Synthesis of Aspirin
Name ________________________________ Date __________ Hour ____
Introduction
Aspirin belongs to a general class of drugs called analgesics, which relieve pain. With
the exception of caffeine, analgesics are the most commonly used drug around the world
today. Americans consume 45 millions pounds per year, which is an average of over 270
tablets per person. Their widespread use has made them a popular remedy for many
ailments, including headaches, muscular aches, fever, colds, influenza, arthritis, and
rheumatism.
The Reaction
The reaction involves mixing solid salicylic acid (C7H6O3) with liquid acetic anhydride
(C4H6O3), while adding some acid as a catalyst, and producing acetylsalicylic acid
(aspirin, C9H8O4) and acetic acid (HC2H3O2). The structures of the reactants and
products are shown below.
Δ
C7H6O3 (s) + C4H6O3 (l)
Salicylic
Acid
Acetic
anhydride
, H+
C9H8O4 (s) + HC2H3O2 (l)
Aspirin
Acetic Acid
Some Considerations
The acetic anhydride is very reactive, and will react with water if any is present.
Therefore, the 125 mL Erlenmeyer flask has to be clean and dry before the experiment
begins. If your Erlenmeyer flask is not clean and dry, notify your instructor.
Salicylic acid will react with acetic anhydride very, very slowly at room temperature
when no catalyst is added. To speed up the reaction, a small amount of acid catalyst is
added in the form of H2SO4. The sulfuric acid provides a source of hydrogen ions (H+),
which is the ion responsible for the catalytic action of the acid. Sulfuric acid is used
instead of hydrochloric acid (HCl) because the chloride ion from HCl will react with the
salicylic acid, producing an unwanted by-product.
Heating of the reaction is also necessary. The direct, intense flame of a Bunsen burner is
hot enough to decompose the aspirin as it forms. By using a water bath, the reaction is
heated more gently, and the maximum temperature will not exceed 100˚C.
Finally, the reaction shows that acetic acid (HC2H3O2) is also formed in the reaction.
Most of the acetic acid is removed upon rinsing, but some may remain. Inexpensive
bottles of aspirin do not always have all of the acetic acid removed, and may have a slight
“vinegar” smell.
Experimental Procedure
Warning:
Wear splash-proof safety glasses throughout this lab. Sulfuric Acid is very
caustic (meaning it reacts with the skin) and very corrosive (meaning it
reacts with metals), so treat it with extreme caution. Wash your hands
before taking off you safety glasses.
1) Mix the Starting Materials and Heat. Weigh and record 5.0 g of salicylic acid in a
plastic weighing tray and transfer to a clean, dry 125-mL Erlenmeyer flask. Cover the
crystals with 10.0 mL of liquid acetic anhydride, which can be found in the hood. Swirl
the flask to wet the salicylic acid crystals. Carefully add 5 drops of concentrated sulfuric
acid (also in the hood) and heat the flask in a hot water bath for a minimum of 10
minutes. All of the solid salicylic acid should dissolve.
2) Cool to Crystallize the Aspirin. Remove the flask from the hot water bath and add
25 mL of cold, distilled water to decompose any excess acetic anhydride. Chill the
solution until the crystallization of the aspirin is complete, swirling occasionally to
decompose residual acetic anhydride. If time permits, go on to step 3. If little time
remains, place the Erlenmeyer flask containing your aspirin on a post-it note with your
name, and set aside until tomorrow.
3) Wash and Transfer the Aspirin to the
Filter Paper and Wash. Setup a vacuum filtraTion apparatus with a funnel, piece of filter paper,
And Erlenmeyer flask. Wet the filter paper with
water from your squeeze bottle. Add 25 mL cold,
distilled water to the flask, swirl and chill again.
Filter the aspirin crystals, and rinse them with
5 mL of cold water. (Aspirin is somewhat soluble
in warm water but is much less soluble in cold water.
If warm water is used for rinsing, a lot of aspirin
will dissolve and will make it through the filter
paper.)
4) Determine the mass of a small plastic container and record. Scrape your aspirin
off the filter paper, and put them in the weighing tray. Using a Sharpie, write your name
and class hour on the tray, and set aside. We will let the aspirin dry overnight.
5) Determine the mass of aspirin formed. Take your aspirin crystals over to the
balance, and determine the mass of the aspirin + tray and record. Place the aspirin in a
zip-lock baggie, and label the bag with your name(s), hour, formula of aspirin, and a
picture of its structure.
Data: (Record all masses to the nearest 0.01 g)
Mass of salicylic acid used =
__________ g
Volume of acetic anhydride =
__________ mL
Mass of tray =
__________ g
Mass of aspirin + tray =
__________ g
Calculations: Make sure to show your work on all calculations!
1) The density of acetic anhydride is 0.792 g/mL. Calculate the mass of the acetic
anhydride you used.
2) Determine the limiting reagent in this experiment.
3) Calculate your theoretical yield of aspirin for this experiment.
4) Calculate your experimental yield of aspirin.
5) Calculate your %Yield for this reaction.
6) Give at least two reasons why your percent yield was less than 100%.
Synthesis of Aspirin Pre-Lab (This must be done before doing the lab tomorrow.)
1) Write the formulas of the following compounds:
salicylic acid
__________
acetic anhydride __________
aspirin
__________
acetic acid
__________
sulfuric acid
__________
2) What role does the sulfuric acid play in this lab? Which ion in particular is needed
from the acid?
3) Why is a hot water bath used to heat the reaction, and not a direct flame from a
Bunsen burner?
4) Hydrochloric acid is much less expensive than sulfuric acid. Why, then, is sulfuric
acid used in this laboratory?
5) What is an analgesic?
6) Some inexpensive bottles of aspirin smell like vinegar. Explain why.
7) Why must the Erlenmeyer flask be dry before beginning the experiment?
Suppose a student conducted an experiment, collecting the data shown below.
Answer questions 8-12 based upon this data. Make sure to show your work on all
calculations. The answers are provided so that you can check yourself as you go.
You may need to refer to the front page of this lab for the balanced reaction.
Hypothetical Data:
Mass of salicylic acid used =
4.21
g
Volume of acetic anhydride =
7.25
mL
Mass of tray =
1.35
g
Mass of aspirin + tray =
5.90
g
8) The density of acetic anhydride is 0.792 g/mL. Calculate the mass of 7.25 mL of
acetic anhydride. (Answer: 5.74 g)
9) Determine the limiting reagent in this experiment.
(Answer: 4.21 g of salicylic acid requires 3.11 g of acetic anhydride, so the salicylic
acid is the LR.)
10) Calculate the theoretical yield of aspirin for this experiment. (Answer: 5.49 g)
11) Calculate the Yield of aspirin. (Answer: 4.55 g)
12) Calculate your %Yield for this reaction. (Answer: 82.9%)