Experiment 2. Recrystallization: The Purification of Crystalline Organic Compounds

Transcription

1 Experiment 2 Recrystallization: The Purification of Crystalline Organic Compounds The method selected for purifying a given organic compound depends on a number of factors such as the physical state of the compound (solid, liquid, or gas), the properties of the compound, and the nature of the impurities. Many crystalline organic solids can be purified by the technique known as recrystallization. This method may be applied to small quantities found in research laboratories (less than a gram) to the large quantities (tons) manufactured in the chemical industry. In principle, the procedure consists of (1) slowly dissolving the solid in an appropriate solvent while heating until a nearly saturated solution is reached, (2) cooling the solution until crystals form, and (3) recovering the crystals by filtration or centrifugation. Solubility Principles Understanding the relationship between solubility and temperature of an organic compound is essential to understanding recrystallization. The solubility of an organic compound in a given solvent depends primarily on the relationship between the polarities of the solvent and organic compound and the crystal lattice energy of the organic compound. Generally speaking, nonpolar solvents such as the hydrocarbons like benzene, toluene, and hexane, dissolve nonpolar solids such as naphthalene, dichlorobenzene, etc. On the other hand, polar hydroxylic solvents, like methanol and ethanol, dissolve polar compounds which contain polar groups such as -OH, -NH, and -CO 2 H. This behavior is sometimes called "like dissolves like". This is a useful principle but it should not be used as the answer to all solubility questions. The energy required to break up the lattice structure of the crystal also affects the solubility. The solubility of a compound may be approximated from its melting point. For example, a compound with a m.p. of 287 o C would have different solubility characteristics than a compound with a m.p. of 42 o C. High melting solids have strong intermolecular forces between the molecules in the crystal lattice. Strong solvent-solute interactions must overcome these forces in order for the compound to dissolve. Assuming a higher melting solid can be dissolved in a hot solvent it is frequently easier to recrystallize because there is a greater tendency for the molecules to form the crystal lattice. If the polarities of a solvent and an organic solid are similar, the compound will tend to be soluble. If the polarities are dissimilar, the compound will tend to be insoluble. For example, naphthalene (m.p o C) is soluble in toluene (nonpolar solid/nonpolar solvent) but it is insoluble in water (nonpolar solid/polar solvent). 46

2 CH 3 soluble in insoluble in H O H naphthalene toluene naphthalene water Between the two extremes of soluble and insoluble, many organic compounds are partially soluble in solvents of intermediate polarity. Ethanol is a polar compound because of its hydroxyl group, -OH, but it also contains the nonpolar ethyl group, CH 3 CH 2 -. In ethanol, the hydroxyl group makes a larger contribution to the overall polarity and ethanol is therefore considered to be a polar compound. When naphthalene is mixed with ethanol a limited amount dissolves. Nonpolar interactions between naphthalene and the CH 3 CH 2 - portion of ethanol are favorable but the interaction of naphthalene and the OH- of ethanol are not. Therefore naphthalene is only partially soluble in ethanol. Compounds that are partially soluble usually show an increase in solubility as the temperature is raised, as shown in the following Temperature-Solubility plot cool 40 o C 20 heat Solubility (grams/10 ml) Purification by recrystallization is based on this temperature-solubility relationship and consists of the following operations: compound A (containing impurities) is placed in a flask and heated while slowly adding a solvent until it just dissolves (usually at the solvent's boiling point). In the preceding figure the solubility of compound A at 80 o C, the boiling point of the solvent, is about 3g per 10 ml. If you begin with 3 g of A, 10 ml of solvent is required to dissolve it at the boiling point. At this point, insoluble impurities, if present, may be removed by filtering the hot solution. The solution is then cooled. The solubility of A decreases. At some point, if the conditions are right, compound A begins to crystallize. 47

3 After placing the flask in an ice bath the solubility of A is approximately 0.5 g per 10 ml. This means that at 0 o C 0.5g of compound A is still dissolved and the remaining 2.5g (3g-0.5g=2.5g) has crystallized. This corresponds to an 83% (2.5/3 X 100%) recovery. Soluble impurities, present in a lower concentration than compound A, will usually remain in solution. Purified compound A is then obtained by suction filtering the crystals and rinsing with a minimal amount of pure, cold solvent. The rinsing with cold solvent removes soluble impurities. If too much solvent is used in the rinse the % recovery will decrease, since even at a low temperature compound A is still slightly soluble. Residual solvent adhering to the crystals is finally removed by evaporation in air or in a vacuum desiccator, with or without heat. Most of the material that you recrystallize will be dried by evaporation of the residual solvent in air. Several recrystallizations, from different solvents, may be required to reach the desired level of purity. In each recrystallization some of the compound is lost because of its inherent solubility, even at ice temperature. These losses can be minimized by careful technique but not totally eliminated. Second crops of crystals may be obtained by evaporating the filtrate and recrystallizing the residue. Choice of Solvent The choice of solvent for recrystallization depends on several factors: 1. The solvent should dissolve a relatively large amount of organic compound at high temperatures and a small amount at low temperatures. 2. The solvent should dissolve the soluble impurities even at low temperatures. 3. The solvent should not chemically react with the compound being crystallized. 4. The solvent should be easily removed (usually by evaporation) from the recrystallized product. A lower boiling solvent is sometimes used to rinse the crystals to remove a higher boiling solvent. The choice of solvent in a recrystallization is determined experimentally. This is done by observing the outcome of several small-scale trial recrystallizations with a variety of solvents, usually done in test tubes. This point cannot be overemphasized! If you try to recrystallize all of a substance with the wrong solvent or use too much of it there may be no going back. Once the appropriate solvent is found the remainder of the material can be recrystallized. Mixed Solvents Mixed solvent systems are sometimes used in recrystallizations to adjust the solvent polarity. For example, a mixture of toluene (relatively non-polar) and ethyl acetate (relatively polar) may have better characteristics than either alone. In this instance the compound is usually dissolved in the solvent of greater solubility and the solvent of lesser solubility is added slowly until the hot solution is near the cloudy point (saturation point). The solution is then cooled and crystals form. A similar result can be obtained by experimenting with samples of the solvent mixed in various proportions. Finding the best solvent mixture can require a considerable amount of effort. 48

4 General Experimental Techniques of Recrystallization Heating The heat source most commonly used in a recrystallization is the steam bath since it provides a constant temperature and will not ignite the highly flammable solvents frequently used in recrystallizations. If steam is not available, an electrically heated hotplate may be used. In some situations the hot plate is preferred over the steam bath because it can reach higher temperatures. A gas burner should only be used when water is the solvent since most organic solvents are very flammable. When heating to dissolve the compound the solvent should not boil at a rate which would boil it off. If solvent is lost during heating more may be added. In some situations a condenser may be used. On the steam bath, solvents with boiling points less than 100 o C are in danger of boiling over. For example, when a test tube containing hexanes is placed on a steam bath the contents will boil vigorously and shoot out the tube! For many recrystallizations it is necessary to add boiling stones to promote smooth boiling. The stones are later removed. The steam bath should be connected as shown above. Steam enters the top hole of the steam bath and the condensate (hot water) drains out the lower hole into the sink. Rings should be removed to provide a large opening for efficient heat transfer to the flask. The steam bath should not be turned up to full force except initially to drive out the condensed water in the line. Just enough steam to heat a solution should be used. The surface of the steam bath is not smooth so care must be taken or the flask may tip over. Caution: Steam and the steam valve are hot! steam in steam/water out Hot Filtration If the hot solution has insoluble material or if decolorizing carbon is used for the removal of colored impurities, the solution must be filtered to remove the insoluble material. This step is done as rapidly as possible to Fluted filter paper minimize crystallization on the funnel. A stemless funnel, heated with steam and dried before use, is fitted with a fluted filter paper (your instructor will demonstrate how to "flute" a piece of filter paper) and stemless funnel (hot) supported on a ring. The hot solution is directed toward the middle of the filter paper with a glass rod, as shown in the diagram at the right. A collection flask is placed under the funnel and heated during the filtration. It is not always necessary to place the collection flask on the steam bath but for those compounds that crystallize out rapidly, the rising warm vapors of solvent help keep the funnel hot. It is good practice, in most cases, to place a few ml of the recrystallizing solvent in the collection flask beforehand. The heat of the steam bath will vaporize it and warm the filter paper and funnel. After the solution has been filtered, the funnel is rinsed with small portions of hot solvent. If crystallization has occurred in the collection flask during the filtration, the flask may be reheated to dissolve the crystals. Excess solvent may also be used so that crystallization is less likely to occur on the funnel. After filtration, the solvent 49

5 can be evaporated to near saturation by passing a stream of air or nitrogen (not burner gas!) into the heated flask until the desired volume is reached. Crystallization The hot solution should then be placed on an insulated surface (wood, folded towel etc.) for crystallization to take place. Once crystals start to form the flask should be left undisturbed. A slow rate of crystallization usually affords larger and purer crystals although extremely slow crystallization may trap solvent and impurities in the crystals. In some instances large crystals may be difficult to rinse and a purer product is obtained with smaller crystals. This is particularly true when the impurity is colored. For example, if slow crystallization gives large brown crystals a faster crystallization may give small white crystals simply because rinsing was more effective with the smaller crystals. With some compounds the initiation of crystal formation is very slow and scratching the inside of the flask under the liquid level with a non fire-polished glass rod or adding a seed crystal may be necessary to initiate crystallization. Each compound/solvent system seems to have its own personality when it comes to crystallization. Some compounds give spectacular displays of large needles, prisms, rosettes, etc. The shape of the crystals depends on the compound, the solvent, and the rate of crystallization. Some compounds may form different crystal shapes depending on the solvent. This may lead to different melting points for the same compound as mentioned in Experiment 1. After the solution reaches room temperature and it appears that no more crystals are forming the mixture should be cooled in an ice bath (ice and water should be used) to reach a minimum solubility and maximum recovery of the crystals. The solvent for rinsing the crystals, in a flask, should also be cooled. The solvent for rinsing is the same as that used in the recrystallization. Filtration Many organic compounds crystallize as fine crystals so that gravity filtration is very slow. With a slow filtration rate the mixture would warm up and dissolve some of the crystals. To facilitate a rapid filtration and effective rinsing, the crystals are filtered by suction on a Büchner or Hirsch funnel. The Hirsch funnel is preferred with smaller quantities (about 1 g or less) since rinsing is more effective and the product is more easily recovered. The Büchner and Hirsch funnels require a piece of properly fitting filter paper that just covers the holes but does not go up the side. The steps to follow in a filtration are: (1) Set up the apparatus using clamps (a filtration flask that is not clamped is easily upset and Büchner funnels are expensive). A three-prong clamp is 50

6 ideal for securing the side arm flask. (2) Place a properly fitted piece of filter paper on the funnel and place the funnel on the rubber adapter on the filter flask. (3) Turn the vacuum on all the way. (4) Add a few milliliters of cold solvent (the same solvent used in the recrystallization) to seat the filter paper. (5) Loosen the crystals in the flask by vigorous swirling or with a glass rod. Pour the resulting slurry onto the filter all at once and push the funnel down on the adapter to ensure a tight fit. To make stubborn crystals come out of the flask it may be necessary to swirl the flask and turn it upside down quickly while tapping (care is needed here!). Avoid decanting the liquid. (6) Transfer any crystals adhering to the flask onto the filter with a minimum amount of cold solvent (the same solvent used in the recrystallization). (7) Rinse the crystals with a minimal amount of cold solvent using a Pasteur pipette. Avoid using too much solvent in attempting to transfer every crystal. Decolorizing Carbon Colored impurities are common unwanted by-products in many organic reactions. They are frequently removed by adsorption on decolorizing carbon (also called activated charcoal) which is added to the hot solution just before the hot filtration. The hot solution should be cooled slightly before adding the carbon since the fine particles of carbon can promote rapid boiling, causing the contents to boil out of the flask! The right amount of carbon to add is determined by experiment but a guide might be 5-10% of the solute weight. Too little carbon will result in a less than white product whereas too much will give a lower yield of product since the carbon also absorbs the compound to be recrystallized. If the filtrate has a gray cast, fine particles of carbon have passed through or around the filter paper. Residual carbon can be removed by refiltration of the heated solution through a clean filter paper or through Celite, a filter agent (see instructor for this eventuality). The Experiment The object of the experiment is to purify an unknown by recrystallization. The experiment consists of two parts: Part (1) selection of appropriate solvent for recrystallization, Part (2) recrystallization of the unknown. NOTE: All waste that contains the unknown should be poured into the waste container in hood Z. This includes the filtrates after performing the recrystallizations. Only take a minimal of solvent (5-10 ml) to your workstation. Do not return solvents to the original bottles. 51

7 Part I. Solubility Tests and Choice of Solvent for Recrystallization. Exchange a clean and dry vial for one containing a ~ 3 gram sample of unknown. Immediately write down the unknown number in your lab notebook. Save a small portion (0.1g) for the melting point by placing it in a vial. In this part of the experiment the best solvent to recrystallize your unknown will be determined. The solvent in which the organic compound is most soluble when hot and least soluble when cold is usually a good solvent for recrystallization. Note that if a solid is soluble in the cold solvent it is also soluble when hot! Place a small amount of the unknown in a test tube and just cover it with solvent. Mix well and observe. If the material dissolves it is soluble at room temperature and clearly the solvent it not suitable. If the material does not dissolve or appears to be partially soluble, heat with the steam bath, mixing well. If it appears to dissolve, even partially, add more solvent and continue to heat. If it dissolves allow the solution to cool. Keep in mind that the unknown contains insoluble impurities so that not everything will dissolve. Be careful not to heat too long and evaporate the solvent. Allow the solution to cool and note the type of crystals. Finding the best solvent and watching the crystals form can be fun. Be aware that an insoluble solid may appear to dissolve when heated if its melting point is below the temperature of the heated solvent. In this case the solid has simply melted and on cooling solidifies. This is not a recrystallization! It is simply melting and solidification. A true recrystallization takes place when crystals are formed directly from solution. They will usually be seen to form from the entire solution-sides/top/bottom. Tests for recrystallization should be done with: water, 95% ethanol, 95% ethanol-water (use in a ratio of ~8:2), methanol, toluene, and heptane. Observations should be recorded for all tests. Some substances may be recrystallized from ethanol but recoveries are poor. This is because these substances are too soluble in ethanol even at cold temperatures. In this case a better recovery may be obtained by using an ethanol-water mixture where water serves to decrease the substance s solubility. Typically 10-30% water is effective. Higher % s can increase the recovery but will lower the purity since the solubility of impurities may also decrease. This may be carried out as follows. Add ethanol gradually to the substance to be recrystallized. A slight excess of ethanol is desired so that there will be a lesser tendency to crystallize during the hot filtration. Perform the hot filtration as usual. Then reheat the filtered solution and gradually add water allowing it time to stay hot. Approximately 10-20% water may be added or until the solution is near saturated as evidenced by a little cloudiness. Then allow the solution to cool and material to crystallize. Part 2. Recrystallization of the Unknown Review the general procedure for recrystallization. The skill of recrystallization requires practice. Since you will recrystallize substances throughout the year you should give attention to 52

8 understanding the principles and operation of this technique. recrystallization, the steps of the technique should be reviewed. Before beginning the Weigh the remaining amount of the unknown, after performing the solubility tests, (weight to the nearest g) into a 50 ml Erlenmeyer flask. Add the solvent selected in Part 1 to the flask to just cover the material. Heat on the steam bath with sufficient time for the solvent to reach the steam bath temperature (exercise care with hexanes since steam bath temperature is above its boiling point). Then slowly add additional solvent until the material just dissolves. In most cases it is important to use a minimum amount of solvent in dissolving the compound otherwise it may not crystallize or may partially crystallize giving a poor % recovery. However, in this case you are doing a hot filtration and it is better to err on the side of a little too much solvent otherwise the solution may crystallize during the hot filtration. The next steps are not always done but can be useful to provide information about the behavior of the solid. Cool the flask by placing it in an ice bath. Stir with a glass rod to break up the crystals. If the flask contents are too thick add more solvent to obtain a slurry of pourable consistency. Setup the equipment for a hot filtration and review the procedure. Reheat the flask on the steam bath, add more solvent if necessary, and perform the hot filtration. It is important to carryout the hot filtration rapidly with the fluted filter so that the solution does not cool and crystallize in the funnel. It may be necessary to begin with a hot solution that has a slight excess of the solvent to minimization crystallization in the funnel during the hot filtration. After filtration allow the filtrate to cool. Crystals should form at this time. If crystals form immediately the flask may be reheated to dissolve them though this is not always necessary. Once the flask has cooled to room temperature, cool it further in an ice-water bath (cool the solvent you will rinse the crystals with at the same time in a 25 ml flask). Break up the solid with a glass rod before the final filtration. If the mixture is too thick at this point add a small amount of solvent and continue to cool; the flask may also be reheated and allowed to cool. Filter on the Büchner or Hirsh funnel rinsing with cold solvent. Remove the "cake" of partially dried crystals and filter paper by sliding a spatula under the filter paper. Place them on a clean, tared piece of paper or watch glass. Peel off the filter paper and allow the solvent to evaporate over the week. Volatile solids may sublime during this time period. A solid is considered volatile if you can smell it! Determine the weight and transfer it to a properly labeled vial. Submit the vial to the instructor by placing the plastic tray on the desk. The % recovery = g purified material/g unpurified material X 100 Be aware that some unknowns now and in the future may require recrystallization from water, for instance, benzoic acid. The water will require a burner to heat it up. When the ventilation system is operating the smell or sound of an open gas jet cannot be detected. When the lab is over the ventilation system is turned off. A gas jet that is left on will eventually fill the room with gas, which could result in a serious explosion. 53

9 The notebook report should contain: 1. Title 2. Purpose of the experiment 3. The procedure (reference and outline) 4. Data and Observations (organized on your notebook page as shown below) Unknown number: Observations Data A. Solubility test table B. Data for the recrystallization solvent used in recrystallization approximate volume of solvent mass before recrystallization mass after recrystallization % recovery mp range before recrystallization mp range after recrystallization 5. Conclusions ASA Questions: 1. What is the purpose of recrystallization? 2. What solvent should be used to rinse crystals in a recrystallization? 3. What precautions should be taken when using hexanes as a recrystallizing solvent? 4. What is the relationship between solubility and temperature for most organic compounds? 54

10 5. The solubility of compound A is 2 g /100 ml at the temperature of a steam bath. What is the minimal amount of solvent required to dissolve 5.0 g of the material? 6. If the solubility of compound A is 0.5 g/100 ml at ice temperature how many grams of compound A (problem 5) may be isolated in a recrystallization? 7. Why might it be useful to recrystallize a substance from two different solvents in sequence? 8. Circle the solvents that are miscible with water: ethanol, hexanes, toluene. 9. Which is more soluble in hexanes, methanol or ethanol? Why? 10. How can you determine if a substance is dissolved in a hot solvent or has melted without actually dissolving? 55