The Synthesis of Aspirin

Purpose: To synthesize aspirin, a common analgesic drug. The experiment actually involves three parts: The synthesis of aspirin, the isolation and purification of aspirin, and the estimation of the purity of the final product.

Synthesis: The synthesis involves the reaction of salicylic acid and acetic anhydride in the presence of a catalyst, phosphoric acid, H3PO4.

Isolation and Purification: Once the aspirin is prepared it must be isolated from the reaction solution and purified. The aspirin is insoluble in cold water, and can be isolated by filtering the chilled reaction solution. Purification is necessary to remove any unreacted salicylic acid and acetic anhydride, as well as the acetic acid product and phosphoric acid. Acetic anhydride is caused to decompose by the addition of water once the formation of aspirin is complete:

The acetic acid and phosphoric acid are water soluble and can be removed by washing the aspirin with chilled water. Salicylic acid is only slightly soluble in water and is not completely removed in the washing step. Final purification is accomplished by the process of recrystallization. The impure aspirin is dissolved in warm ethanol. The solution is then cooled slowly, and the aspirin crystallizes out of solution leaving the salicylic acid and other impurities behind.

Estimation of Purity: The melting point of a compound can be used to identify it and also to estimate its purity. Generally an impure compound will exhibit a melting point which is lower than that of the pure compound. Therefore, if your aspirin melts at a temperature below the accepted melting point two possibilities exist: either your product is impure or it is not aspirin. A pure substance will melt sharply over a range of 1 or 2 degrees celcius. That is, the temperature at which melting first occurs is only 1 or 2 degree less than the temperature at which the sample is completely melted. An impure compound will melt over a wider temperature 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. Still in the hood, 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. Preweigh a watch glass, and place the filter paper with the product on it and weigh. Obtain the weight of the aspirin by subtracting the weights of the filter paper and the watch glass from the total weight.
  9. At this point the aspirin contains traces of water and salicylic acid, and further purification is required. Place a small amount of this impure aspirin in a small labeled beaker, cover with a tissue, and set aside to dry until next week. The amount to be placed in the beaker, enough to measure a melting point, will be indicated by the instructor.
  10. transfer the remainder of the impure aspirin to a 100 mL beaker. Add 10 mL of 95% ethanol to the beaker and warm (do not boil) the mixture in a water bath to dissolve the crystals. If the crystals do not all dissolve, add 5 mL more of the ethanol and continue to warm the mixture. When the crystals are all dissolved, add 20 mL of warm water, cover the beaker with a watch glass, and let the solution cool slowly. Crystals of aspirin will form. Complete the recrystallization by cooling in an ice bath.
  11. Collect the purified aspirin by filtration as before. Dry the crystals by pulling air through them for about 15 minutes. Put the product into a small beaker, cover with a tissue, and set aside to dry further until next week.
  12. Measure the melting points of both the impure aspirin, and the recrystallized aspirin.

Conclusion:

1. In this experiment, the amount of acetic anhydride reactant used is in excess over the amount needed to react with all of the salicylic acid.

a) If all (0.022 mole) of the salicylic acid reacts, how many moles of aspirin would you expect to get?
b) How many moles did you actually get? The molar mass of aspirin is 180.2.
c) What percent of the expected amount did you get?

2. Compare the melting points of the impure aspirin and of the recrystallized aspirin with that of pure aspirin, 138-140oC. What do you conclude?