Organic Synthesis of Aspirin Chemistry Formal Lab.

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Experiment Four: Organic Synthesis of Aspirin


The purpose of this experiment is to synthesize a common organic product called acetylsalicylic acid (aspirin), and to become familiar with the optimum conditions needed for successful yields. Aspirin is produced from an acid catalyzed reaction between salicylic acid with acetic anhydride. The crystalline aspirin is synthesized and purified by recrystallization, although there is not a hundred percent yield due to sources of error.


Aspirin is a medicine commonly found in households around the world. It also is one of the least expensive and most useful drugs in the market. A Chemist named Felix Hoffmann first synthesized aspirin, otherwise known as acetylsalicylic acid, in 1897 from salicylic acid.1

Salicylic acid is a naturally formed substance. For example, willow trees have been recognized since ancient times as a natural source. Salicylic acid is found mainly in the willow's leaves and bark. The pure acid possesses several useful medicinal properties. It is an antipyretic (a fever reducer), an analgesic (a pain reducer), and an anti-inflammatory (a swelling reducer).

Unfortunately, pure salicylic acid makes for a particularly unpleasant medicine. The salicylic acid molecule contains two acidic functional groups, the phenol group and the carboxylic acid group. These groups make salicylic acid irritating because it burns the sensitive linings of the mouth, throat, esophagus, and stomach. 2

These harsh qualities are alleviated by replacing the acidic hydrogens with less reactive groups of atoms: the acetyl group (COCH3). This results in acetylsalicylic acid or "aspirin".

Acetylsalicylic acid is synthesized in labs and does not occur naturally. It is produced from adding acetic anhydride to salicylic acid in the presence of sulfuric acid (H2SO4), an acid catalyst: (Diagram from 1)

The impurities left over (unreacted salicylic acid, acetic acid and sulfuric acid) are removed by the process of recrystallization. This process separates pure aspirin by precipitating it out of the impure solution.

Pure acetylsalicylic acid contains only one acidic functional group which bypasses most of the digestive system without causing burns.2 Ultimately, acetylsalicylic acid hydrolyses with water, mainly in the bloodstream, to regenerate salicylic acid: (Diagram from 1)

There are four mathematical calculations given by the instructor. They are:

1. Density = Mass / Volume

2.# of moles of the limiting reagent = Weight of the limiting reagent (grams) / Molecular weight limiting reagent (grams/mole)

3.Theoretical Yield = # of moles of limiting reagent X

Of aspirinMolecular weight of aspirin (Acetylsalicylic Acid)

4.Percent actual Yield = (Actual yield / Theoretical Yield) X 100%


Chemicals Used:

*Salicylic Acid: C7H6O3(S)(Provided)

*Acetic Anhydride: CH3COOCOCH3(l)(Provided)

*Sulfuric Acid: Conc. H2SO4(l)(Provided)


*Acetic Acid:C2H4O2(l)(Prepared)

*Acetylsalicylic Acid:C9H8O4(S)(Prepared)


Followed the procedures listed in the University of Winnipeg, Department of Chemistry Laboratory Manual, page 43.



PropertyMeasurement Method of Determination

Mass of salicylic acid1.498 gAnalytical Balance

Volume of acetic anhydride5.0 mLGlass Graduated Cylinder

Density of acetic anhydride1.08 g/mLGiven by Lab Manual (page 45)

Weight of acetic anhydride5.4 gSee #1 in Calculations below

Molecular weight of salicylic acid138.12 g/molGiven by Lab Manual (page 45)

Molecular weight of acetic anhydride102.09 g/molGiven by Lab Manual (page 45)

Molecular weight of acetylsalicylic acid180.16 g/molSee #2 in Calculations below

Mass of filter paper + aspirin2.103 gAnalytical Balance

Mass of filter paper0.962 gAnalytical Balance

Mass of aspirin1.141 gSee #3 in Calculations below

Percent yield of aspirin58.42%See #4 in...
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