PART 1: RECRYSTALLIZATION OF AN ORGANIC SOLID

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1 EXPERIMENT 2 PART 1: RECRYSTALLIZATION OF AN ORGANIC SOLID THEORY Recrystallization is a purification technique that is very frequently used for organic compounds. When solid reaction products are isolated, they are seldom "pure" but are usually contaminated with small amounts of impurities such as reaction by-products, unreacted starting material, etc.. The process of recrystallization is used to "purify", or separate, the desired compound from the impurities. If you want to dissolve an ionic (salt-type) or very polar material, you use a polar, ionizing solvent such as water. If you want to dissolve a non-polar material (many organic compounds) you use a non- or relatively non-polar (organic) solvent. For intermediate cases where the material is moderately polar, water and/or organic solvents such as the alcohols, ethers, etc., may be suitable. In most cases the solubility of solids in liquids increases with increasing temperature. In crystallizations (or recrystallizations) we take advantage of this phenomenon by dissolving the solid in the minimum amount of hot solvent, rapidly filtering the hot solution to remove any insoluble impurities, and then allowing the filtrate to cool slowly and undisturbed so that large, well-formed crystals are obtained with the minimum number of imperfections and minimum surface area to trap and hold impurities. After cooling slowly to near room temperature, the material is chilled, in ice, to get maximum recovery of crystals. The solid is then collected by suction filtration and dried. If desired or necessary, the filtrate can be concentrated to give a second crop of crystals. DEFINITIONS AND TERMS: Pure: All molecules of the sample are identical (an impossible goal!) Impurities: Other undesired compounds present in the sample of desired compound. The impurities are usually unreacted starting materials or by-products from the reaction. Solvent: A liquid, either a single compound or a mixture, which will dissolve the solid to be purified. Solute: The material (impure) that is dissolved in the solvent.

2 - 2 - Mother liquor: The solution remaining after the "pure" product has been removed by suction filtration. This solution contains some desired product that did not crystallize but now has a higher proportion of impurities. Saturated solution: A solution that has the maximum possible concentration of solute that can exist at equilibrium with the solid substance, at a given temperature. Solubility: The concentration (in g/100 ml, mol L -1, etc.) of solute in a saturated solution. Solubility generally increases with increasing temperature, though there are exceptions. Crystallization: The process of cooling, without the loss of solvent, a hot, nearly saturated solution of a compound so that as the temperature falls, the solubility of the solute decreases and crystalline material is deposited from the solution. These crystals, that can be collected by filtration, will be of higher purity than the starting solute. Recrystallization: Repetition of the above process using the crystalline product that came out of solution, to achieve further purification. GENERAL Unless an impurity is very insoluble in the chosen solvent, it may remain in solution through the whole process. Since it is only a minor component, there may not be enough of it to form a saturated solution, even after cooling. A very soluble component in small proportions will certainly not come out of solution on cooling. The crystallized sample will only contain any of the soluble impurities if any remaining mother liquor evaporates from the crystals at the end of the cold suction filtration step. The purified product will probably still contain some of the impurities, but at a much lower level. 1. Solvent: The choice of a solvent is an art! In this course, you will be told what solvents to use. In general, one follows the adage "like dissolves like" (e.g. polar solids tend to dissolve in polar solvents, non-polar solids in non-polar solvents), but the choice of a good solvent is much more complex. Some factors to consider for a solvent follow: It must have a reasonably low boiling point, to aid in easy removal from the isolated "pure" product. There should be a wide range between the boiling point of the solvent and the melting point of the product. The solvent should boil well below the melting point of the solute, otherwise the product will not crystallize but separate as an oil as the saturated solution cools. The solvent must not react with the product and must be able to dissolve the product when hot, but not dissolve much of it at 0 C. Frequently, mixtures of miscible solvents (e.g. ethanol and water) are employed. If a solid is very soluble in hot ethanol and still rather soluble in cold ethanol, most of the desired product will remain in solution on cooling and yields will be low. If this solid is insoluble in water, an ethanol-water mixture can be found that has the

3 - 3 - desired properties, e.g. high solute solubility when hot, low solubility when cold. Mixed solvents are used in two ways: Either the mixture is made up before the crystallization then used as if it were a pure solvent, or, more frequently, the solute is dissolved in the hot solvent in which it is very soluble and then the second solvent is added, slowly, to the hot solution (after hot filtration) to force the solute out of solution. This process requires skill; check with your instructor the first time it is attempted. 2. Dissolving the solute: The solute is placed in an Erlenmeyer flask, boiling stones are added and the MINIMUM amount of boiling solvent is added to dissolve the sample. The solution is boiled and if all the solid does not dissolve, more solvent is added and the solution boiled again. This procedure is continued carefully until all the solute is dissolved or until no more remaining solid will dissolve. (The remaining solid could be insoluble impurities.) Use of too much solvent will cause the desired product to remain in solution on cooling, rather than crystallizing. Use of too little solvent will cause premature crystallization in the flask or in the filter paper during the hot gravity filtration. When all of the solute has been dissolved, a few ml of extra solvent (maybe 10% of the solution volume) should be added to compensate for solvent loss that occurs during subsequent heating/boiling. 3. Charcoal: If the solution (solvent + solute) is coloured by high molecular weight impurities, activated charcoal may be added, as it tends to adsorb these impurities and thus remove the colour. The charcoal is removed during the hot gravity filtration. Charcoal particles will cause boiling solvents to foam, so remove the solution from the heat and when the boiling stops (~20 30 seconds), add the charcoal (~0.2g). The solution should be boiled gently for a minute or two before filtering. 4. Hot filtration: This procedure removes insoluble impurities from the solution. Hot gravity filtration employs a fluted #4 (615) filter paper in a glass funnel. The funnel and flask are preheated on the steam bath and are supported in or on the steam bath (in the bath for large flasks or high boiling solvents, on the bath for small flasks or low boiling solvents) during the filtration to avoid cooling the solution and thereby preventing premature crystallization in the filter paper, funnel or flask. When the solvent is non-aqueous, it is advisable to dry the glass funnel, with a towel, just before use. The filter paper should also be kept as full as possible during the filtration by continually adding hot (boiling) solution. This filtration is not done with suction as the vacuum will cause the hot solution, which is to be retained, to boil vigorously which could cause loss of or contamination of the product. 5. Cooling: The hot filtered solution is cooled to cause the crystallization of the product. First the solution is cooled slowly to room temperature and then chilled in an ice bath for 10 15

4 - 4 - minutes. Allowing the solution to stand undisturbed, so that slow steady cooling occurs, allows the formation of large crystals, while swirling the solution causes more rapid cooling, resulting in smaller crystals. 6. Ice bath: For efficient cooling of a flask and its contents, an ice bath should contain enough water to form a slurry of ice cold (0 C) water and ice around the flask. Using only ice results in inefficient cooling as the flask is cooled only at the points of ice contact rather than over the whole immersed surface. 7. Suction filtration: This is used to collect the purified crystals on a flat filter paper (#5 or 610, hardened) in a Buchner funnel placed on a thick walled suction flask that is connected to a water aspirator. The water aspirator, when turned on full, achieves a partial vacuum that pulls the solvent from the crystals quickly. Before pouring the cold crystals/solvent mixture into the funnel, the filter paper, wetted with solvent (read note 8 and/or consult a demonstrator), is sealed onto the funnel by turning on the vacuum. The crystals/solvent mixture should be swirled and poured carefully, but quickly, into the funnel. The "mother liquor" may be used to rinse any remaining crystals out of the crystallizing flask and it may be used to obtain a second crop of product so the suction flask must be clean. It is a good practice to clean out the suction flask before each filtration. If necessary, pressing the crystals with a clean spatula will speed up removal of solvent. 8. Sealing the filter paper: Before the solution is filtered, the filter paper must be sealed onto the funnel. The paper must be sealed by pouring some of the solvent through the paper, placed in the funnel, and applying the suction so that the paper is sealed. If the solvent used is water, the paper will easily be sealed, but if the solvent used is a more volatile organic solvent, such as ethanol, problems can arise. If the solvent used is ethanol, the paper may not seal very easily because the ethanol may evaporate from the paper very rapidly and the seal will be broken. If filtration is attempted after the seal has been broken, the crystals will be sucked through the funnel and will be lost. To prevent this, one of the following methods can be employed. Either Pour a little water through the funnel, apply the suction to seal the paper and then pour some ethanol through to wash the water from the paper. The filtration must then be performed fairly rapidly, before the paper becomes unsealed. Also note that if all of the water was not rinsed from the paper, the paper will likely become clogged with product as the solution is filtered and the filtration will proceed slowly. Or

5 - 5 - Pour a little cold ethanol through the paper, apply the suction until all of the ethanol has been pulled through the paper (and the paper is sealed) and immediately filter your crystals before the paper can become unsealed. 9. Washing crystals: Washing is necessary to remove mother liquor from the solid product after suction filtration. Washing is best done with small volumes of fresh, ice-cold crystallization solvent, but note that this will cause some loss of crystals as the product will still have some solubility in the cold solvent. It is useless simply to pour solvent through the crystals under suction as the solvent goes through so quickly that little is achieved. The proper procedure is to break the vacuum either at the aspirator or by lifting the funnel, add the ice-cold, fresh solvent to the crystals in the funnel, carefully swirl so the solvent mixes with the crystals but does not disturb the filter paper, then reapply the vacuum to remove the washing solvent. Consult an instructor before washing crystals. NOTE: Never turn off the aspirator without breaking the vacuum or water may suck back through the aspirator into the suction flask. 10. Product: The product crystals can be dried on the Buchner funnel by suction for minutes if the solvent is volatile. If not, it is best to scrape the crystals onto a clean dry watch glass and allow them to dry in your locker over the week. Note: Some solid products will sublime if left open in this way, so if you are unsure, check with an instructor. When the crystals are dry, weights and melting points can be determined.

6 - 6 - RECRYSTALLIZATION OF TRIMYRISTIN The crude trimyristin (from lab 1) should be in your locker in a 250 ml round bottom flask. Before proceeding, make sure that you know the mass of the crude product from experiment one. Boil a portion of absolute ethanol. To the crude sample, add hot absolute ethanol (about ml) and heat the solution to boiling (boiling stones!) using a steam bath. If the crude lipid is not completely dissolved, add more ethanol, a few ml at a time, until no more solid will dissolve. (The amount of ethanol needed will vary, depending on the yield of crude material you have obtained, but you should not need more than 15 ml). Remove from the steam, let stand for seconds, then slowly add 0.2 g activated charcoal. Boil the solution gently for 2 minutes on the steam bath. Heat your short stemmed funnel on the steam bath while boiling the sample. Filter the hot solution by gravity through a fluted filter paper into a clean dry Erlenmeyer flask (placed on the steam bath to prevent premature cooling and crystallization during filtration). The flask containing the filtered solution should be set aside to cool to room temperature. As the solution cools, a white crystalline solid should form. When crystals are present you may cool the flask quickly (e.g. in an ice bath) to complete the recrystallization. Chill a small sample of absolute ethanol (10-15 ml in a sample vial) for use in the washing step. Set up the apparatus for a suction filtration, using a clean, dry suction flask and a Buchner funnel (#5, hardened filter paper). Before filtration, ensure that the filter paper is sealed using ethanol (See notes 7 & 8 on page 22). Filter the cold solution by suction through the Buchner funnel. Use some ice-cold absolute ethanol to transfer the remaining crystals out of the Erlenmeyer flask into the funnel and wash them with a small volume of ice-cold ethanol. Use a small clean spatula to squeeze excess solvent from the product and allow the aspirator to draw air through the solid for a few minutes. This will have the effect of drying the crystals. Spread the crystals on to a small pre-weighed watch glass and leave them in your locker for a week to complete drying. At this point, proceed to PART 2 of this experiment. After the product has been drying for a week, weigh the product and determine the melting point of the sample. Crush a small sample of trimyristin on a watch glass and place it into a melting point capillary. You should look up the expected melting point to help determine the appropriate power level. Place the capillary in the melting point apparatus and determine the melting range for the sample and add this to the raw data for lab #2. Compare the experimental melting point to the literature value. Comment on the purity of the sample. After the melting point has been measured, transfer the purified trimyristin to sample vial, label it and store it in your locker until it is needed in lab period 10 and 11.

7 - 7 - PART 2: MELTING POINTS THEORY The melting point is the temperature at which the solid and liquid phases of a compound are in equilibrium. The term "melting point" should be more appropriately be referred to as a "melting range" because even for very pure substances, there is a small temperature range between the start and completion of melting. Pure substances will melt over a narrow range (1-3 C). Impure substances tend to melt at a lower temperature and over a broader range. Both the size of the range and the actual melting point are thus useful indicators of the purity of a compound. PROCEDURE Before coming to the lab, look up the expected melting points for trans cinnamic acid and for urea and read appendix 1 for details on the procedure for determining a melting point. Crush a small sample of trans cinnamic acid on a watch glass and place it into a melting point capillary. Repeat this procedure for a small sample of urea. Combine a sample of your crushed cinnamic acid with a sample of crushed urea on a watch glass and thoroughly mix them together. Place this sample into a third capillary. Place all three capillaries in the melting point apparatus and determine the melting ranges for the three samples. You can melt all three samples during one trial, since the melting point device can accommodate up to three samples at one time. In the Discussion section, compare the experimental melting points to the literature values and comment on the purity of the samples. REPORT In the purpose section, explain (briefly) how the recrystallization procedure removes impurities from the sample. Your explanation should explain how impurities are removed by each of the following steps: the use of charcoal, the hot gravity filtration and the cold suction filtration. Calculate the percentage recovery for the recrystallized product and comment on sources of error. Comment on the melting points of the recrystallized sample.

8 - 8 - QUESTIONS (FALL 2010) 1. A sample of benzoic acid contaminated with borax and sand is to be recrystallized. Compound Solubility in 100 C Solubility in 0 C in g / 100 ml in g / 100 ml Benzoic acid Borax Sand insoluble insoluble The sample initially contained 3.0 g of benzoic acid, 1.5 g of Borax and 1.0 g of sand and was treated with 50 ml of boiling water, filtered by gravity and then cooled in ice to obtain a precipitate which was isolated by suction filtration. How much benzoic acid should be formed and will it be pure (in theory)? Use calculations to aid in your explanation. 2. After the suction filtration, the filtrate in the suction flask contains (among other things) some dissolved product, which had failed to crystallize. Could you process this filtrate to obtain a sample of the product? Explain. Would the second crop of crystals be as pure as the first crop? Explain. 3. Why is 2-methyl-2- propanol rarely used as a solvent for recrystallization? (Look up some of it's physical properties such as boiling point, melting point) QUESTIONS (WINTER 2011) 1. A sample of benzoic acid contaminated with NaCl and sand is to be recrystallized. Compound Solubility in 100 C in g / 100 ml Solubility in 0 C in g / 100 ml Benzoic acid NaCl Sand insoluble insoluble The sample initially contained 3.0 g of benzoic acid, 2.0 g of NaCl and 1.0 g of sand and was treated with 50 ml of boiling water, filtered by gravity and then cooled in ice to obtain a precipitate which was isolated by suction filtration. How much benzoic acid should be formed and will it be pure (in theory)? Use calculations to aid in your explanation.

9 You are given a mixture of solids A,B and C. In the solvent you are using, compound A is slightly more soluble than compound B, while C is essentially insoluble. Could you use the technique of recrystallization to obtain a sample of pure A? Explain. 3. During the suction filtration, the recrystallized product is washed with ice cold ethanol. Explain why this washing solution must be ice-cold. Since the recrystallized sample is not soluble in water, why not use ice-cold water as the washing solvent?

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