The chief disadvantage of the shortest processing time rule is

Lab 5 Purifying Acetanilide through Recrystallization

Objectives • Identify appropriate recrystallization solvent for acetanilide • Purify acetanilide by recrystallization • Calculate percentage recovery by recrystallization

Background

Recrystallization Often during the process of synthesis, there will be undesirable compounds, or impurities, remaining with the synthesized compound of interest. To obtain a pure compound, the impurities need to be removed. One common purification technique is recrystallization. Recrystallization is based on variable solubility depending on temperature. A compound that is only soluble in a solvent at higher temperatures will crystallize out into a pure form when the solution is cooled. The impurities can then be removed along with the solvent.

Choosing a Solvent for Recrystallization A crucial step in recrystallization is choosing the right solvent. In general, the compound being purified should not be soluble in the solvent at room temperature but should dissolve when the solvent is heated. The impurities, if known, should ideally be soluble in the solvent at all temperatures so that they can easily be removed after the pure desired compound crystallizes out. In addition, the boiling point of the recrystallization solvent should not be close to the melting point of the compound being purified. If the boiling point is too close to the melting point or even above the melting point, the solid could melt and appear as an oil deposit, rather creating a solution.

If a suitable solvent cannot be found after testing, a solvent pair is often used. The pair of solvents must be miscible in each other but have opposite abilities to dissolve the compound. The desired compound should be very soluble in one of the solvents but not soluble in the other.

Once a suitable solvent is found for recrystallization, a minimum amount of the solvent should be used to dissolve the compound to be recrystallized. A solution near saturation is ideal to form crystals of the solid. However, an additional 5% solvent volume might be added to ensure that the compound doesn’t come out of solution prematurely.

Removing Impurities during Recrystallization During a recrystallization, there might be impurities that impede the successful crystallization of the pure product. For example, the heated solution might have an incorrect color, indicating other dissolved species. In this case, compounds such as activated carbon are often used to absorb the impurity from the solution. The solid activated carbon can then be filtered out.

Another common issue with recrystallization is an impurity that remains solid after dissolving the desired compound. This impurity can be removed using hot filtration, in which the glassware is

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Lab 5

heated to keep the desired compound in solution while the solid impurity is filtered out. Hot filtration is also often used in conjunction with activated carbon to avoid premature crystallization of the solid from solution during the removal of the activated carbon.

Initiating Crystal Formation Even with the best solvent and other impurities removed, the compound does not always form crystals upon cooling. There are a couple methods to initiate crystal formation, if needed.

The first method is gently scratching the side or bottom of the flask with a glass stirring rod. The small scratch provides a place for crystals to start growing. If scratching does not work, adding a small bit of the solid compound can act as a seed crystal for the rest of the crystals to grow around.

Isolating the Recrystallized Solid Once crystals form, they need to be removed from the solution. There are two types of filtration that can be used to achieve this.

Gravity filtration uses gravity to pull the liquid through filter paper placed in a funnel, isolating the solid on the filter paper, as shown in Figure 1. The filter paper can either be fluted with multiple folds as in the diagram or folded into quarters and opened into a cone shape. The solution is often decanted, pouring off the liquid first, into the filtration set-up so that solid doesn’t build up and block the flow through the filter paper. Gravity filtration is the preferred method for hot filtration as all the parts can easily be heated and manipulated when hot.

Figure 1 Gravity filtration set-up with fluted filter paper and a beaker being decanted into it

Vacuum filtration is a little more complicated as it uses a vacuum to help pull the liquid through the filter. The vacuum source may be a vacuum line or an aspirator attached to a water line. The vacuum flask is attached to the vacuum tubing and needs to be clamped in place to avoid the set- up toppling over, as shown in Figure 2. The Büchner funnel is connected to the flask through a rubber adapter for a good vacuum seal. The filter paper also needs to sit flat in the funnel, covering all the holes in the funnel. Vacuum filtration is most often used in recrystallization to isolate and dry the product crystals.

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Figure 2 Vacuum filtration set-up with cutaway of Büchner funnel and filter paper that will sit in it

During a recrystallization filtration, make sure you know where the desired product is located. In a gravity filtration to remove impurities, the product is still in solution, so the compound is in the collected liquid, known as the filtrate. When isolating the crystal product, the compound is the solid collected on the filter paper. The filtrate liquid at this step is often referred to as the mother liquor, as it was the liquid from which the crystals formed.

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Lab 5

Materials • Acetone • Ethanol • Petroleum ether • Water • Acetanilide • Activated carbon • Test tubes • Hot plate • Sand bath

• Ice bath • 25-mL Erlenmeyer flask • 100-mL beakers • Microspatula • Boiling chips • Pasteur pipette • Gravity filtration set-up • Vacuum filtration set-up • Melting point apparatus

Safety goggles are required!

All work should be performed in the fume hood.

Acetanilide is toxic and an irritant. Acetone and ethanol are flammable irritants; wash thoroughly if skin contact is made. Activated carbon added to hot solvent can cause the solution to boil over.

Procedure

Purifying Acetanilide through Recrystallization

Choosing a Solvent for Recrystallization 1. Label 4 test tubes as acetone, ethanol, petroleum ether, and water. 2. Place approximately 0.1 g of acetanilide in each test tube. 3. Use a microspatula to grind the acetanilide to a fine powder, which will help it dissolve. 4. Add 2.0 mL of each solvent to the respective labeled tube and shake well. Record your

observations on the data sheet. 5. Heat to boiling any test tubes that still have solid acetanilide in them in the sand. Record

your observations on the data sheet.

6. Allow the heated solvents to cool to room temperature. 7. Prepare an ice bath using a 250 mL beaker with equal parts ice and water. 8. Place all four test tubes in the ice bath. Record your observations on the data sheet. 9. Based on your observations, choose a solvent to recrystallize the acetanilide. Verify with

your instructor or teaching assistant that you should use the solvent that you selected. 10. Discard the contents of the four test tubes into the non-halogenated waste container. Wash

the test tubes with soap and water.

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Lab 5

Dissolving the Solid 1. Place 15 mL of the chosen solvent in a 25-mL Erlenmeyer flask with two to three boiling

chips and put it on a hot plate so that it can heat to a boil. 2. Measure out approximately 0.5 g of acetanilide into a different 25 mL Erlenmeyer flask.

Record the exact mass on your data sheet. 3. Use a Pasteur pipette to transfer approximately 1 mL (20 drops) of your boiling solvent to

the flask containing the acetanilide and mix well.

4. Place the acetanilide flask back on the hot plate to keep the solvent boiling. 5. Continue adding 0.5 mL at a time and mixing well until all of the acetanilide dissolves.

There may be a small amount of a solid impurity left in the flask despite multiple additions. DO NOT add all of the solvent that is left.

6. Remove both flasks from the hot plate and allow them to cool to room temperature. 7. Measure the amount of solvent that is left in the solvent flask and subtract it from the 15.0

mL you started with to find the volume added to the acetanilide. 8. Calculate 5% of the volume added to the acetanilide. 9. Add the calculated 5% volume of solvent to the acetanilide flask, which creates a 5%

excess, and mix well.

Removing Impurities during Recrystallization 1. Assemble the gravity filtration system as shown in Figure 1. 2. Measure out 0.050 g of activated carbon. 3. Place approximately 20 mL of recrystallizing solvent into a 100 mL beaker. 4. Heat the beaker on the hot plate until the solvent begins to boil. 5. Pre-heat the filtration system by filtering the boiling solvent through the system and place

the filtration system on the hot plate so that it remains hot. 6. Add the 0.050 g of activated carbon to the Erlenmeyer flask with the acetanilide solution

and reheat the solution to boiling. 7. Working quickly and while the filtration system is still hot, pour the boiling solvent from

the filtration system into the other 100 mL beaker. Then, filter the contents of the flask including the activated carbon through the filtration system. Record your observations, including color of the solution on your data sheet.

Crystal Formation and Isolating the Recrystallized Solid 1. Allow the filtered solution to cool to room temperature for about five minutes. If no crystals

start to form in this time, use a glass stirring rod to gently scratch the flask. 2. Place the cooled solution into the ice water bath for 5 minutes to complete crystallization. 3. Assemble the vacuum filtration system as in Figure 2.

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Lab 5

4. Cool 5 mL of the recrystallizing solvent in a test tube in the ice water bath at the same time as the solution.

5. Measure and record the mass of a piece of filter paper to go into the vacuum filtration system.

6. Turn on the vacuum source, probably water for the aspirator, and wet the filter paper with a few drops of the cool recrystallizing solvent to keep it in place.

7. Swirl and pour the flask containing the crystals and filtrate from the gravity filtration into the Büchner funnel using a stirring rod to direct crystals to the middle of the paper.

8. Remove the Büchner funnel and pour the filtrate back into the flask. Replace the Büchner funnel.

9. Rinse the acetanilide crystals on the filter paper with 5 mL portions of the filtrate into the filter system.

10. Slowly wash the crystals in the funnel through the filter system with the cold recrystallizing solvent from the ice bath.

11. Keep the vacuum pulling through the solid for five minutes to help dry the crystals and the filter paper.

12. Turn the water off and disassemble the filter system, making sure not to lose any of your recrystallized acetanilide.

13. Measure and record the mass of the dried acetanilide crystals and the filter paper. 14. Measure and record the melting point of your acetanilide crystals. 15. Place all liquids in the non-halogenated waste container. Place the acetanilide crystals in

the recrystallized acetanilide container. Filter paper should go in the trash after removing the crystals. Wash all glassware that was used with soap and water.

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Lab 5

Pre-Lab Questions Prepare for lab by completing and understanding the answers to these questions. Refer to the Background or another resource, such as your textbook, if necessary.

1. What are the dangers of adding activated carbon to a hot solution?

2. Why is activated carbon added to the solution in this experiment?

3. Give two possible ways to start crystal formation if crystals do not start upon cooling.

4. What properties does a good recrystallization solvent need to have?

5. What is the formula for percent recovery?

6. What is the literature value for the melting point of acetanilide? Provide your reference.

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Lab 5 Report Sheet

Lab 5: Purifying Acetanilide through Recrystallization Report Sheet

Name _______________________ Date _______________________

Section _______________________ Instructor _______________________

Purifying Acetanilide through Recrystallization

Choosing a Solvent for Recrystallization Solvent Solubility at 25 oC Solubility at boiling point

acetone ____________________ ____________________

ethanol ____________________ ____________________

petroleum ether ____________________ ____________________

water ____________________ ____________________

Chosen recrystallization solvent: ______________________

Dissolving the Solid Volume of solvent to dissolve solid ___________________________

5% of solvent added ___________________________

Space for calculations:

Mass of initial acetanilide ___________________________

Removing Impurities during Recrystallization Observations during recrystallization and impurity removal:

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Lab 5 Report Sheet

Crystal Formation and Isolating the Recrystallized Solid Mass of filter paper ___________________________

Mass of filter paper with acetanilide ___________________________

Mass of recovered acetanilide ___________________________

Percent recovery of acetanilide ___________________________

Space for calculations:

Melting point of recovered acetanilide ___________________________

Post-lab Questions 1. What would it mean if the melting point had a wide range in temperature rather than a

narrow range of one to two degrees?

2. Is your percent recovery of crystals within reason of what you would expect? Explain.

3. If you had chosen petroleum ether as your recrystallization solvent, what would have

happened?

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Lab 5 Report Sheet

4. Would there have been a change in your results if activated carbon was not added to the solution before recrystallization?

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