Preparation of an Ester
Acetylsalicylic Acid (Aspirin)
To become familiar with the techniques and principle of esterification.
Aspirin is a drug widely used as an antipyretic agent (to reduce fever), as an analgesic agent (to reduce pain), and/or as an anti-inflammatory agent (to reduce redness, heat or swelling in tissues). Chemically, aspirin is an ester. Esters are the products of reaction of acids with alcohols, as shown in the following equation using type formulas:
R – C – OH + R’ – OH R – C – O – R’ + H2O
ACID ALCOHOL ESTER WATER
The symbol R refers to the hydrocarbon portion (radical) of the molecules aside from the
functional group. In an organic acid, R – C – OH, the functional group is the carboxyl
group (-COOH) or –C-OH. The type of formula for an alcohol is R-OH, where the functional group is the hydroxyl group (-OH). The symbol R’ indicates that the two R-groups in the ester formula need not be the same. It has been shown by radioactive tracer methods that in the mechanism of the esterification reaction, the –OH group is split from the acid and the –H from the alcohol.
Aspirin can be made as follows:
C – OH
C – OH
CH3 – C – OH + HO – CH3 – C – O– + H2O
The use of acetic anhydride instead of acetic acid, however, is a better preparative method, because the anhydride with the water to form acetic acid tends to drive the reaction to the right as shown below. An acid catalyst also is used to speed up the reaction.
C – OH
C – OH
+ HO – CH3 – C – O– + CH3 – C – OH
Acetic acid (138.12 g/mol) (180.15 g/mol)
The theoretical yield of aspirin in grams is the mass calculated from the above equation, with salicylic acid being the limiting reagent. The actual yield of aspirin is obtained by carrying out the synthesis: item (3) in the Report form. The percentage yield is found by the following relationship:
In this exercise, you will prepare a sample of aspirin, determine your theoretical, actual, and percentage yields, and estimate the purity of your product based on its melting point.
DANGER: A C E T I C A N H Y D R I D E
Vapor of liquid causes severe burns. Harmful if inhaled. Combustible. Do not get liquid or vapor in eyes, skin or clothing. Do not breathe vapor. If contacted, remove contaminated clothing at once.
Immediately flush skin or eyes with plenty of water.
For eyes get medical attention. Wash clothing before re-use.
Obtain a clean, dry 50-mL Erlenmeyer flask. Add 2.0 g of salicylic acid, 5.0 mL of acetic anhydride (Caution: Avoid contact with skin), and 5 – 10 drops of concentrated sulfuric acid to the Erlenmeyer flask. Mix the contents by swirling gently until the reactants are in solution.
Then heat the flask and contents by clamping it in a water bath maintained at 80oC for 15 minutes.
Now remove the flask from the water bath and immediately, but cautiously, add 2 mL of distilled water to decompose any remaining excess acetic anhydride.
Allow the flask to cool for a few minutes and then add 20 mL of water.
Place the flask in an ice-water...
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