CHM 1025C/CHM 1032C Experiment 14: Synthesis of an Acid
Purpose: To synthesize an acid to observe the
process of purification
via
re-crystallization
Objectives:
1. The student
will gain some knowledge of synthetic methods.
2. The student
will learn the techniques of crystallization and re-crystallization to purify a
synthesized substance.
3. The student
will re-enforce the technique of determining melting points on the unknown
acid.
4. The student
will learn to collect a solid product on a Buchner
funnel by vacuum filtration.
Equipment
and Chemicals:
|
1
thermometer, 260° C 1 250 ml
Erlenmeyer flask 1 125
Erlenmeyer flask Buchner funnel Filter
flask Gooch
ring Filter
paper Capillary
tubes Meltemp Apparatus Stirring
rod Ring
stand Iron ring |
Phosphoric
acid Acetic
anhydride Salicylic
acid 1% Ferric
chloride solution Commercial
aspirin |
Introduction:
In this experiment you will synthesize
acetylsalicylic acid. The procedures for
synthesizing the acid appear in the procedure section below.
Acetylsalicylic acid is also called
aspirin. It is an anti pyretic substance
(has the ability to lower high body temperatures to normal) and an analgesic
substance (able to relieve minor pain). It
is a moderately effective agent in the treatment of rheumatoid arthritis. Thirty million pounds usually in 5 grain
tablets are used annually in the
Synthesis:
The synthesis of aspirin is classified as an esterification
reaction. Salicylic acid is treated with acetic anhydride, an acid derivative,
causing a chemical reaction that turns salicylic acid's phenol group into an acetyl
group, (R-OH → R-OCOCH3). This process yields aspirin and acetic acid, which is considered a byproduct
of this reaction. Small amounts of sulfuric acid (and occasionally phosphoric
acid) are almost always used as a catalyst.
Procedure: Acetylsalicylic Acid (Aspirin)
(1) Place 2.5-3.0
grams of salicylic acid in a 125 ml Erlenmeyer flask,
(2) Add 8 ml of
acetic anhydride and 15 drops of 85% phosphoric acid (H3PO4)
Caution: Acetic anhydride has a peppery
smell and is an eye irritant. If you
spill
acetic anhydride on the desk, do not wash it with water.
(3) Stir the
mixture well.
(4) Heat the
flask in a water bath for 5 minutes.
(5) Remove the
flask from the bath and, while still hot, cautiously add 3 ml of water in one
portion.
Caution: The solution may boil from the
heat of decomposition of the excess
acetic
anhydride; handle the flask carefully!
(6) After the
decomposition is complete, add 40 ml of distilled water and stir the solution
until crystals begin to form.
(7) Cool the
mixture in an ice bath to complete the crystallization process.
(8) Collect the
acid by vacuum filtration.
|
|
(9)Wash with
cold water and allow the product to dry on the Buchner
funnel. (10)Take a small sample of the aspirin and place it in a test
tube and 1 ml of water. (11) Add 1 drop of 1% Ferric chloride solution to test for the
phenolic hydroxyl group. Note
the color and repeat the test on both commercial aspirin and pure salicylic
acid. If your aspirin is dark purple
in color from this test, wash the aspirin with more cold water until the test
is less purple than the commercial aspirin. (12) Remove a sample a dry it and determine the melting point
of the aspirin. (13) Weigh the dry sample next week . (14) Save the sample for you to take home and remember this
class by keeping it in your medicine cabinet |
Calculations
for your Lab Notebook:
1. When the
sample is dry (next week), weigh the sample.
2. Write the
equation for the reaction
3. Calculate the
theoretical yield of the acid you synthesized from the beginning amount of
Salicylic acid used.
4. Using the actual
mass of the synthesized acid and the theoretical yield, calculate the percent
yield for this experiment.