallization is precipitated, the solid is extracted and extruded with a glass plug to remove the mother liquor as much as possible. Open the piston on the safety bottle, add a small amount of water to the funnel, wet the crystal completely, and then pump it dry again. Repeat this for 1 or 2 times. Finally, the crystal is transferred to the surface dish, spread out and dried under an infrared lamp.
The melting point of dried benzoic acid was determined, weighed and the recovery was calculated.
