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Aspirin Experiment Fifteen

Synthesis and Characterization of Aspirin
One of the simpler organic reactions that can be carried out is the formation of an ester from an acid and an alcohol. This reaction proceeds as follows:
O + HO R' + H2O R OH R OR' an alcohol an acid an ester In the equation, R and R’ are H atoms or organic fragments like CH3, C2H5, or more complex aromatic groups. Thereare many known esters in organic chemistry that can be synthesized from organic acids and alcohols. The driving force for the reaction is in general not very great, resulting in an equilibrium mixture of the formed ester, water, acid, and alcohol. O

There are some esters which are solids because of their high molecular weight or other properties. Most of these esters are not soluble in water, sothey can be separated from the mixture by crystallization. This experiment involves an ester of this type, a substance commonly called aspirin (or acetylsalicylic acid). Aspirin is the active component in headache pills and is one of the most effective, relatively nontoxic, pain killers. Aspirin can be made by the reaction of the hydroxyl group (-OH group) in the salicylic acid molecule with thecarboxylic acid group (-COOH group) in acetic acid. The reaction proceeds as follows: OH OH O O O O + H2O + H3C HO H3C OH O
aspirin salicylic acid acetic acid A better preparative method, which we will use in this experiment, employs acetic anhydride in the reaction instead of acetic acid. The anhydride can be considered to be the product of a reaction in which two acetic acid molecules combine,with the elimination of a molecule of water. The anhydride will react with the water produced in the esterification reaction and will tend to drive the reaction to the right. A catalyst, normally sulfuric or phosphoric acid, is also used to speed up the reaction. The reaction occurs as follows:
OH O H3C O O CH3 + O HO salicylic acid H3PO4 H3C OH O O O aspirin + H3C O OH

acetic anhydrideacetic acid

Chemistry M01B Laboratory Manual pp. 87

Aspirin Experiment Fifteen The aspirin you will prepare in this experiment is somewhat impure and should certainly not be taken internally, even if the experiment gives you a bad headache. We will attempt to purify the aspirin via recrystallization with ethanol. The purified compound will be characterized by its melting point and by infraredspectroscopy. For further theory on IR spectroscopy, see the “Introduction to IR and NMR Spectroscopy” lab located at the end of this manual. Safety During the synthesis steps of the procedure, wear gloves and safety goggles. Both phosphoric acid and acetic anhydride are corrosive and will cause burns if spilled on your skin. Salicylic acid is a skin irritant. Procedure Prepare a 250 mL beaker ofapproximately 1/4 full of water. Place it on a hot plate and heat to 80 ºC. Weigh out approximately 500 mg salicylic acid in a 25 mL Erlenmeyer flask. Perform the next operation in the fume hood: pipet 1.0 mL of acetic anhydride and pour it into the flask in such a way as to wash any crystals of salicylic acid on the walls down to the bottom. Add 5 drops of 85% phosphoric acid to the mixture toserve as a catalyst. Clamp the flask in the water bath, and immerse it in the hot water bath for 10 minutes, stirring the liquid in the flask occasionally with a stirring rod. Once the reaction is complete, remove the flask from the water bath, and CAUTIOUSLY add 10 – 20 drops of water to the mixture to destroy any excess acetic anhydride. There will be some hot acetic acid vapor evolved as a resultof the decomposition of any unreacted acetic anhydride. Let the flask cool for a few minutes in air, during which time crystals of aspirin should begin to form. Put the flask in an ice bath to hasten crystallization and increase the yield of product. If crystals are slow to appear, it may be helpful to scratch the inside of the flask with a glass rod. Collect the aspirin by vacuum filtration....
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