Aspirin Synthesis

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, aspirin is one of the most inexpensive drugs available and is produced in vast amounts. In fact, industry makes 43,000,000 pounds of the drug every year.

This microscale experiment will permit you to make acetylsalicylic acid, or aspirin, by reacting salicylic acid with acetic anhydride. You will make an amount of acetylsalicylic acid equivalent to about half of an aspirin tablet, but your aspirin will not be in a form that can be ingested because it will still be contaminated with harsh chemicals from the reaction.

The reaction is given below; remember that each corner of the hexagon represents one carbon atom with enough hydrogen atoms to provide four bonds to each carbon atom.

salicylic acid + acetic anhydride  acetylsalicylic acid + acetic acid

Purpose:

To synthesize acetylsalicylic acid on a microscale basis.

Materials/Equipment:

Analytical balance, Filter paper, Stirring rod, Hot plate, Salicylic acid, Acetic anhydride, Boiling chips, Conc. Phosphoric Acid, Distilled water, 100ml beaker, Thermometer, Vacuum pump, Ice/Styrofoam cup, Microscale kit:Test tube, Syringe, Filtering flasks

Safety Considerations:Wear goggles and an apron at all times in the lab.Several of these chemicals will irritate your skin and eyes. If you spill anything 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 eye wash station.Do not ingest the aspirin you produce. It still contains harmful chemicals and is not fit to be taken.

Procedure:

  1. A hot water bath should be started in a central location using a 400 ml beaker filled ¾ with water. Heat to between 70o and 90oC. Once it reaches 70o C, cut it off. It will continue to heat up.
  2. Place a filter paper on the balance, tare it; add between .1350 and .1400g of salicylic acid to the paper. Record the mass (all four decimal places!) in the data table.
  3. Transfer all of the acid to a reaction tube (test tube) from the microscale kit.
  4. Get the mass of one boiling chip. Add the chip and one drop of conc. phosphoric acid to the test tube.
  5. Using the syringe from your 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 test tube when adding the acetic anhydride. Be sure to rinse the syringe well with water to clean it after use.
  6. Shake the reaction tube vigorously side to side to mix the reactants thoroughly.
  7. Fill a styrofoam cup about one-third full with the hot water from the central source, making a hot water bath.
  8. Place the test tube in the water bath. You are trying to dissolve the salicylic acid and may need to agitate the test tube while it is in the water bath.
  9. While waiting write your name in pencil on a piece of filter paper. Determine its mass using an analytical balance, and record it in the data table (all of the digits).
  10. Once the acid is dissolved, heat the solution in the reaction tube for 2 minutes more then cautiously add 0.5 mL of distilled water using your syringe.
  11. Remove the test tube from the water bath and allow it to cool to room temperature.
  12. Once crystallization has started, cool the test tube in a cup of ice water for several minutes until crystallization is complete. Be careful not to let the tube tip over and spill into the ice water.
  13. Place your massed filter paper in the funnel on top of the filtration flask. Transfer all of the contents of the test tube to the funnel. Rinse the test tube with ice water using your syringe to be sure you transfer all of your product to the funnel. Do not let the syringe touch the inside of the tube. Go to step # 14 if your liquid is NOT going through the filter. Otherwise, move on to step #15
  14. (Attach your vacuum pump to the filtering flask. It does not need to go on all the way. Have someone hang on to the flask, as it will probably tip over. Filter your product using the vacuum pump by pumping it slowly until the liquid is removed.)
  15. Allow the product to dry in the funnel overnight.
  16. To clean your lab station, rinse all glassware completely. You may need to roll a paper towel carefully to wipe clean the inside of the reaction tube. Once your product is dry, find the mass of the filter paper and product. Record its mass in the table below then calculate the mass of the dry product.

Data:

Mass of salicylic acid used
Mass of boiling chip
Mass of filter paper
Mass of filter paper, chip, and product
Mass of product

Calculations:

You reacted salicylic acid and acetic anhydride to form aspirin, or acetylsalicylic acid. You were directed to use an excess amount of the acetic anhydride so that the amount of aspirin produced would be determined by the amount of salicylic acid reacted.

  1. Calculate the molar mass of salicylic acid, C7H6O3.
  2. From the mass of salicylic acid measured out, calculate the moles of salicylic acid used.
  3. Using the balanced equation, determine the number of moles of acetylsalicylic acid produced by the moles of salicylic acid calculated above.
  4. Calculate the mass of aspirin that should have been produced, the theoretical yield. (hint: First calculate the molar mass of the acetylsalicylic acid.)
  5. Calculate the percent yield for your reaction.
    percent yield = x 100

Questions:

  1. What is the chemical formula for aspirin?
  2. What are some factors that would affect your percent yield?
  3. What chemicals may still be present that would contaminate your aspirin product?
  4. How could you use a Mel-temp (a melting point apparatus) to check the purity of the aspirin you produced?