Preparation, Purification and Analysis of Aspirin (acetylsalicylic acid)
Synthesis:
We will prepare aspirin by reacting salicylic acid 1 with an excess of acetic anhydride 2
to produce aspirin 3 and acetic acid 4 :
OOH
OH
O
O
O
OOH
O
O
O
OH
H3PO4 (cat.)
++
1 2 3 4
The solid aspirin product will be separated from the reaction mixture by suction filtration
and further purified by recrystallization. A variety of techniques will then be used to
assess the purity of the product.
1) Start by creating a warm water bath: Fill a 250-mL Beaker with approximately 50-mL
of water, heat it on a Heater/Stirrer to 70-80 oC.
2) Weigh 2.0 g of salicylic acid into a 50-mL Erlenmeyer flask, then using a 10-mL
graduated cylinder measure out 4-mL acetic anhydride. Add the anhydride drop wise to
the flask containing acid using a Pasteur pipette. Dissolve the solid acid by gently
swirling the mixture, then add 10 drops of concentrated (85%) phosphoric acid (H3PO4)
to the solution.
3) Place the Erlenmeyer flask containing the solution in the warm water bath and heat
your mixture for 10 min. at 70-80 oC; be careful that the flask does not tip over into the
water bath. The solution should be agitated when heating by gently swirling the flask or
by using a glass stir rod. Add 20 drops of dd water to the Erlenmeyer flask and heat for
another 5 min.
4) Cool the mixture on an ice/water bath. Filter off the crystals that form by suction
filtration, then wash the collected solid with 2-3-mL portions of ice-cold water. Leave the
crystals on the suction apparatus to aid in drying them. Meanwhile, place a small amount
of your crystals (the tip of a spatula, ca. 50-mg) in a 3” test tube and add a few drops of
the iron chloride solution (FeCl3). If your solution turns purple it indicates starting
material salicylic acid remains in your product and it needs to be further purified by
recrystallization. Proceed to the step #5 below. If the FeCl3 remains yellow, even after
stirring and waiting a few minutes skip step #5 and go on to the Analysis section.
5) Remove your crystals from the Buchner funnel/filter paper (after turning of your
suction filtration) by scraping them into a 50-mL beaker using a spatula. Dissolve the
crystals in a minimum amount of warm ethanol, add the ethanol drop wise, using a
Pasteur pipette, while heating on the hotplate, about 10 drops of ethanol should be
