Experiment: Esterification: The Synthesis of Methyl Benzoate
Esters, along with acid chlorides, acid anhydrides, and amides, belong to a class of compounds
frequently referred to as carboxylic acid derivatives. As a group, these compounds are
important. Acid chlorides and anhydrides, the most reactive of the group, are not found in nature
but are important tools in the synthesis of other compounds. Esters and amides, less reactive
than the other two, are the basis for many biologically important molecules. Many lipids are
esters, while polypeptides, or polyamides, provide the structural backbone for all proteins.
Esters are also important industrially as solvents, and, because they often have pleasant odors,
are important as synthetic flavoring agents and fragrances. (You will observe the pleasant smell
of the ester you will prepare in today’s experiment.) Amides are also important commercially in
the pharmaceutical industry, while polyamides are important synthetic fibers.
The reactions that all acid derivatives undergo are nucleophilic acyl substitution reactions. For
acid chlorides and anhydrides, these substitution reactions are very rapid. For esters, the
reactions are slower, and for amides slowest of all. Carboxylic acids themselves also undergo
these substitution reactions, and the synthesis you will be doing in this experiment is the acid
catalyzed nucleophilic acyl substitution reaction of benzoic acid with methanol to produce the
ester, methyl benzoate.
+
H
C6H5COOH + CH3OH  C6H5COOCH3 + H2O
The mechanism for this reaction is the acid catalyzed nucleophilic acyl substitution mechanism.
In it, the acid’s carbonyl group is protonated to ease the addition of the poor nucleophile,
methanol. Once the methanol has added to the carbonyl, proton transfer leads to an intermediate
with a protonated hydroxyl group which leaves to form a carbocation and water. Deprotonation
of this carbocation yields the methyl benzoate product. The two carbocations formed throughout
this mechanism each have extensive resonance stabilization. As part of your post-lab
assignment, you will be asked to write the mechanism for this reaction.
The reaction above is called a Fischer esterification, and as an equilibrium reaction, it will give a
poor yield of ester. To obtain a good yield of ester, we must do something to shift the
equilibrium to the right. As you remember from your study of LeChatelier’s Principle in general
chemistry, there are several ways to shift this equilibrium to the right, producing more methyl
benzoate. One way would be to remove one of the products as it is formed. In some equilibrium
reactions, this is done by distilling off the products formed, forcing the equilibrium to shift to
produce more. For this equilibrium, this technique won’t work because the methyl benzoate
product has a higher boiling point than methanol. The methanol will distill off before the methyl
benzoate, so the equilibrium will shift to the left. The technique used in this experiment to shift
the equilibrium is to add an excess of one of the reagents, methanol. By doing this, the
equilibrium will shift to the right to get rid of much of the excess. However, the equilibrium will
not shift all the way. There will be some benzoic acid left over at the end of the reaction.
During the purification steps of this experiment, you will extract out any unreacted benzoic acid
with aqueous sodium bicarbonate. If there is enough benzoic acid remaining at the end of your
reaction, you will recover it.
 Pre-lab Preparation
Before coming to lab, write the following assignment in your lab notebook.
1.
Review the material about extraction and washing in your CH 203 lab manual,
your discussion notes from CH203 or from the internet.
2.
In tabular form write the relevant physical constants (bp for liquids and mp for
solids, solubility in ether and water) for benzoic acid, methanol, and methyl
benzoate. (You will find methyl benzoate in the Handbook of Chemistry and
Physics under benzoic acid, methyl ester.)
3.
Answer the following questions about the procedure.
a. Calculate the moles of the reactants in this experiment and use this
information to explain why this equilibrium reaction will shift to the right to
form more methyl benzoate.
b. At the end of the reflux period, water and diethyl ether are added to the
reaction mixture, forming two layers. Consider the solubility data in your
reference table and decide what materials will be found in the ether layer.
c. What is the purpose of washing the ether layer with aqueous sodium
bicarbonate? Write an equation showing the reaction that takes place.
Experimental Procedure
! Safety Considerations
! Methanol is an extremely toxic compound which can cause blindness or death if drunk in small
quantities or absorbed in large quantities through the skin. If you spill small amounts on the
skin, rinse it off immediately with water. If you spill a larger quantity on your clothing or skin,
notify your instructor promptly.
! Sulfuric acid is a very strong acid. If you spill any on yourself, rinse it immediately with
water. Sulfuric acid spills should be cleaned immediately with water so that other students will
not be burned by it. Even a few drops on the side of a reagent bottle can cause severe burns on
the fingers of anyone picking up that bottle.
! You will be working with diethyl ether in this experiment. Ether is very volatile and
flammable. All extraction and drying involving the ether solutions must be done in a fume
hood. No glassware that has contained ether will be removed from the hood until the odor of
ether has disappeared. The only time that you may remove ether from the hood is for the final
distillation. Then, stopper the flask with a greased, ground glass stopper before carrying it
back to your desk.
! In the washing step with sodium bicarbonate, a gas will be produced. Vent your separatory
funnel more frequently than usual during this washing step.
! During the extraction and washing steps of the purification, all aqueous layers except for the
sodium bicarbonate layer may be discarded in the sinks in the hood. Because ether is
somewhat soluble in water, do not discard these aqueous layers in any other sinks. You will be
told what to do with the sodium bicarbonate layer in the lab procedure that follows. As with all
extractions and washings, before discarding any layer, be certain that you have the layer you
need to keep.
The experiment is a long and involved one. It is always important to be thoroughly prepared for
any lab experiment, but it is especially so for this one. Plan your work carefully before coming
to lab.
1.
Into a 50-mL round bottom flask add 4.0 grams of benzoic acid. The benzoic acid is very
light and fluffy but it will fit into the flask. Then add 10 mL of methanol to the flask.
Add it in such a way as to wash the benzoic acid from the ground glass neck of the flask.
If any benzoic acid remains inside the ground glass neck of the flask, wipe it out. Swirl
the flask to mix the two reagents. Not all of the benzoic acid will dissolve in the alcohol.
Do not be concerned with this. It will dissolve once heat is applied.
2.
Cool the mixture in an ice-water bath.
3.
Measure out 1.2 mL of concentrated sulfuric acid into a graduated cylinder and slowly
add it to the cold reaction mixture. After the addition is complete, swirl the flask to mix
the contents and add one or two boiling stones.
4.
Attach a water cooled reflux condenser as shown below and begin heating the reaction
mixture. Reflux the reaction mixture for thirty minutes. (Remember, when the sample is
refluxing, you will see a ring of solvent condensing about half-way up the water-cooled
condenser. Don’t start timing the reflux until you see this occur.)
condenser
with clamp
water goes in the
bottom and out
the top
50 mL rb flask
5.
While the reaction is refluxing, your instructor may want to use part of this time to talk
with you about the reaction, the mechanism, or the experimental procedure. Use any
other time available to set up your final distillation, a simple distillation with a 25-mL
distilling flask. Make certain that all glassware used in the setup is absolutely dry. Be
sure that the distillation apparatus is well greased and tightly connected.
6.
At the end of the reflux period, cool the reaction mixture to room temperature and
transfer it to a 50-mL separatory funnel in a fume hood. Rinse the reaction flask with 15
mL of ether and add it to the reaction mixture. Then add 10 mL of water to the reaction
mixture.
7.
Stopper the separatory funnel, vent it, and then shake the mixture with frequent venting.
Allow the layers to separate and draw off the water layer.
8.
Wash this ether layer with 15 mL of water. Separate the layers and retain the ether layer.
9.
Wash the ether layer with 10 mL of 0.5M sodium bicarbonate solution. Gas is evolved
during this wash so vent the funnel early and often during this washing. Separate the
layers. Put the aqueous layer in a beaker and set it aside for later in the procedure.
Retain the ether layer.
10.
Wash the ether layer with 10 mL of saturated sodium chloride solution. Separate the
layers and retain the ether layer. Note: The purpose of washing with saturated sodium
chloride is to remove water from the ether later. Because ether has a highly
electronegative oxygen atom in the molecule, water can hydrogen bond with it.
Consequently, water is slightly soluble in ether and vice versa. A saturated salt solution,
because it is so concentrated, will seek to dilute itself by extracting water from the wet
ether solution. For this reason, ether extracts are often washed with saturated salt
solutions as the last of several washings with aqueous solutions.
11.
In a small Erlenmeyer flask, dry the ether solution with anhydrous calcium chloride
pellets. Add enough calcium chloride so that the most recently added pellets don’t clump
with the ones added earlier in the drying. Stopper the flask and let it sit in the hood for at
least ten minutes.
12.
While the ether solution is drying, go back to the aqueous layer from the sodium
bicarbonate wash. This solution contains sodium benzoate formed from the acid base
reaction with unreacted benzoic acid. Add 6M HCl to this solution until Congo Red test
paper turns blue. Filter with suction any benzoic acid that precipitates. Allow it to dry
for at least 24 hours. Then weigh it.
13.
Decant the drying ether layer into a 25-mL flask containing one or two boiling stones.
Stopper it, carry it back to your desk and attach it to the distillation apparatus that you
have already set up. Distill off the ether into a round bottom flask.
14.
After the ether has been distilled off, the collection of liquid will stop as the remaining
product is heated to its boiling point. In a separate tared round bottom flask, collect
material in the correct boiling point range based on the information given in you pre-lab.
Weigh your product and save your product for use in the Grignard Reaction experiment.
 Post-Lab and Report Requirements
1.
Using structural formulas, write a balanced equation for this reaction.
2.
Write a detailed mechanism for this reaction. Your mechanism must show all
intermediates and required curved arrows.
3.
Based on calculations you did in pre-lab question 3a, you can now determine the
theoretical yield in grams and the percentage yield of methyl benzoate for this
reaction by calculating the following:
a. Give the mass of benzoic acid recovered from the sodium bicarbonate wash.
b. Calculate the moles of benzoic acid that actually reacted based on the fact that
you recovered some unreacted benzoic acid. Now calculate the theoretical yield
and percentage yield of methyl benzoate based on this adjusted quantity of your
limiting reagent.