V. Results and Discussion One of the objectives of this exercise is to synthesize acetylsalicylic acid (aspirin) from salicylic acid. The mechanism for this synthesis is through nucleophilic acyl substitution. Acetic anhydride was the acetylation reagent used with the salicylic acid. The mechanisms and the reaction involved in the synthesis are seen in the following figure. 1.00 gram of fine white salicylic acid powder was weighed in a clean, dry 125mL Erlenmeyer flask. 3mL of strong smelling, clear colorless acetic anhydride liquid was then measured in a 10mL graduated cylinder in the fume hood, and poured into the Erlenmeyer flask. The flask was then gently swirled while 5 drops of 85% phosphoric acid was added to the flask. During this time, the solution in the flask was whitish and cloudy.
Phosphoric acid was used in the synthesis to become a source of H+ ions that would catalyze the reaction. Phosphoric acid is a liquid that doesn 't contain much water, since water will cause side reactions and reduce the yield of acetylsalicylic acid. A side reaction is the production of acetic acid from the water in phosphoric acid reacting with the acetic anhydride. Another reason for the use of phosphoric acid is that it creates a relatively strong conjugate base, since the acid catalysis step needs to be reversible. Sulfuric acid is also used in acetylsalicylic acid synthesis since it is 98% pure. In using concentrated sulfuric acid, there would be less of the side reactions,