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Protocol

Chemistry

Recrystallization

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  1. Recrystallization of Acetanilide

    In this part of the lab, you'll purify acetanilide by recrystallization from an aqueous solution. Acetanilide is moderately soluble in boiling water, but it's much less soluble in room temperature and cold water. Aniline, a potential impurity, is much more soluble in room temperature water. Thus, you'll first make a saturated solution of acetanilide in boiling water. Then, you'll slowly cool the solution to room temperature.

    As the acetanilide's solubility decreases, it will slowly form flat colorless or white crystals. Water-soluble impurities will remain in solution. You will then cool the solution to further decrease the solubility of acetanilide. Finally, you'll recover the crystals with vacuum filtration and measure the melting point of your crystals.

    • Before you start the lab, put on a lab coat, safety glasses, and nitrile gloves.
    • To begin, bring a 125-mL Erlenmeyer flask and your lab notebook to the analytical balance to obtain your acetanilide.
    • Measure 2 g of acetanilide in a tared weighing boat and record the precise mass in your lab notebook. Add the acetanilide to the flask and bring it back to your fume hood.

      Table 1: Recrystallization of acetanilide

      Compound Mass (g) Melting point (°C)
      Acetanilide
      Seed crystal (if used)
      Recrystallized acetanilide
      (subtract seed crystal if used)
      Percent recovery
      Click Here to download Table 1
    • Then, measure 30 mL of deionized water and carefully pour it into the flask of acetanilide.
    • Place the flask on a stirring hotplate, add a stir bar, and set the hotplate to medium heat and stirring. Let the mixture boil for 4 – 5 min to dissolve the acetanilide.
    • If you see solid acetanilide after the solution has boiled for 5 min, measure another 10 mL of deionized water. Use a Pasteur pipette to add water dropwise until either the solid dissolves or you've added all 10 mL.
    • Let the solution boil for at least a minute after each milliliter before adding more water. Note: It's important to add water slowly so that you don't cool the solution too much or add more water than you need.
    • If you still see some solid after that, those are impurities that you must remove by hot filtration. If you do not see solid, continue the lab at step 14.
    • To perform hot filtration, place about 15 mL of deionized water and one or two boiling chips in a 50-mL beaker and start heating it on a hotplate.
    • Clamp another 125-mL Erlenmeyer flask on a hotplate and place a wide stem or stemless glass funnel in the flask. Fold a large piece of circular filter paper in quarters, place it point down in the funnel, and open it into a cone.
    • Once the water boils, use a pipette to wet the filter paper with hot water. Then, use tongs to pick up the flask of acetanilide solution and pour the hot liquid into the funnel, being careful not to let it overflow. Note: If the stir bar falls into the funnel, retrieve it with tweezers and rinse it with hot water.
    • Then, rinse the inside of the first flask with a few milliliters of hot water, pour the contents into the funnel, and dissolve any crystals that form with hot water.
    • Once the funnel is empty, use tongs to move it to the first flask. Carefully unclamp the second flask. You can now continue with the lab as usual.
    • Once there is no visible solid in your hot acetanilide solution, turn off the heat and stir motor and use tongs to place the flask on the floor of the hood.
    • Leave the solution undisturbed as it cools to room temperature, which usually takes at least 15 min. Moving or shaking the flask will make it harder for crystals to form.
    • While you wait, place a small amount of the starting acetanilide material in a weighing boat and bring it to the melting point apparatus.
    • Pack a capillary tube with the acetanilide and measure and record its melting point.
    • Once the solution cools, look for crystals without moving the flask too much. If you don't see any, use a glass rod to gently scratch the inside of the flask under the solution. It's easier for crystals to start to form on scratched or rough surfaces.
    • If crystals still haven't formed after 10 more min, ask your instructor for a seed crystal of high purity acetanilide. Measure and record the mass of the seed crystal, and then drop it into the room temperature acetanilide solution. Crystal growth will then occur on the seed crystal.
    • Once you see several crystals in the flask, prepare two ice baths in 600-mL beakers. One ice bath should nearly fill the beaker, and the other should occupy about half of the beaker.
    • Measure 5 mL of deionized water and place it in the full ice bath. Confirm that the flask is cool to the touch and that several crystals have formed before gently placing it in the half-full ice bath. Leave the flask undisturbed to improve crystal growth.
    • Check the crystal growth progress every 15 min. Once it looks like most of your acetanilide has crystallized, or you don't see much crystal growth over a 15-min period, set up for a vacuum filtration using a 125- or 250-mL filter flask.
    • Wet the filter paper with 1 mL of ice-cold water. Then, start the vacuum and pour the contents of the flask into the Büchner funnel of the vacuum filtration setup.
    • Scrape crystals from the flask into the funnel using a glass rod or flexible spatula and rinse the remaining crystals into the funnel with about 1 mL of cold deionized water.
    • Then, stop the vacuum, break the seal, and pour the rest of the cold deionized water into the funnel. Gently stir the crystals with the glass rod for a few seconds to wash them, being careful not to tear the filter paper.
    • Turn on the vacuum to drain the funnel. Then, stop the vacuum, break the seal, and empty the filtrate into a waste beaker. Turn the vacuum back on, and leave the washed crystals in the funnel while the vacuum pump pulls air through them until they are dry, which usually takes about half an hour. You'll start working on your next recrystallization while you wait.
  2. Recrystallization of trans-Cinnamic Acid

    In the second half of the lab, you'll recrystallize trans-cinnamic acid, which we'll just call cinnamic acid. Cinnamic acid is soluble in ethanol and nearly insoluble in water. So, you'll use a mixture of 95% ethanol and 5% water so that cinnamic acid is still soluble at high temperatures but less soluble at low temperatures.

    • To get started, fill a 5- or 10-mL graduated cylinder with deionized water and bring it to your hood.
    • Then, weigh 0.5 g of cinnamic acid, record its mass, and place it in a 25-mL Erlenmeyer flask.

      Table 2: Recrystallization of trans-Cinnamic acid

      Compound Mass (g) Melting point (°C)
      trans-Cinnamic acid
      Seed crystal (if used)
      Recrystallized trans-cinnamic acid
      (subtract seed crystal if used)
      Percent recovery
      Click Here to download Table 2
    • Next, obtain 4 mL of 95% ethanol, and pour it into the flask containing cinnamic acid. Place the flask on a hotplate set to medium heat and stir the solution with a glass rod until the cinnamic acid dissolves, which usually takes 7 – 10 min.
    • Then, turn off the heat and use a Pasteur pipette to add 5 drops of deionized water to the solution. This reduces cinnamic acid solubility. Note: If you don't see precipitation, add up to 5 more drops of water one by one. Stop adding water if you see solids starting to form.
    • Then, remove the flask from the hotplate and let it cool to room temperature undisturbed.
    • While you wait, get a small amount of the starting cinnamic acid and measure its melting point.
    • Once the flask has cooled to room temperature, inspect it for crystals. If you don't see any, gently scratch the inside of the flask under the solution with the glass rod and wait for 10 min. Note: If that still doesn't work, get a seed crystal of cinnamic acid from your instructor. Remember to weigh the seed crystal and record its mass before you add it to your solution.
    • Once the flask is cool to the touch and contains several crystals, remake the ice baths with one ice bath shallow enough that the 25-mL flask will not be completely submerged.
    • Refill the graduated cylinder with deionized water. Place the flask of cinnamic acid crystals in the shallow ice bath and the water-filled graduated cylinder in the other ice bath. Let the crystals grow for at least 15 min.
    • Your acetanilide crystals should be dry by now, so measure their mass and melting point while you wait for your cinnamic acid to finish crystallizing. Turn off the vacuum and break the vacuum seal.
    • Gently place the Büchner funnel in a clean beaker and bring it to the balance. Measure and record the mass of your pure acetanilide crystals. Remember to subtract the mass of the seed crystal if you used one.
    • Then, bring your crystals to the melting point apparatus and measure and record the melting point.
    • Now, place a clean, dry Büchner funnel with a new piece of filter paper in the filter flask and check on your cinnamic acid. Once it looks like most of the cinnamic acid has crystallized, wet the filter paper with cold deionized water.
    • Start the vacuum and pour the contents of the cinnamic acid flask into the funnel. Scrape the remaining crystals out of the flask with the glass rod and rinse the flask with about 1 mL of cold water.
    • Now, turn off the vacuum, break the seal, and pour the rest of the cold water into the funnel. Gently stir the crystals with the glass rod to wash them, being careful not to damage the filter paper.
    • Turn on the vacuum to drain the funnel. Leave the vacuum running until the cinnamic crystals are dry.
    • Place the Büchner funnel in a beaker and bring it to the balance. Measure and record the mass of your pure cinnamic acid crystals. Remember to subtract the mass of the seed crystal if you used one. Then, measure the melting point of your recrystallized cinnamic acid.
    • Now, dispose of your acetanilide and cinnamic acid in the designated solid waste container. Flush the filtrate down the drain with running water and pour leftover ice and water into the sink.
    • Clean your glassware using your usual methods and put away your other lab equipment.
    • Lastly, discard your used pipettes in the glass waste and throw out used filter paper and other trash in the lab trash container.
  3. Results
    • First, calculate the percent recovery for each compound. We expect that some of each compound stayed in solution, so these percentages will be below the reported purity of your starting material.
    • Now, look at the differences in melting points between the starting material and the crystals. For each compound, the less pure starting material has a lower melting point and a broader range than the recrystallized compound.
    • Lastly, compare your results to the literature values. The melting point of acetanilide is 114 °C, and the melting point of trans-cinnamic acid is 133 °C. The melting points of your crystals should span 1 – 2 °C and should be at or just below those values.

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