WEEK 14:
HYDROLYSIS OF METHYL BENZOATE
PURPOSE:
This experiment will use microwave energy to cause a
hydrolysis of the ester methyl benzoate to benzoic acid using a
5% aqueous sodium hydroxide solution. After the reaction
mixture cools, the solution will need to be acidified to cause
the precipitation of benzoic acid.
IMPORTANT REACTIONS:
COOCH3
1) 5% NaOH
COOH
2) conc. HCl
Methyl Benzoate
Mol Wt 136.16
bp 198-199oC
Benzoic Acid
Mol Wt 122.12
mp 121-123oC
BACKGROUND INFORMATION:
The hydrolysis of an ester is an easy and important
reaction in organic chemistry. One can use an acid or a base to
shorten the time needed for the reaction to occur.
The acid catalyzed hydrolysis of an ester gives the product
in the free acid form without further pH adjustment. While this
may seem preferable, it is seldom done as this reaction is
reversible and the equilibrium process significantly lowers the
yield.
The base promoted reaction forms the sodium salt of the
carboxylic acid which cannot enter into an equilibrium reaction
so the reaction can proceed to completion giving a high yield of
product. Note that the base, sodium hydroxide, is not a
catalyst in this reaction as it is consumed in forming the
sodium salt. The free acid form is recovered from the sodium
salt quite easily by a simple pH adjustment. In this reaction,
concentrated hydrochloric acid will be used to acidify the basic
reaction product. Since benzoic acid is moderately water
soluble, it is necessary to limit the amount of water in the
solution.
Microwave heating will be used in this experiment. As
mentioned in an earlier experiment, the use of microwave energy
for heating dates back to the 1960’s and became widespread in
the 1980’s. Since microwave energy can penetrate deep within a
molecule, the entire molecule can be heated at the same time.
Conventional heating is generally done by conduction and heats
from the outside to the inside of the sample. Therefore,
microwave heating can be faster than conventional heating.
However, for heating to occur, the microwave energy needs to be
absorbed by the bonds in the molecule. Absorption of energy is
better in polar bonds and poor in nonpolar bonds. Hence, water
heats rapidly in a microwave oven. It is said that alkanes do
not heat well with microwave energy.
The design of a microwave oven presents some challenges to
the designer. It is hard to have a uniform distribution of
microwave energy in the cabinet. Further, if no absorption of
microwave energy occurs a reflection of energy in the unit can
destroy to apparatus. Some manufacturers warn that operating a
microwave oven when empty can ruin the oven.
Microwave ovens can also cause fires. Many people who have
made popcorn in a microwave oven have seen burnt popcorn kernels
at one time or another. It is important to remember that three
things are needed for a fire to start: a source of ignition such
as microwave energy, a combustible substance such as many
organic compounds, and a source of oxygen such as air. It is
important that one follows the experimental details carefully to
work in a safe manner.
Microwave energy can penetrate the human body and can
interfere with certain electronic devices that may be implanted
for health reasons (such a cardiac pacemakers). If you have
implanted electronic devices, let the Instructor know and avoid
the lab while the microwave unit is turned on.
Please make careful observations during this experiment and
comment on any success or failure in your report.
EXPERIMENTAL PROCEDURE:
You will use the laboratory microwave in this experiment.
Be especially careful when handling the reaction tubes as they
are delicate and expensive to replace.
Place methyl benzoate (1.00 ml) in a microwave reaction
tube and add 5 % aqueous sodium hydroxide (10 ml). Add a
magnetic stirrer bar, then screw on the cap and place your tube
in the microwave turntable. Record the position number of your
reaction tube on the turntable. When all of the tubes have been
added, your instructor will place the turntable into the
microwave unit, close the door and select the correct filesetting for your reaction (Ester.rot). The program will take 12
minutes to run, followed by a 5 min. vent and cool down program.
On completion, your instructor will open the microwave door and
remove the turntable. Carefully remove your reaction tube, and
empty the contents into a beaker. Cool the contents to room
temperature, and acidify the solution with about 2 ml of
concentrated HCl. As acidification proceeds, solid benzoic acid
will precipitate. Be sure to check that the final solution is
acidic by using pH paper. Cool the mixture in an ice bath and
suction filter the benzoic acid. After the benzoic acid has
dried sufficiently, weigh the solid, take its melting point and
calculate the percent yield.
IMPORTANT INFORMATION ABOUT THE REPORT:
The report for this experiment will follow the usual format
for synthesis reactions. Be sure the percent yield calculation
is carefully done. Also, record the melting point range of the
final product and compare that melting point to the reported
melting point of acetylsalicylic acid. Using these data,
discuss the relative success on the experiment. Also, be sure
to critique the effectiveness of using microwave heating to
cause the reaction to occur.
END OF EXPERIMENT.
© 2007 STEPHEN ANDERSON AND ROBERT SHINE