The Synthesis of Aspirin

Purpose: Synthesize aspirin and determine the feasibility of the synthetic methodology using percent yield. The purity of the product is confirmed by measuring its melting point range.

Safety Considerations: This experiment uses salicylic acid, acetic anhydride and phosphoric acid. The salicylic acid and aspirin may cause irritation to your skin or eyes, but are basically not hazardous. An excess of these can be disposed of in the sink or if packaged, in the trash. If you spill some, wipe it up with a wet paper towel and throw the towel in the trash. The acetic anhydride and phosphoric acid can cause bad burns. Use them in the hood. Be sure to wear gloves and safety goggles when using these chemicals. Excess chemicals must be disposed of in the plastic tub of water. This will convert the acetic anhydride to vinegar and dilute the phosphoric acid. If you spill a lot of either of these, notify your instructor.

Procedure:

  1. Weigh out 3.0 g of salicylic acid and place in a 250 ml Erlenmeyer flask.
  2. Measure out 6.0 ml of acetic anhydride and add this to your flask. Be sure to do this in the hood and wear your goggles.
    Don't let the acetic anhydride contact your skin and don't get the vapors in your eyes.
  3. Carefully add 5 to 10 drops of 85% phosphoric acid, a catalyst, to the flask and swirl to mix everything thoroughly.
  4. Still in the hood, heat the mixture for about 10 min. in a beaker of warm water (70-80 oC).
  5. After heating, cautiously add 20 drops of distilled water.
  6. Next add 20 ml of distilled water and cool in an ice bath. You can do this at your bench. If crystals do not appear, you can scratch the walls of the flask with a stirring rod to induce crystallization.
  7. Filter the solid aspirin through a piece of pre-weighed filter paper using a Buchner funnel and the aspirator. Wash the crystals with 2-3 ml of chilled water. The liquid is mostly water and can be washed down the sink. Allow the air to be drawn through the solid and filter paper for 15 minutes. Be sure to record the filter paper weight in your notebook.
  8. Place the filter paper with the product in a watch glass and put it in the oven at 100 oC for about 30 min. until dry.
  9. Put the dry aspirin and the filter paper into a pre-weighed plastic bag and weigh again.
  10. Measure the melting point range with the "Meltemp" Apparatus (your instructor will demonstrate) and record it in your notebook.  Compare your value to the actual melting point range for pure aspirin, 138-140oC.
  11. Calculate the weight of your product by subtracting the weight of the paper and bag from the total. The theoretical (maximum) yield is 3.9 grams of pure aspirin. What percent of this amount (3.9g) did you actually prepare? This is your percent yield. Record it in your notebook and turn in your product to the Stockroom.

In your conclusions, assess (1) the feasibility of your synthesis using your percent yield and (2) the purity of your products using your melting point range.

Revised by SLP on 10/12/99.