Recrystallization

Recrystallization

Experiment 2: Recrystallization &
Melting Point

• Most important method for the purification of organic solids
• Separation of compounds based on differences in solubility between the compound of interest and its contaminants
• Basic technique:
1. dissolve impure sample in an "appropriate" hot solvent

Part A: Choosing a Solvent
Part B: Purification of Phenacetin

2. cool solution slowly to induce crystal growth
3. filter resulting mixture to isolate crystals

Reading: Mohrig, Hammond & Schatz
Ch. 15 pgs 183-197
Ch. 10 pgs 104-113
Ch. 14 pgs 174-182

• Scale: 5-10 mg discovery based research - a new material prepared in a lab
1,000 kg + commercial applications - sugar refining, synthesis of pharmaceutical agents, etc.

Recrystallization

Recrystallization

• Molecular selection - based on size, shape, & functionality

• Molecular selection - based on size, shape, & functionality

pure substance aggregation begins

molecules deposit on growing surface in orderly manner, excluding those of different size of shape

pure substance if deposition occurs too quickly, an impure substance can result crystal defects

incorporated impurities

Recrystallization Steps
1. Choose an appropriate solvent
- compound (solid) should be soluble when solvent is hot
- compound should be insoluble when solvent is cold
- may require some trial & error
2. Dissolve impure compound in the minimum amount of hot solvent
- too much solvent & compound may not come out when cool
3. Decolorize solution if needed with activated charcoal (Norit)
- skip this step if no/few colored impurities are present
- be sure your compound is not supposed to be colored!
4. Filter off any insoluble materials
- insoluble impurities and/or activated charcoal
- done while solution is hot

Melting Point

Recrystallization Steps
5. Slowly cool the resulting solution to induce crystallization
- first cool to room temperature, then