Read Lab 5 ­ Recrystallization text version

Recrystallization

Most of the organic substances recovered from organic reactions, or collected from natural sources, are impure and require purification to obtain the desired pure product. One technique commonly used to purify solid organic substances is called crystallization. The general technique involves dissolving the material in a minimal amount of a suitable hot solvent. This creates a saturated solution. Upon slowly cooling the solution to room temperature, the solid will come out of solution. The dissolved material has a decreased solubility at the lower temperatures and crystallizes out of solution. If the cooling process is allowed to proceed slowly, pure crystals of the compound will form, and this is called crystallization. However, if the solution is cooled too quickly, nonselective crystals will form, and this is called precipitation. Crystallization is a purification process because as the crystals slowly form, the growing crystal selects only those same molecules which fit correctly into the crystal lattice. The undesired molecules (impurities) which do not fit into the crystal lattice remain in solution. Preparation: Mayo pg 90-93, 110-115; View the "Recrystallization" DVD in the Vasche Library prior to this experiment. Purpose: The purpose of the experiment is to introduce you to a technique called recrystallization. This technique can be used to purify solids. You will perform two recrystallizations which simulate two common situations. Crude benzoic acid contains an impurity which is more soluble in the chosen solvent than pure benzoic acid. The other situation is crude fluorene, where both soluble and insoluble impurities are present. Procedures: 1. Benzoic Acid from Water Weigh approximately 500 mg of crude benzoic acid, and place this material and a boiling stick in a 25 mL Erlenmeyer flask. Add approximately 15 mL of water. Heat the mixture on a hot plate until the solid has gone into solution. You will likely have to bring the water to reflux before all the crude dissolves. If necessary, use a Pasteur pipette to add additional water until all of the crude benzoic acid dissolves. After all the crude benzoic acid dissolves, take the mixture off the hot plate and let it slowly cool to room temperature. It is advisable to place the hot flask on an insulated surface, such as a textbook, or the edge of your cork ring, so that cooling is slow and uniform. You may want to clamp the flask onto a ring stand, suspending it in air, away from cold surfaces. Watch the flask closely for crystal growth which will likely take 10-15 minutes (be patient!). Do not disturb the flask during crystallization as this may result in smaller crystals. If no crystal growth has occurred after 15 minutes and the flask is at or near room temperature, scratch the inside-bottom of the flask with a glass rod, and/or place it in an ice bath to induce crystallization. If crystals do grow at room temperature, place the flask in an ice bath anyway for 5-10 minutes to maximize crystal growth and recovery of the solid.

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Filter the crystals using the Hirsch funnel (Figure 1) fitted with filter paper and apply a vacuum.

(a) (b) (c)

Figure 1. Hirsch funnel filtration apparatus (a) Hirsch funnel (b) f unnel adapter (c) filter flask

A vacuum can be generated by connecting the thick walled rubber hose to the side arm of the filter flask and the other end to the yellow house-vacuum inlet in the fume hood. Rinse the Erlenmeyer flask 2-3 times with ice-cold water to retrieve a majority of the crystals, and pour the rinses over the crystals on the Hirsch funnel to wash them. Dry the crystals on the funnel by applying the vacuum for at least 5 minutes. Further dry your purified benzoic acid in the lab oven. Weigh your purified sample, calculate the % recovery and determine the melting points of the crude and purified benzoic acid. Submit your purified benzoic acid sample according to the directions on the Report Form. 2. Fluorene from 95% Ethanol Prepare a glass wool filter-funnel by cramming a small amount of glass wool into the constriction of your glass or plastic funnel (Figure 2).

(a)

Figure 2. Hot f iltration funnel (a) glass wool lodged in funnel constriction

This will serve to filter and remove the insoluble impurity from the crude fluorene. Do not use too much glass wool and do not make the glass wool plug too tight, or the hot solution will not flow easily. If the flow is too slow, the hot, dissolved crude may cool and crystallize in the funnel. You may want to test the flow of your funnel by running through a few mL of the 2:1 acetone:water recrystallization solvent. Suspend the funnel through an iron ring, or clamp it to a ring stand. Place a small beaker or Erlenmeyer flask (50 mL) under the funnel. Weigh approximately 250 mg of crude fluorene, and place this material and a boiling stick in a 25 mL Erlenmeyer flask. Add 15 mL of the 2:1 acetone:water solution

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to the crude fluorene. Heat the crude fluorene mixture just until it boils. Realize the crude contains both soluble and insoluble impurities, so you should not expect everything to dissolve. Once you are confident that all of the fluorene and the soluble impurity have dissolved, quickly pour the hot crude solution into the glass wool filter-funnel, collecting the filtrate in the small beaker. Return the filtrate to the hotplate, with the boiling stick, and heat again to reflux. Remove the flask from the hotplate and allow it to slowly cool as you did with the benzoic acid sample. Watch the flask closely for crystal growth which will likely take 10-15 minutes (be patient!). Do not disturb the flask during crystallization as this may result in smaller crystals. If no crystal growth has occurred after 15 minutes and the flask is at or near room temperature, scratch the inside-bottom of the flask with a glass rod, and/or place it in an ice bath to induce crystallization. If crystals do grow at room temperature, place the flask in an ice bath anyway for 5-10 minutes to maximize crystal growth and recovery of the solid. Filter the crystals using the Hirsch funnel fitted with filter paper and apply a vacuum. Rinse the flask 1-2 times with ice-cold 2:1 acetone:water to retrieve a majority of the crystals, and pour the rinses over the crystals on the Hirsch funnel to wash them. Dry the crystals on the funnel by applying the vacuum for at least 5 minutes. Further dry your purified fluorene in the lab oven. Weigh your purified sample, calculate the % recovery and determine the melting points of the crude and purified fluorene. Submit your purified fluorene sample according to the directions on the Report Form.

References: Mayo, D. W.; Pike, R. M.; Trumper, P. K. Microscale Techniques for the Organic Laboratory, 2nd ed.; John Wiley & Sons: New York, 2001.

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CHEM 3012 - Spring 2009 Organic Chemistry Laboratory I

Name ______________________________ Partner _____________________________

Recrystallization ­ Report Form Raw Data Benzoic Acid mass of crude: __________ g m.p. of crude: __________ °C Fluorene mass of crude: __________ g m.p. of crude: __________ °C mass of purified: __________ g m.p. of purified: __________ °C mass of purified: __________ g m.p. of purified: __________ °C

Submit both purified samples in separate, labeled, sealed bags, stapled to the front, upper, left-hand corner of this page. Partners should halve each purified sample for submission. Observations appearance of crude benzoic acid:

appearance of purified benzoic acid:

appearance of crude fluorene:

appearance of purified fluorene:

Results % recovery of benzoic acid (show calculation):

% recovery of fluorene (show calculation):

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Discussion (Must be duplicated in notebook. Limit yourself to the available space.)

Conclusion (Must be duplicated in notebook. Limit yourself to the available space.)

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Questions (Answer in complete, full sentences. Limit yourself to the available space.) 1. Why is it advantageous to use a stemless funnel, instead of a long-stem funnel, when filtering hot recrystallization solutions?

2. What are some properties of an ideal recrystallization solvent?

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3. What would have happened if, instead of 2:1 acetone:water, you attempted to recrystallize the crude fluorene from 100% acetone? Explain your answer.

4. Imagine you failed to dry your purified benzoic acid sample completely. Tell how the error will affect your experimental results (yield, purity, or both) and explain why.

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Lab 5 ­ Recrystallization

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