3. Acetylsalicylic acid
Systematic names: 2-acetoxybenzoic acid, 2-acetoxybenzenecarboxylic acid
It is prepared by a simple acetylation of salicylic acid with acetic anhydride.
Scheme of preparation:
OHO
OH
CH3
O CH3
O O
OHO
O CH3
O
CH3 OH
O
+
H2SO4
+
Chemicals:
salicylic acid 6.9 g (0.05 mol)
acetic anhydride 14 g (0.13 mol)
(Calculate and measure volume of acetic anhydride; density data can be found on bottle label, if
not, see a laboratory chemicals catalogue. Ask a lecturer for checking of your calculated volume
before you start the preparation.)
concentrated sulfuric acid 10 drops
Procedure:
6.9 g of salicylic acid is mixed with 14 g of acetic anhydride in an Erlenmayer flask and shaken
well (but carefully !) until the solid is completely dissolved. Then 10 drops of sulfuric acid are
added and the mixture is well shaken again. Its temperature should spontaneously increase to
approx. 45°C and crystals of acetylsalicylic acid should start to form. If not, the forming of crystals
can be initiated by rubbing of the flask wall with a glass rod. The content of the flask is then left to
crystallize for the additional 30 minutes and then poured into 140 ml of water in a beaker. Then it is
shortly heated on an electric hotplate and stirred with a glass rod until dissolution. If some solid
remains undissolved the mixture must be filtered. The solution (or filtrate) is then allowed to cool
down to laboratory temperature until crystals of acetylsalicylic acid are formed again. After
reaching laboratory temperature, the mixture can be refrigerated for several minutes. Crystals of
acetylsalicylic acid are then isolated by suction and washed with water on the filter until pH of the
filtrate is greater than 3. Acetylsalicylic acid can be recrystallized from aqueous ethanol. Yield is
approx. 90 % of theory. It needs to be well dried in a hot-air dryer before identity confirmation by
its melting point determination on a capillary melting point apparatus. Its purity or, more
accurately, absence of starting salicylic acid, is then confirmed by thin layer chromatography
(TLC) on a silica gel sheet and reversed-phase high performance liquid chromatography
(HPLC). Salicylic acid is used as a comparison compound in both chromatography procedures. Try
the mixture ethyl acetate/petroleum ether 1:1 as a mobile phase for TLC.
Confirmation of purity of acetylsalicylic acid by HPLC
Chromatography conditions:
Mobile phase: probably methanol : water : acetic acid 50 : 49 : 1 (is prepared in advance by the
technician)
Flow 0.6 ml.min-1
Detection UV, λ=254 nm
Column: a reversed-phase column (probably C18 or C16 with inserted amide moiety, ask a
lecturer)
Sample loop volume: 20 µl
A suitable integration hardware and software
Switching the chromatograph on/off and adjusting of parameters and mobile phase is performed by
a lecturer or a laboratory technician.
Chromatography procedure:
The solution of the proper sample of acetylsalicylic acid of concentration 0.1 mg.ml-1
in the actual
mobile phase is prepared in a volumetric flask. The comparison solution of salicylic acid of the
same concentration is prepared similarly. Approximately 0.3 ml of an analysed solution is aspired
into a syringe equipped with a suitable needle, the syringe piston is pulled down and whole the
inner surface of the syringe is washed with the solution. Then the syringe is emptied into a waste
bottle. This procedure is at least three times repeated. Approx. 0.3 ml of the solution is then aspired
again, the syringe is oriented with needle to top and the air bubble is sprayed out. The solution is
then injected to the needle port of the manual sample injector. At least 2 drops which drop out from
the waste capillary indicate that the sample loop is full (the handle is turned to Load). Then the
handle is turned to Inject and the sample solution begins to stream into the column and,
simultaneously, the observing of the analysis starts. The handle can be returned to Load after 2
minutes. The analysis proceeds about 10 minutes, then it is stopped. The chromatogram is
automatically evaluated and peaks are assigned with retention times and areas. Ideally, a
chromatogram could contain only one peak of acetylsalicylic or salicylic acid respectively. If not,
per cents of area of the particular peak in the total area are expressed. Every sample is analysed
three times, chromatograms are printed and mean retention times and per cents of total area are
calculated.
The report must contain in addition to record of synthesis:
• actual chromatographic conditions (composition of mobile phase, flow, wavelength of
detection, specification of column
• mean retention times of acetylsalicylic and salicylic acids
• mean per cents of total area which belong to acetylsalicylic and salicylic acids respectively
(the area of eventual solvent peak near the start of analysis is not taken into account)
Properties:
Acetylsalicylic acid forms white needle-shaped crystals of m.p. 135-137°C, odourless, of weakly
acid taste. The compound is slightly soluble in water, freely soluble in ethanol 96%.
Usage:
Acetylsalicylic acid has been used as analgesic, antipyretic and anti-inflammatory drug for more
than 110 years. More recently, its anti-platelet and thus antithrombotic effect is employed. All its
effects are mediated by cyclooxygenases inhibition.