n today’s experiment, we will be using acetic anhydride, (CH
3
CO)
2
O, instead of acetic ac id. This reaction is shown in equation (3). Acid anhydride, which is made by the removal of a water molecule from two acetic acid molecules, reacts with alcohols in much the same way as acids to form the ester.
Drug manufacturers prepare aspirin from salicylic acid by this method, which is a much better synthetic method than shown in (2). Using an excess of acetic anhydride has the advantage that the water that is produced by the reaction will react immediately with acetic anhydride. This will shift the reaction to the right, improving the yield of aspirin in the reaction. Second, unwanted side reactions are controlled well. In both reactions, a catalyst, normally sulfuric acid or phosphoric acid, is used to increase the reaction rateOrganic reactions seldom yield pure substances. In many instances, several reactions proceed simultaneously and mixtures of products result.
Likewise, the reaction may be incomp lete, and a portion of the starting material will be present in the product. It is for these reasons that one need s to purify the product.
Recrystallization
is a method of purification based on solubility of substances. Ideally, it is the dissolution of impure crystals into a hot solution an d, upon cooling, the collection of the pure component crystal’s. In today’s experiment, you will purify aspirin by this method.
You will also determine the melting point of aspirin.
Melting