Introduction

Acetylsalicylic acid is a wonder drug par excellence. It is widely used as an analgesic (pain reliever) and fever depressant; it also reduces inflammation and may even prevent heart attacks. It has a few side effects for some people, yet it is safe enough to be sold without a prescription. Because it is easy to prepare, acetylsalicylic acid – better known by its trade name, aspirin – is one of the most inexpensive drugs available and is produced in vast amounts. In fact, the drug industry makes about 20,000 metric tons (43 million pounds!) of aspirin every year.

In this microscale experiment, you will make acetylsalicylic acid – aspirin – by reacting salicylic acid with acetic anhydride. Structurally, the reaction looks like this:

/ + / /  / / + /
salicylic acid / + / acetic anhydride /  / acetylsalicylic acid / + / acetic acid

You will be making acetylsalicylic acid equivalent to half of an aspirin tablet, but your aspirin will not be in a form that can be taken. It will still be contaminated with harsh chemicals from the reaction.

Objectives

By doing this lab, students will be able to:

  • synthesize acetylsalicylic acid on a microscale basis
  • describe the dangers of ingesting substances made in the lab
  • demonstrate proper use of microscale chemistry equipment

Materials and Equipment

Microscale organic kitThermometerConcentrated phosphoric acid

Analytical balanceStyrofoam® cupDistilled water

Weighing paperStirring rodIce

Hot plateSalicylic acidFilter paper

100-mL beakerAcetic anhydrideVacuum pump

Safety Considerations

  • As always, you should wear safety goggles/glasses when working in the lab area. If aprons are available, wear one.
  • Several of these chemicals will irritate your skin and eyes. If you spill any of the chemicals on you, wash the area thoroughly with soap and water.
  • Do not rub your face or eyes, because you may get chemicals in your eyes. If your eyes start to burn, rinse them at the eyewash station.
  • Do not ingest the aspirin you produce. It still contains harmful chemicals and is not fit to be taken.

Procedure

  1. Pour about 40 mL of tap water into the 100-mL beaker. Place the beaker on the hot plate and turn on the hot plate to a power setting of 5 (or about halfway).
  2. Using a pencil, write your name on a piece of filter paper. Determine the mass of the paper using an analytical balance and record the mass on the data table.
  3. With the filter paper still on the balance, press the TARE button. Weigh out between 135 and 140 milligrams (0.1350 – 0.140 g) of salicylic acid directly onto the filter paper. Record the mass (all four decimal places) in the data table. Place the acid in a test tube from the microscale kit.
  4. Add one drop of concentrated phosphoric acid to the test tube.
  5. Using the syringe from the microscale kit, add 0.3 mL of acetic anhydride to the test tube. Try to rinse all the other ingredients to the bottom of the tube when adding the anhydride.
  6. Use a stirring rod to mix the reactants thoroughly.
  7. When the water bath – the water in the beaker – is between 70 and 90 C, place the test tube in the water bath to heat its contents. You are trying to dissolve the salicylic acid and may need to stir the contents of the test tube while it is in the water bath.
  8. Once the acid is dissolved, heat the tube for 2 more minutes. Then, carefully add 0.5 mL of distilled water to the tube.
  9. Remove the test tube from the water bath and allow it to cool to room temperature. Crystals of acetylsali-cylic acid should form inside the tube. If crystallization does not occur at room temperature, use a stirring rod to scratch the inside of the test tube.
  10. Once crystallization has begun, cool the test tube in a cup of ice water for several minutes, until crystalliza-tion is complete. Be careful not to let the tube tip over and spill into the ice water.
  11. Line the Hirsch funnel from your microscale kit with the filter paper you weighed earlier. Using a spatula, transfer the contents of the test tube to the funnel. Rinse the test tube with ice water to make sure you transfer all your product to the funnel.
  12. Using the vacuum pump, filter your product. Your instructor will demonstrate how to do this.
  13. Leave the filter paper and acetylsalicylic acid to dry overnight. Make sure to leave it in a place where it will not be disturbed.
  14. Once your product is dry, weigh the filter paper and product and record the mass in the data table. Calculate the mass of the dry product.

Cleanup and Disposal

  • To clean up your lab station, rinse all the glassware and the funnel and return them to the microscale kit.
  • Once you have recorded the mass of the filter paper and acetylsalicylic acid, throw the paper and acid away in the trash can. Under no circumstances should you even think about ingesting the product!
  • Wash your hands thoroughly with soap and water when you finish making the aspirin – and when you’re through weighing it.