The most common method of purifying solid organic compounds is by recrystallization. In this technique, an impure solid compound is dissolved in a solvent and then allowed to slowly crystallize out as the solution cools. As the compound crystallizes from the solution, the molecules of the other compounds dissolved in solution are excluded from the growing crystal lattice, giving a pure solid.
Crystallization of a solid is not the same as precipitation of a solid. In crystallization, there is a slow, selective formation of the crystal framework resulting in a pure compound. In precipitation, there is a rapid formation of a solid from a solution that usually produces an amorphous solid containing many trapped impurities within the solid's crystal framework. For this reason, experimental procedures that produce a solid product by precipitation always include a final recrystallization step to give the pure compound.
The process of recrystallization relies on the property that for most compounds, as the temperature of a solvent increases, the solubility of the compound in that solvent also increases. For example, much more table sugar can be dissolved in very hot water (just below the boiling point) than in water at room temperature. What will happen if a concentrated solution of hot water and sugar is allowed to cool to room temperature? As the temperature of the solution decreases, the solubility of the sugar in the water also decreases, and the sugar molecules will begin to crystallize out of the solution. (This is how rock candy is made.) This is the basic process that goes on in the recrystallization of a solid.
The steps in the recrystallization of a compound are:
- Find a suitable solvent for the recrystallization;
- Dissolve the impure solid in a minimum volume of hot solvent;
- Remove any insoluble impurities by filtration;
- Slowly cool the hot solution to crystallize the desired compound from the solution;
- Filter the solution to isolate the purified solid compound.
Choosing a solvent
The first consideration in purifying a solid by recrystallization is to find a suitable solvent. There are four important properties that you should look for in a good solvent for recrystallization.
- The compound should be very soluble at the boiling point of the solvent and only sparingly soluble in the solvent at room temperature. This difference in solubility at hot versus cold temperatures is essential for the recrystallization process. If the compound is insoluble in the chosen solvent at high temperatures, then it will not dissolve. If the compound is very soluble in the solvent at room temperature, then getting the compound to crystallize in pure form from solution is difficult. For example, water is an excellent solvent for the recrystallization of benzoic acid. At 10°C only 2.1 g of benzoic acid dissolves in 1 liter of water, while at 95 °C the solubility is 68 g/L.
- The unwanted impurities should be either very soluble in the solvent at room temperature or insoluble in the hot solvent. This way, after the impure solid is dissolved in the hot solvent, any undissolved impurities can be removed by filtration. After the solution cools and the desired compound crystallizes out, any remaining soluble impurities will remain dissolved in the solvent.
- The solvent should not react with the compound being purified. The desired compound may be lost during recrystallization if the solvent reacts with the compound.
- The solvent should be volatile enough to be easily removed from the solvent after the compound has crystallized. This allows for easy and rapid drying of the solid compound after it has been isolated from the solution.
Finding a solvent with the desired properties is a search done by trial and error. First, test the solubility of tiny samples of the compound in test tubes with a variety of different solvents (water, ethanol, methanol, ethyl acetate, diethyl ether, hexane, toluene, etc.) at room temperature. If the compound dissolves in the solvent at room temperature, then that solvent is unsuitable for recrystallization. If the compound is insoluble in the solvent at room temperature, then the mixture is heated to the solvent's boiling point to determine if the solid will dissolve at high temperature, and then cooled to see whether it crystallizes from the solution at room temperature.
Dissolving the solid
Once a suitable solvent is selected, place the impure solid in an Erlenmeyer flask and add a small volume of hot solvent to the flask. Erlenmeyer flasks are preferred over beakers for recrystallization because the conical shape of an Erlenmeyer flask decreases the amount of solvent lost to evaporation during heating, prevents the formation of a crust around the sides of the glass, and makes it easier to swirl the hot solution while dissolving the solid without splashing it out of the flask.
Keep the solution in the Erlenmeyer flask warm on a hot plate or in a water bath, and add small volumes of hot solvent to the flask until all of the solid just dissolves. Swirl the solution between additions of solvent and break up any lumps with a stirring rod or spatula. Occasionally there will be impurities present in the solid that are insoluble in the chosen solvent even at high temperature. If subsequent additions of solvent to the solution do not seem to dissolve any of the remaining solid, stop adding solvent to the solution (as this will decrease the percent recovery of the desired compound) and filter or decant the hot solution to remove the insoluble impurities.
Using decolorizing carbon
Colored impurities are sometimes difficult to remove from solid mixtures. These colored impurities, often due to the presence of polar or polymeric compounds, can cause a colorless organic solid to have a tint of color even after recrystallization. Decolorizing or activated carbon is used to remove the colored impurities from the sample. Decolorizing carbon is very finely divided carbon that provides high surface area to adsorb the colored impurities.
Very little decolorizing carbon is needed to remove the colored impurities from a solution. You must be judicious in your use of decolorizing carbon: if too much is used, it can adsorb the desired compound from the solution as well as the colored impurities. After the impure solid sample is dissolved in hot solvent, a small amount of decolorizing carbon, about the size of a pea, is added to the hot solution. This must be done carefully to avoid a surge of boiling from the hot solution. The solution is stirred and heated for a few minutes and then filtered hot to remove the decolorizing carbon. The resulting filtrate should be colorless and the recrystallization process continues as before.
Crystallizing the solid
After the insoluble impurities have been removed, cover the flask containing the hot filtrate with a watch glass and set it aside undisturbed to cool slowly to room temperature. As the solution cools, the solubility of the dissolved compound will decrease and the solid will begin to crystallize from the solution. After the flask has cooled to room temperature, it may be placed in an ice bath to increase the yield of solid. Do not rapidly cool the hot solution by placing the flask in an ice bath before it has cooled to room temperature-this will result in a rapid precipitation of the solid in an impure form because of trapped impurities.
Sometimes the dissolved compound fails to crystallize from the solution on cooling. If this happens, crystallization can be induced by various methods. One way to induce crystallization is by scratching the inner wall of the Erlenmeyer flask with a glass stirring rod. This is believed to release very small particles of glass which act as nuclei for crystal growth. Another method of inducing crystallization is to add a small crystal of the desired compound, called a seed crystal, to the solution. Again, this seed crystal acts as a template on which the dissolved solid will begin crystallizing. If neither of these two techniques results in crystallization, the compound was probably dissolved in too much hot solvent. If you believe that you may have too much solvent for the amount of dissolved compound, reheat the solution to boiling, boil off or distill some of the solvent, and then allow the solution to cool to room temperature again to effect crystallization.
Isolating the solid by suction filtration
Once the compound has completely precipitated from the solution, it is separated from the remaining solution (also called the mother liquor) by filtration. Typically this is done by vacuum or suction filtration using a Büchner funnel. Line the bottom of the Büchner or Hirsch funnel with a piece of filter paper that is large enough to cover the holes in the bottom plate of the funnel without curling up on the sides of the funnel. Place a neoprene adapter on the stem of the funnel and insert it in the top of a filter flask (a thick-walled Erlenmeyer flask with a side-arm) that has been securely clamped to a ringstand.
Using a piece of thick-walled vacuum tubing, connect the side-arm of the filter flask to a water aspirator. Turn the water to the aspirator on full force to create a vacuum through the system. If necessary, carefully adjust the piece of filter paper so that it covers all of the holes in the funnel, and then dampen it with a small volume of cold solvent; this will create a better seal between the filter paper and the plate in the funnel, preventing any solid from getting under the filter paper and passing through the funnel. Slowly pour the recrystallization solution into the funnel and allow the suction to pull the mother liquor through. Rinse the Erlenmeyer flask with a small volume of cold recrystallization solvent to remove any remaining solid. Add this solvent to the funnel and then wash the solid in the funnel, called the filter cake or residue, with a few milliliters of fresh, cold recrystallization solvent to remove any remaining mother liquor and dissolved impurities.
Leave the aspirator on for a few minutes and allow air to pass through the crystals to dry them. After pulling air through the crystals for a brief time, remove the vacuum from the system by disconnecting the vacuum tubing from the aspirator before turning the water off. If you turn the aspirator water off first, water can be sucked into the filter flask and may contaminate the product. The filter cake is removed from the funnel by carefully prying it from the filter using a spatula. The cake of crystals will still be slightly wet with solvent and should be allowed to dry thoroughly before measuring the weight or melting point of the solid material.
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