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Purpose:
The purpose of this experiment is to prepare acetylsalicylic acid, a.k.a. aspirin.
Introduction:
An important part of chemistry is the preparation, or synthesis, of compounds from simpler start- ing materials. The compounds so prepared can be anything from industrial commodities such as sulfuric acid to specialty chemicals and drugs. All such preparations are carried out first at a lab scale. In this experiment, you will prepare aspirin (acetylsalicylic acid) from salicylic acid and acetic anhydride. The reaction is shown below, with modified Lewis structures.
C
C C
C
C C
C
O
O
O
H
H
H H
H
H
+ C
C
O
O
C
C
O
H
H H H H
H
C
C C
C
C C
H H
H
H C
O
C
C
O
H
H H
O
O H
+ C
C
O
H O H
H H
Salicylic acid
Acetic anhydride
Acetylsalicylic acid
Acetic acid
The acetic anhydride will serve as both reactant and solvent, a common technique in synthesis. When all the salicylic acid has been converted to aspirin, water will be added. This converts any unreacted acetic anhydride to acetic acid; the reaction is shown below. The reaction is an example of hydrolysis, the splitting (lysis) of a substance with water.
C
C O
O
C
C
O
H
H H
H
H H
+ H O^ H
C
C
O
H O
H H
H + C
C
O
O
H H
H H
Acetic anhydride
Water 2 Acetic Acid
Aspirin is not very soluble in water, so it crystallizes as the solution cools. It is then isolated by vacuum filtration. Most of the acetic acid is removed by the filtration, but enough remains to heavily contaminate the product. In fact, the mass of the crude (unpurified) product may exceed the calculated theoretical yield! The crude product is purified by recrystallization in hot water. This removes most of the remaining acetic acid, but it also hydrolyzes some of the product, giving salicylic acid and more acetic acid. On balance, however, recrystallization improves the
purity of the product. It does cause some loss of material, so one does not obtain 100% of the aspirin that could form from the starting materials. Therefore, one calculates both a theoretical yield and a percent yield.
Aspirin is the most commonly used drug in the world. It treats pain (is an analgesic), swelling (is an anti-inflammatory), and fever (is an antipyretic). Its anticoagulant activity was not noticed until the 1940’s. The precursors to aspirin were originally found in willow (genus salix, from which the name salicylic acid is derived), spirea (from which the name aspirin was derived), and wintergreen. These were known to the ancients. In 200 BC, Hippocrates recommended that his patients chew the bark and leaves for pain relief. By the mid 1800’s, the sodium salt of sal- icylic acid was being prepared for medicinal use. It had a nauseating taste, and was irritating to the stomach. A chemist in Germany developed acetylsalicylic acid to help his father, who had rheumatism, tolerate the drug. The chemist worked for a dye-manufacturing firm, Friedrich Bayer & Co. Aspirin was one of the first important pharmaceuticals produced by the company.
Safety and Waste Disposal:
In this experiment, you will use acetic anhydride and phosphoric acid, both of which are corro- sive and acidic. Avoid contact with them. If you get any on your skin, immediately wash thor- oughly with soap and water. Acetic anhydride has irritating vapors (the hydrolysis reaction occurs on the mucous membranes in your nose and throat), so it will be dispensed in the fume hood. Avoid inhaling its vapors. The aspirin is not pure enough for use as a drug. It should be disposed of in the waste jar in the supply area. The liquid waste can be flushed down the sink with water.
Procedure:
- Obtain a 50 mL Erlenmeyer flask, a filter flask, a Buchner funnel, a watch glass, a thermo- meter, a weigh boat, and two pieces of filter paper from the supply area. If the Erlenmeyer flask contains water droplets, heat it on a Bunsen burner to remove the moisture.
- Obtain the mass of the watch glass and record it on your data sheet.
- Weigh about 2 grams of salicylic acid into the Erlenmeyer flask. Do not try to weigh out exactly 2.000 grams of the chemical; your results will be fine with +/– 0.250 grams of salicylic acid. Since you won’t need the mass of the flask for further work, zero the bal- ance with the flask in the pan. Then remove the flask from the balance and dispense the amount of chemical you need. Do not leave the flask on the balance pan and try to get the salicylic acid into it, you’ll make a mess.
- Measure about 5 mL of acetic anhydride (density 1.082 g/mL) into a graduated cylinder, then pour this into the flask with the salicylic acid. Acetic anhydride will be in the fume hood, with a graduated cylinder to measure it in. Swirl the contents of the flask gently to mix them. The solid may not all dissolve. Add 5 drops of 85% phosphoric acid (H 3 PO 4 ). This will also be in the hood.
- Set up a Bunsen burner and ring stand. Use a 250 mL beaker to make a water bath. Clamp the Erlenmeyer flask in the water bath.
- Heat the water bath to about 75°C. Maintain this temperature for 15 minutes. Move the burner away from the bath when the water gets too hot. A few degrees of temperature fluctuation will not hurt the reaction, but excess heat will release acetic anhydride vapor.
