Synthesis of Aspirin

Experiment 19
Preparation of Aspirin
Goals:
Concept:
1. Synthesize aspirin.
2. Familiarize students with vacuum filtration and melting point
techniques.
3. Be able to calculate percent (%) yield of the synthesized product.
4. Test the purity of the product.
Acetylsalicylic acid is the chemical name for aspirin. It is a drug with
many uses. It is used to reduce fever (antipyretic) and relieve pain and
inflammation (analgesic). It contains an ester group and is synthesized by an
esterification reaction. In this experiment, aspirin will be synthesized from
salicylic acid and acetic anhydride using an acid, H2S04, as a catalyst. The
reaction is shown below.
~
~O~OH +
Salicylic acid
(SA)
R 0 H+
CH3C-O-1b.CH3-
Acetic anhydride
o
~
II
C-OH
~CH3
~
Acety~alicylic acid
ASA=aspiriin
Calculation of possible theoretical yield from salicylic acid:
o
II
+ CH3C-OH
acetic acid
vinegar acid
mass (g) of
salicylic acid
(SA)
1 mole of SA
1 mole of ASA 180 g ASA _ grams ofasprin
x
x
x - (ASA)
138 g of SA
1 mole of SA I moleASA
possible product!
(molecular weight) (molecular weight) theoretical yield/mass
The purity of the aspirin will be determined by using two techniques,
melting point, (a physical property) and the addition ofFeCh, (a chemical
property) for the un-reacted starting material, salicylic acid.
Melting points are reported as ranges; temperature when the solid starts
melting to the temperature it completely becomes liquid. Pure substances usually
have narrow range (I-3°C) while impure substances have a wider melting range
and the observed melting point is lower than that of the pure substance.
The use of Ferric chloride, FeCh is also used to give a qualitative
indication of the purity of the aspirin. The orange colored FeCh forms a purple
color with a phenolic group (~OHdirectly attached to the benzene ring). Salicylic
acid has a phenolic group while the product aspirin does not. Therefore, the
formation of a purple complex with FeCh shows the presence of un-reacted
salicylic acid.
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Materials:
Salicylic acid, acetic anhydride, cone. H2S04,commercial aspirin,
1 % FeC13,400mL beaker, hot plate, filter paper, watch glass, 125 mL
Erlenmeyer flask, glass stirring rod, Buchner funnel, suction flask, rubber
hose (vacuum hose), ice, melting point apparatus, mercury thermometer
Wear safety goggles and do all parts of this experiment in the hood. Dispose of
chemicals in proper containers.
Procedure:
A. Synthesis of aspirin
1. Set up a hot water bath using a 400-mL beaker half-filled with water on a
hot plate as a source of heat. The water temperature in the beaker must be
maintained at 100°C.
2. Weigh a filter paper. Record the mass.
3. Weigh a 125 mL Erlenmeyer flask. Record the mass.
4. Add between 4.4 and 4.6 grams of salicylic acid (SA) to the flask and
reweigh the flask containing the salicylic acid. Record the mass.
5. Calculate the mass of the salicylic acid.
6. Add 6 mL of acetic anhydride to the Erlenmeyer flask. Mix in the hood.
Note: Acetic anhydride is an irritant to the nose and sinus. Be careful
handling it.
7. Slowly add (in the hood) 6-8 drops concentrated H2S04 to the flask,
stirring the mixture with a glass stirring rod.
8. Place the Erlenmeyer flask containing the mixture in the hot water bath.
Continue to stir until the salicylic acid is completely dissolved.
9. Remove the Erlenmeyer flask from the water bath and let it cool in cold
water to room temperature.
10. Add 20 mL of ice water (ice in distilled water). Place the flask in an ice
bath, stir, and observe the formation of aspirin.
11. Collect the crystals by vacuum filtration (refer to page 9B). First moisten
the pre weighed filter paper with water and fit it in the Buchner funnel.
Attach the funnel to the flask with a side arm. Attach one end of the
vacuum hose to the side arm of the flask and the other end to the aspirator.
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-- - - ---

12.
Turn on the faucet and transfer the crystals from the Erlenmeyer flask to
the funnel. Wash the crystals twice with 10mL of ice water each time.
Continue the vacuum filtration until the crystals are dry. Carefully remove
the filter paper from the funnel and place it in a watch glass in a secure
place until next week.
13. Weigh the filter paper with the crystals and calculate the actual mass
(yield) of your synthesized aspirin.
14. Calculate the theoretical yield and then the % yield of your aspirin.
B. Test the product for purity:
1. Meltini! point: Instructor, please show students how to use the melting point
equipment. The melting of pure aspirin is 13SoC. Take the melting point of
your aspirin and compare it with that of a commercial aspirin.
2. The use of FeCI}: Place a pinch (amount covering end ofthe microspatula) of
your prepared aspirin and crushed commercial aspirin in 2 separate test tubes.
Add 5 drops ofFeCb solution to each test tube. Compare the color ofthe two test
tubes. The darker the purple color is, the more un-reacted salicylic acid is present
in the aspirin.
Data Table:
Mass of filter paoer
grams
rams
rams
rams
ams
Theoretical yield (mass) of ASA, possible yield from
stoichiometry (see calculation on the first page of lab)
Actual yield (mass) of ASA from the exoeriment
grams
grams
x 100 %
Meltin
Meltinl! ooint of commercial asoirin
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Questions:
L
1. A student used 3.50g of salicylic acid. Calculate the following:
a) Theoretical yield (mass)
b) The actual yield (from the experiment) was found to be 1.50
grams less than the theoretical yield (mass), calculate the %
yield.
c) The melting point range of an unknown substance was
determined to be 120o-12SoC.What can you tell about the
purity of the unknown based on this information
L
2. Aspirin can irritate the stomach. What is added to the commercial
aspirin to reduce its side effect.
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