Purpose: To synthesize aspirin, a common analgesic drug, via nucleophilic acid-catalyzed substitution reaction of salicyclic acid with acetic anhydride. The whole reaction is catalyzed by phosphoric acid. (The experiment involved three parts: The synthesis of aspirin, the isolation and purification of aspirin, and the estimation of the purity of the final product. ) [pic] Procedure[1]:
A mixture of salicylic acid (0.21g, 1. 52 mmol) and acetic anhydride (0. 52g, 5. 1mmol, 0. 48 ml) with one drop of concentrated phosphoric acid in a conical vial was heated (50oC) till salicylic acid was dissolved while stirring (conical vial was added with a magnetic spinvane and attached with an air condenser). The solution was then heated for another 10 minutes and cooled to room temperature while the air condenser and spinvane was removed.
The solution was left to crystallize in a beaker of crushed ice while water was added (3. 0mL). The crystals were collected by vacuum filtration and washed with cold water (2 x 1. 0mL) then let dry to yield aspirin (0. 1732, 63%) as solid form. Mp. 133-136 oC (lit[2] 134-136 oC). Discussion: The reaction of synthesis of aspirin with salicylic acid an acetic anhydride with phosphoric acid as catalyst formed acetylsalicylic acid in 63% yield as crystalline solid.
The melting point of the crystals was determined to be 133-136 oC, which is within the reported literature range and supports the identity of the desired reaction product. The narrow range of the melting point indicated the purity of the compound. The reaction yield may be improved by adjusting the amount of reactant or product in the reaction. For instance, by increasing the amount of acetic anhydride reactant, the reaction equilibrium will favor the product, which will produce a greater amount of aspirin crystals.
Or, by removing the acetic acid product, the reaction equilibrium will favor the product to increase the yield of aspirin. Percent yield may also be increased by increasing the temperature and amount of catalyst. Both would serve the purpose of higher energy and produce more product as crystals [1] Callam, Christopher.
Organic Chemistry Lab- Chemistry 254, Iowa: McGraw Hill, 2008-2009, 115-120. 2 Gokel, George W. Dean’s Handbook of Organic Chemistry: McGraw Hill, 2004, 135.