BEST METHOD 4 PREPARATION OF ASPIRIN

 EXPERIMENTAL PROCEDURE FOR SYNTHESIS OF ASPIRIN

1. PUT ON YOUR CHEMICAL SPLASH-PROOF SAFETY GOGGLES!

2. Adding The Starting Materials

A. Using a weigh boat, weigh out 5.00 g (+ 0.01 g) of salicylic acid (C7H6O3). Transfer this to a 125 mL Erlenmeyer flask using a powder funnel. Record this mass on the Data Sheet.

B. Using the graduated cylinder located under the hood, measure out 7.00 mL of acetic anhydride and add this to the flask. Be sure to do this in the hood and wearing your goggles. Don't let the acetic anhydride contact your skin and don't get the vapors in your eyes.

C. Carefully add 8 drops of concentrated sulfuric acid (18 M H2SO4), a catalyst, to the flask.

3. Heating The Starting Materials

A. At your lab bench, assemble a hot water bath using a 600 or 800 mL beaker and place the flask in the water bath as shown in Figure 1. Make sure that the water bath is located directly under the hood at your lab bench.

B. Place the flask in the water bath and heat. After the water begins to boil, heat for an additional 15 minutes. (NOTE: The hot water bath will be used again later in the procedure.)

 .

4. Cooling The Reaction Mixture

A. After heating, turn the bunsen burner off and CAREFULLY remove the flask from the water bath (remember it is hot!) and allow the flask and contents to cool on the lab bench for about 3 minutes.

B. After the flask has cooled for about 3 minutes, CAUTIOUSLY add 15 mL of room temperature water to the flask to facilitate the decomposition of the excess acetic anhydride. Swirl the flask to mix the contents.

C. Label your flask containing the reaction mixture and place it in an ice bath and cool until the crystallization of the aspirin appears complete (approx. 15 min.). If crystals do not appear, you can scratch the walls of the flask with a stirring rod to induce crystallization.

5. Isolating The Product

A. Collect the solid aspirin using a Buchner funnel and filter paper as shown in Figure 2. Be sure to seat the filter paper in the filter with a small amount of water.

B. Rinse the flask twice with 3 mL of ice cold water to remove any residual crystals.

C. Discard the filtrate left in the filter flask into the waste container under the hood.

6. Recrystallizing The Aspirin

A. Transfer as much of the solid as possible from the Buchner funnel to a clean, dry 250 mL beaker.

B. Add 10 mL of 95% ethanol to the beaker and if necessary, warm (do not boil!) the mixture in the water bath to dissolve the crystals. If the crystals do not all dissolve, add 2 mL more of the ethanol and continue to warm the mixture to dissolve the crystals.

C. When the crystals are all dissolved, add 10 mL of deionized water, cover the beaker with a watch glass, and allow the solution to cool slowly on the lab bench undisturbed for about 10 minutes.

D. After the 10 minutes cooling on the lab bench, complete the crystallization by placing the beaker and contents in the ice bath. (Label your beaker!) Crystals should form. If an "oil" appears instead of a solid, reheat the beaker in the hot water bath until the oil disappears. If crystals do not appear, you can scratch the bottom of the beaker with a stirring rod to induce crystallization.

7. Drying The Purified Aspirin

A. Using a clean circle of filter paper, collect the purified aspirin by suction filtration as before.

B. Dry the crystals by pulling air through them for about 15 minutes. (Discard the filtrate left in the filter flask into the waste container under the hood. Rinse the filter flask with water and discard the rinse water into the waste container under the hood.)

C. Place the aspirin onto a doubled piece of paper towel and set aside to dry while performing the qualitative analysis of the aspirin. (Wash the filter funnel with water and discard the rinse water into the waste container under the hood.)