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.
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?