Recrystallization is a widely used laboratory technique for purifying solid organic compounds based on differential solubility. The process separates a desired compound from impurities by dissolving the substance in a hot solvent and then allowing it to cool. The temperature change causes the target compound to crystallize while impurities remain dissolved in the liquid, known as the mother liquor. This method requires selecting a solvent where the compound’s solubility changes significantly with temperature, and this article focuses on using ethanol.
Why Ethanol is an Effective Solvent
Ethanol is an appropriate choice for many organic compounds due to its chemical properties. The molecule possesses medium polarity, stemming from both its nonpolar ethyl group and its polar hydroxyl (\(\text{OH}\)) group. This dual nature allows it to effectively dissolve a broad spectrum of organic compounds, including those too polar for nonpolar solvents and those too nonpolar for water.
A practical advantage of using ethanol is its relatively low boiling point of approximately \(78^\circ\text{C}\). This moderate volatility means the solvent can be heated easily to promote dissolution. More importantly, it can be removed easily from the final purified crystals via simple evaporation. For the technique to be successful, the compound must be highly soluble in hot ethanol but only sparingly soluble when the solvent is cold. This dramatic change in solubility drives the compound into a pure crystalline form upon cooling.
Step-by-Step Recrystallization Procedure
The process begins by dissolving the impure solid in the minimum necessary amount of boiling ethanol. Using the least amount of solvent possible ensures the solution becomes saturated quickly. This maximizes the amount of product that will crystallize out later. The solid is added to a flask, and small portions of hot ethanol are added incrementally until the solid just dissolves, often requiring heating to a gentle boil.
Once dissolved, the solution must be filtered while still hot to remove insoluble impurities. This step, known as hot gravity filtration, is performed quickly using a pre-heated funnel. This prevents the dissolved compound from crystallizing prematurely in the filter paper or funnel stem. If crystals form during filtration, a small amount of additional hot solvent can be added to redissolve them.
The filtered solution is then allowed to cool very slowly and undisturbed, ideally to room temperature first. Slow cooling promotes the formation of large, well-ordered crystals. This allows the compound’s molecules time to arrange themselves into a pure lattice structure. Rapid cooling, such as using an ice bath immediately, should be avoided as it can cause the compound to crash out as a fine powder, trapping impurities.
After cooling is complete, the purified crystals are isolated from the mother liquor using cold vacuum filtration. A Büchner funnel fitted with filter paper is used, and a vacuum is applied to draw the solvent through, leaving the solid crystals. To wash away residual liquid containing soluble impurities, the crystals are rinsed with a minimal amount of ice-cold ethanol. Using cold solvent for the wash minimizes product loss due to dissolution.
Ensuring Purity and Maximizing Yield
Sometimes, the compound separates from the cooling solution as an oil, known as “oiling out.” This occurs if the compound’s melting point is below the temperature at which it becomes insoluble, or if the sample is highly impure. If this happens, the oil must be redissolved by reheating and adding a small amount of extra solvent. The cooling process must then be attempted again, often at a much slower rate.
If the solution cools but crystals fail to form, the solution may be supersaturated. Crystallization can often be initiated by mechanically stimulating the solution. Gently scratching the inner wall of the flask at the liquid level with a glass stirring rod provides a rough surface that acts as a nucleation site. Alternatively, adding a tiny “seed crystal” of the pure compound can also induce crystallization.
Drying is the final step before analysis, and all traces of solvent must be removed. Residual ethanol will contaminate the sample and affect purity analysis. The crystals should be left to dry thoroughly, either by drawing air through them on the vacuum filter or by spreading them on a watch glass to air dry.
Final Drying and Analysis
For complete removal of the solvent, a vacuum desiccator can be used to pull off the last traces of ethanol. The success of the purification is confirmed by determining the melting point of the dried product. A sharp melting point that closely matches the known literature value indicates a high level of purity has been achieved.