Cliffs AP Chemistry, 3rd Edition

Experiment 22: Synthesis, Purification and Analysis of an Organic Compound

Background: When acids react with alcohols, an ester and water is formed:

Aspirin, also known as acetylsalicylic acid (ASA), can be synthesized when the carboxyl group
in acetic acid (−−−COOH) reacts with the (−−−OH group in the salicylic acid molecule:

However, the driving force in this reaction is not large, so one usually ends up with an equilib-
rium mixture of water, salicylic acid, ASA and acetic acid. A better approach to produce ASA
(aspirin) is to react acetic anhydride with salicylic acid in the presence of phosphoric or sulfu-
ric acid acting as a catalyst:

The driving forces in this reaction produce a much higher yield of ASA. One method to deter-
mine purity involves determining the melting point of the aspirin produced and comparing it
with the known value. If the product is pure, it will have a very distinct melting point. If the
sample is impure, there will be a resulting melting range. The final melting point will be lower
than the known value by an amount roughly proportional to the amount of impurity present.

A much more definitive method of determining the purity of the aspirin is to analyze the sam-
ple through colorimetry. Salicylic acid, not being very soluble in water, is probably the major
impurity in the ASA produced. In the presence of Fe3+ion, salicylic acid forms a highly colored
magenta or purple complex. By measuring the absorbance of this light and comparing it to
known absorbances, it is possible to determine the percent salicylic acid present in the ASA.
For a discussion of colorimetry and Beer’s law, see Experiments 15 and 17.

+

acetic anhydride salicylic acid

C

C

C O

O OH

O

O HO

aspirin acetic acid

H 3 PO 4

+

C

OH

O

CH 3 C

CH 3

CH 3

O

OCH 3 C

O

OH

acetic acid salicylic acid aspirin

C

OH

O

CH 3 COH

O

++HO

C

OH

O

CH 3 C

O

OH 2 O

RR'C H 2 O

an acid an alcohol an ester

OH++OH

O

RCOR'

O

Part III: AP Chemistry Laboratory Experiments