Crystallization (or recrystallization) is the most important method for the purification of organic compounds. The process of eliminating impurities by crystallization consists of dissolving a compound in a suitable hot solvent, allowing the solution to cool and become saturated with the compound being purified, and it gives the opportunity for the solution to crystallize, it is isolated by filtration, its surface is washed with cold solvent to remove residual impurities, and it is left to dry.
This is a detailed guide on how to crystallize organic compounds. It is best done in a controlled environment such as a well-ventilated area in a chemistry lab. Note that this procedure has many applications, including large scale commercial purification of sugar, by crystallizing the product from brown sugar, which leaves impurities.
Step 1. Choose the appropriate solvent
Remember the Latin saying, Similia similibus solvuntur, which means "like dissolves like." For example, sugar and salt are soluble in water, but not oil, and nonpolar compounds like hydrocarbons will dissolve with nonpolar hydrocarbon solvents like hexane.
The ideal solvent should have the following properties:
- It will dissolve the compound when the solution is hot but not when it is cold.
- Impurities may not dissolve, which may be filtered out when the impure compound dissolves, or may dissolve very well, so that they remain in solution when the desired compound crystallizes.
- It will not react with the compound.
- It is not flammable.
- It is not toxic.
- It is cheap.
- It is very volatile, so it can be easily separated from crystals.
Generally, it is difficult to determine which will be the best solvent. The solvent is generally chosen through experimentation, or by using the most common non-polar solvent available. Familiarize yourself with the following list of common solvents, from most polar to least polar. Note that adjacent solvents are miscible, they will dissolve each other. Commonly used solvents are in bold.
- Water (H2O) It is non-flammable, non-toxic, inexpensive, and will dissolve many polar organic compounds. Its drawback is a high boiling point (100 degrees C), which makes it relatively non-volatile and difficult to separate from crystals.
- The Acetic acid (CH3COOH) It is useful for the oxidative reaction, but it will react with alcohol and amines and is therefore difficult to separate. Its boiling point is 118 degrees C.
- The Dimethylsulfoxide (DMSO), Methylsulfoxide (CH3SOCH3) It is used primarily as a solvent for reactions, rarely for crystallizations.
- The Methanol (CH3OH) It is a useful solvent that will dissolve compounds with a higher polarity than other alcohols.
- The Acetone (CH3COCH3) it is an excellent solvent. Its drawback is a low boiling point of 56 degrees C, which allows a small difference in the solubility of the compound at its boiling point and at room temperature.
- The 2-Butanone or methyl ethyl ketone, MEK (CH3COCH2CH3) It is an excellent solvent with a boiling point of 80 degrees C.
- The Ethyl acetate (CH3COOC2H5) or ethyl ethanoate, it is an excellent solvent with a boiling point of 78 degrees C.
- The Dichloromethane or methylene chloride (CH2Cl2) It is useful as a solvent when used in conjunction with ligroin, but its boiling point of 35 degrees C is too low to be a suitable solvent for crystallization.
- The Ethyl ether or diethyl ether (CH3CH2OCH2CH3) it is a useful solvent if used in conjunction with ligroin, but its boiling point of 40 degrees C is too low to be a good solvent for crystallization.
- The tert-butyric methyl ether (CH3OC (CH3) 3) it is cheap, it is a good substitute for ethyl ether since its boiling point is higher, 52 degrees C.
- The Dioxane (C4H8O2) it is easy to separate from crystals, it is a mild carcinogen, forms peroxides, its boiling point is 101 degrees C.
- The Toluene (C6H5CH3) It is an excellent solvent for the crystallization of aryl compounds and has replaced benzene (a mild carcinogen) that was once common in use. Its disadvantage is its high boiling point of 111 degrees C, which makes it difficult to separate it from the crystals.
- Pentane (C5H12) It is widely used for non-polar compounds. It is often used as a solvent in conjunction with another solvent.
- The Hexane (C6H14) It is used for non-polar compounds, it is inert, it is often used together with another solvent, its boiling point is 69 degrees C.
- The Cyclohexane (C6H12) It is similar to hexane, but it is cheaper, and has a boiling point of 81 degrees.
- Petroleum ether is a mixture of saturated hydrocarbons of which pentane is the main component; It is cheap and its use is interchangeable with pentane, its boiling point is 30 to 60 degrees C.
Ligroin is a mixture of saturated hydrocarbons that has the properties of hexanes.
Steps to choose the solvent:
- Put a few crystals of the impure compound in a test tube and add just one drop of the solvent, letting it run down the wall of the tube to the bottom.
- If the crystals dissolve immediately at room temperature, "reject the solvent" because much of the compound will continue to dissolve at a low temperature, and try another solvent.
- If the crystals do not dissolve at room temperature, heat the tube in a hot sand bath and observe the crystals. Add one more drop of the solvent if they don't dissolve. If they dissolve at the boiling point of the solvent and then recrystallize when cooled to room temperature, you've found the right solvent. If not, try another solvent.
If after running through trial and error, you cannot find a satisfactory single solvent, combine together with an added solvent. Dissolve the crystals in the best solvent (the one in which the crystals can easily dissolve), and add the weaker-acting solvent to the hot solution until it appears cloudy, indicating that the solution is saturated with the solute. The solvent pair must be miscible with each other. Some of the solvent combinations are acetic acid-water, ethanol-water, acetone-water, dioxane-water, acetone-ethanol, ethanol-diethyl ether, methanol-2-butanone, ethyl acetate-cyclohexane, acetone-ligroin, acetone ethyl ligroin, diethyl ether ligroin, dichloromethane ligroin, toluene ligroin.
Step 2. Dissolve the impure compound:
To do this, put the compound in a test tube. Crush large crystals with a glass rod to help them dissolve. Add the solvent drop by drop. To remove solid, insoluble impurities, use excess solvent to dilute the solution and filter impurities from the solid at room temperature (see step 4 to review the filtration procedure), then evaporate the solvent. Before heating, put a wooden applicator in the tube to prevent it from overheating (heating a solution above the boiling point without actually boiling). Air trapped in the wood will flow out of the "core" to allow for even boiling. Alternatively, you can use pieces of boiling porous porcelain. After you have removed the solid impurities and the solvent has evaporated, add the solvent drop by drop, while stirring the crystals with a glass rod and heating the tube with a sand or steam bath, until the compound is completely dissolved. with a minimal amount of solvent.
Step 3. Discoloration of the solution
If the solution is colorless or just pale yellow, skip this step. If the solution is colored, which results from the creation of high molecular weight products through chemical reactions, add the excess solvent and activated charcoal and boil the solution for a few minutes. Color impurities will be absorbed on the surface of the activated carbon, due to its high degree of microporosity. It uses filtration to separate the charcoal with the impurities that it has absorbed, as described in the next step.
Step 4. Remove the solids by filtration
Filtration can be done by gravity, by decantation, or by removing the solvent using a pipette. Do not use vacuum filtration in general, as the hot solvent will cool during the process, allowing the product to crystallize on the filter.
- Gravity Filtration - This is the method of choice for removing fine carbon, dust, lint, etc. Heat three Erlenmeyer flasks on a steam bath or on a hot plate: one flask or flask should contain the solution to be filtered, another should contain a few milliliters of the solvent and a stemless funnel, and the third should contain several milliliters of the crystallized solvent to be used for rinsing. Place the folded filter paper, which will be useful since we are not going to do it under vacuum, in a stemless funnel (it is preferred that it does not have a stem to prevent the saturated solution from cooling and cover the neck of the funnel with crystals) over the second Erlenmeyer flask. Bring the solution to be filtered to a boil, take it with a towel, and pour the solution on the filter paper. Add the solvent that is boiling from the third Erlenmeyer flask to any crystals that have formed on the filter paper and to rinse the first Erlenmeyer flask that contained the solution to be filtered, adding the rinse to the filter paper. Bring the filtered solution to a boil to remove any excess solvent.
- Settling: This is used to separate large solid impurities. Simply pour (decant) the hot solvent leaving only the insoluble solids in the flask.
- Removing a solvent using a pipette: Used for a small amount of solution and if the impurities in the solid are large enough. Insert a square-tipped pipette into the bottom of a test tube (round bottom), and remove the liquid by suction, leaving the solid impurities in the bottle.
Step 5. Crystallize the compound of interest
This step assumes that any color impurities and insoluble impurities have been removed with one of the previous steps. Remove any excess solvent by boiling or blowing it off with a gentle stream of air. It begins with a saturated solution with the solute at the boiling point. Let it cool slowly to room temperature. It should start to crystallize. If not, start the process by adding a seed crystal or by scraping into the tube with a glass rod at the liquid-air interface. Once crystallization has started, be careful not to disturb the container to allow large crystals to form. You can insulate the container with paper or cotton towels to promote slow cooling, which allows large crystals to form. Larger crystals are easier to separate from impurities. Once the container has completely cooled to room temperature, chill it on ice for 5 minutes to maximize the amount of crystals that can be obtained.
Step 6. Collect and wash the crystals:
To do this, separate the crystals from the cold solvent by filtration. This can be done using a Hirsch funnel, a Büchner funnel, or by extracting the solvent using a pipette.
- Filtration using a Hirsch funnel: Place the Hirsch funnel with the unfolded filter paper in a hermetically sealed flask. Place the vacuum flask on ice to keep the solvent cool. Wet the filter paper with the crystallization solvent. Connect the flask to the aspirator, turn it on, and check that the filter paper is pulled into the funnel by the vacuum. Pour and scrape the crystals into the funnel, and break the vacuum as soon as all the liquid has been separated from the crystals. Use a few drops of cold solvent to rinse the crystallization flask and pour it into the funnel while you aspirate again, breaking the vacuum as soon as all the liquid is separated from the crystals. Wash the crystals a few more times with cold solvent to remove any residual impurities. At the end of the wash, leave the vacuum cleaner to dry the crystals.
- Filtration using Büchner funnel: Place an unfolded piece of filter paper at the bottom of the Büchner funnel, and soak it in the solvent. Seal the funnel inside a vacuum flask with a simple rubber or synthetic rubber adapter that allows for vacuum aspiration. Pour and drag the crystals into the funnel, and break the vacuum as soon as the liquid passes into the flask, while the crystals remain on the paper. Rinse the crystallization flask with cold solvent, add this to the washed crystals, reapply vacuum, and break the vacuum when the liquid has separated from the crystals. Repeat and wash the crystals as many times as necessary. Leave the vacuum on to dry the crystals last.
- Wash using a pipette: it is used to wash a small amount of crystals. Insert a pipette with a square tip into the bottom of the test tube (round bottom), and remove the liquid, leaving only the already washed solids.
Step 7. Dry the washed product:
the final drying of a small amount of crystallized product can be done by pressing the crystals between the filter paper sheets, and allowing them time to dry on a watch glass.
- Crystallization can occur rapidly as the solution cools, if too little solvent has been used. When crystallization happens too quickly, impurities can become trapped in the crystals, defeating the purpose of purification by crystallization. On the other hand, if too much solvent is used, crystallization may not even occur. It is best to add just a little more of the solvent beyond saturation at the boiling point. It takes practice to find the perfect balance.
- If too much solvent is added that small crystals form on cooling, evaporate some of the solvent on heating and repeat cooling.
- Perhaps the most important step is to wait for the hot solution to cool slowly and allow the crystals to form. It is extremely important to be patient and allow the solution to cool without interruption.
- When trying to find the ideal solvent by trial and error, start by simmering the most volatile solvents first, as they can be removed the fastest.