recrystallization
Summary
Recrystallization purifies a solid by dissolving it in a minimum of hot solvent and cooling slowly, so the desired compound crystallizes while most impurities stay in solution. Key point: the ideal solvent dissolves the compound well when hot and poorly when cold.
Detail
Meaning: The crude solid is dissolved in the smallest practical amount of hot solvent. On cooling, solubility falls and the solution becomes supersaturated, so the desired compound builds an ordered crystal lattice that tends to exclude molecules of different shape or polarity. Small amounts of impurities stay dissolved in the cold solvent (the mother liquor). Insoluble impurities are removed by hot filtration; crystals are collected by vacuum filtration and rinsed with a little ice-cold solvent.
Equation: Percent recovery = (mass of pure solid recovered ÷ mass of crude solid) × 100%, with masses in kg (g in the lab).
On the MCAT: Passages ask for the best solvent: the compound should be very soluble hot and poorly soluble cold, while impurities should stay dissolved when cold or be insoluble even when hot. Expect questions on why slow cooling gives purer crystals, why excess solvent lowers yield, and why recovery is typically below 100%. A purer product has a higher, narrower melting range.
Common confusion: Faster cooling is not better. Crystals that form quickly trap impurities, so slow cooling gives a purer product. Also, a good solvent should not dissolve the compound well at room temperature; if it did, little would crystallize.
Sources
- Organic Chemistry by Brown, Iverson, Anslyn and Foote
- Organic Chemistry by Klein
- Organic Chemistry by Wade
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.