Preparing Isopentyl Acetate
by the Fischer Esterification
PURPOSE OF THE EXPERIMENT Prepare isopentyl acetate from isopentyl alcohol and
acetic acid by the Fischer esterification.
BACKGROUND INFORMATION Esters are derivatives of carboxylic acids in which the
acyl carbon bears an alkoxy substituent (—OR) rather than the hydroxyl substituent (—OH) of
the acid. Simple esters tend to have pleasant, fruity odors and are widely used as flavors and
fragrances.
Table 1 Composition of the
volatile oil of ripe pineapple
constituent
total volatile oil
ethyl acetate
ethyl alcohol
acetaldehyde
ethyl acrylate
ethyl i-valerate
ethyl u-caproate
percent
0.1900
0.1196
0.0605
0.0014
0.0008
0.0004
0.0008
The volatile compounds in natural fruits and flowers are
usually complex mixtures of compounds, where esters
frequently predominate. Many artificial flavorings contain
esters or mixtures of esters. For example, the volatile oil
of ripe pineapple contains several esters, as shown in
Table 1. Table 2 shows the flavors or fragrances
associated with some other esters.
Isopentyl acetate is known as banana oil because of its
characteristic odor; you will notice the product in this
reaction has a strong aroma of banana. This ester has also
been shown to be one of the active substances in the
alarm pheromone of the honeybee. When a honeybee
worker stings an intruder, an alarm pheromone is secreted
along with the venom. The pheromone causes other bees to become aggressive and attack the
intruder.
Esterification generally refers to the formation
of esters from alcohols and carboxylic acids,
as shown in Equation 1:
RCOOH +
R'OH  RCOOR' + H2O
The reaction proceeds by way of a
nucleophilic substitution at the acyl carbon of
the carboxylic acid. When catalyzed by a
strong acid, usually sulfuric acid or
p-toluenesulfonic acid, the reaction is called
the Fischer esterification.
The reaction mechanism is shown in
Equations 2 - 6. (I have used general or
generic structures here to save space).
Table 2 some common esters found in
natural fruits and oils.
ester
fragrance
isopentyl acetate
isobutyl formate
isopentenyl acetate
isobutyl proprionate
n-propyl acetate
methyl butyrate
methyl anthranilate
methyl salicylate
ethyl butyrate
ethyl phenyl acetate
benzyl acetate
banana
raspberry
juicy fruit
rum
pear
apple
grape
wintergreen
pineapple
honey
peach
1
Equation 2 shows the protonation of the acyl oxygen of the carboxylic acid. The protonation
activates the acyl carbon toward nucleophilic attack. Equation 3 shows the nucleophilic attack at
the acyl carbon by the alcohol oxygen atom to form a tetrahedral intermediate. Equation 4
shows a proton transfer to the hydroxyl oxygen of the carboxyl group. This protonation converts
the hydroxyl group into the good leaving group, water. Equation 5 shows the loss of water
forming the protonated ester. Finally, Equation 6 shows the proton transfer to a base, resulting in
the formation of the ester.
Protonation (equation 2):
+
O
R
OH
+
H
R
C
C
OH
OH
carboxylic acid
protonated
carboxylic acid
Nucleophilic attack (equation 3):
OH
+
OH
R
R
C
C
OH
O
OH
+
R
R1 OH
H
tetrahedral intermediate
Proton Transfer (equation 4):
OH
R
OH
C
O
OH
R
+
C
+
OH2
O
R1
H
R1
Dehydration (equation 5):
+
OH
OH
R
+
C
OH2
R
+
C
O
O
R1
H2O
R1
protonated ester
Deprotonation (equation 6):
O
+
OH
R
R
C
+
C
O
O
R1
+
H
R1
2
Each step of the reaction mechanism is reversible and, therefore, the reaction reaches an
equilibrium. Le Chatelier’s principle suggests that the amount of ester produced in an
equilibrium reaction might he increased either by using an excess of one of the reactants or by
removing one of the products. In practice, an excess of carboxylic acid or alcohol, whichever is
more readily available, is added, and/or water is removed as the reaction proceeds. A
water-absorbing substance such as molecular sieves might he included in the reaction mixture, or
the water might be removed as part of an azeotropic with benzene or toluene.
The mechanism suggests that steric effects might he important in Equation 3, the step involving
the attack by the alcohol at the acyl carbon of the carboxylic acid. Indeed, alkyl-group branching
at the  or  carbon of the acid slows the rate of esterification. For example, the relative rates of
esterification with methanol, CH3OH follow the order
CH3CO2H > CH3CH2CO2H > (CH3)2CHCO2H > (CH3)3CCO2H
Sterically hindered alcohols also react more slowly in the esterification reaction. The relative
rates for esterification of alcohols with acetic acid, CH3CO2H, follow the order
CH3OH > CH3CH2OH > (CH3)3CCH2OH
In this experiment, you will prepare isopentyl acetate by reacting an excess of acetic acid with
isopentyl alcohol. After the reaction is complete, you will remove the excess acetic acid from the
reaction mixture by extraction with sodium hydrogen carbonate (NaHCO3). You will use
sulfuric acid to catalyze the reaction (Equation 7):
O
+
H
O
+
OH
HO
O
+
H2O
3
MATERIALS
Reagents and Properties_______________________________________________________________
substance
quantity
molar mass
bp
d (g/mL)
glacial acetic acid
isopentyl acetate (product)
isopentyl alcohol
sodium chloride,
saturated solution
sodium hydrogen
carbonate, 5%
sodium sulfate,
anhydrous
sulfuric acid,
concentrated
8.5 mL
4.37 g
60.05
130.19
88.15
142
130
1.049
0.876
0.809
25 mL
50 mL
1.5 g
142.04
1.0 mL
98.08
PROCEDURE
Chemical Alert!
glacial acetic acid—corrosive
isopentyl acetate—flammable and irritant
isopentyl alcohol—irritant
sodium sulfate—irritant and hygroscopic
sulfuric acid—corrosive and oxidant
Caution: Wear departmentally approved safety goggles at all times while in the chemistry laboratory!
1. Refluxing the Reaction Mixture
Set up a hot plate at your desk and begin heating a 600 mL beaker of water; you will use this for
your boiling water bath in the second step.
Caution: Isopentyl alcohol (3-methyl-butan-1-ol) is irritating. Glacial acetic acid is corrosive.
Concentrated sulfuric acid (H2SO4) is corrosive and oxidizing. Prevent eye, skin, clothing, and
combustible material contact. Avoid inhaling vapors and ingesting these compounds.
Place 4.37 g (5.4 mL) of isopentyl alcohol into the 125-mL Erlenmeyer flask. Add 8.5 mL of
glacial acetic acid and 1 mL of concentrated H2SO4. Add one acid-resistant boiling chips. A
glass bead will work well as an acid resistant boiling chip, if available. Cover the opening of the
flask with aluminum foil and wrap the foil around the neck of the flask.
Heat the reaction in a boiling water bath for 30 minutes, swirling occasionally. The solution
does not need to boil during this time, but the water bath needs to be maintained at the boiling
point. If the solution begins to boil, reduce the heat. After heating for 30 minutes, turn off the
heater and remove the flask from the hot water bath.
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Note: you may observe a reddish color develop during the beginning of the reaction.
This is not the product (isopentyl acetate is clear); this color is probably one of the
reactive intermediates of the reaction, possibly the diol cation formed in step 3 of the
mechanism. I know from experience that some students will observe this color, and
others will not… I’m not sure why!!!!
Allow the flask to cool to room temperature. Prepare an ice-water bath in a 400-mL beaker. Cool
the flask in the ice-water bath. Place 30 mL of distilled or deionized water in a medium sized test
tube; you can use two test tubes if yours are too small. Cool the test tube(s) in the ice-water bath.
2. Separating and Washing the Product Layer
Pour 20 mL of the cold water into a 125-mL separatory funnel. Carefully pour the cooled
reaction mixture from the Erlenmeyer flask into the funnel containing the water. Be careful not
to transfer the glass bead or boiling chip! This will clog the separatory funnel! Rinse the
reaction flask with the remaining 10 mL of cold water and pour the rinse water into the
separatory funnel, once again taking care to retain the boiling chip in the flask. Swirl the mixture
gently, stopper the separatory funnel, and shake once. Vent the funnel carefully.
note: Vigorous shaking may result in an emulsion.
Label a 150-mL beaker “Aqueous Layers”. Drain the lower aqueous layer from the funnel into
the beaker. (It is a good idea to keep all extraction layers in a labeled container until the
experiment is finished; this may prevent you from accidentally discarding your product!)
Caution: Neutralizing acids with sodium hydrogen carbonate (NaHCO3) generates CO2 gas.
Vent the funnel frequently so that pressure does not build up in the stoppered separatory
funnel. You should never leave a separatory funnel stoppered for more than 30 seconds or so,
particularly in this reaction!
Carefully add 25 mL of 5% NaHCO3 to the separatory funnel. Swirl the funnel gently until
bubbles no longer appear. Stopper the funnel and shake it cautiously. Vent the funnel. Shake the
funnel vigorously and vent it immediately.
Return the funnel to the ring stand and immediately remove the stopper. Allow the layers to
separate. Remove the stopper and drain the lower aqueous layer into the same labeled beaker.
Wash the organic layer in the funnel with a second 25-mL portion of 5% NaHCO3. As before,
take care to frequently vent the funnel. Again, drain the lower aqueous layer into the labeled
beaker.
Finally, wash the organic layer with 25 mL of saturated NaCl. Drain the lower aqueous layer
into the labeled beaker. Place about 1.5 g of anhydrous sodium sulfate (Na2SO4) into a
50 mL Erlenmeyer flask. It is not necessary to weigh the sodium sulfate; use about two scoops
with your spatula and that will be sufficient. Pour the organic layer into the Erlenmeyer flask.
Dry the organic layer over anhydrous Na2SO4 for 5 min.
Decant the ester into a clean, dry, pre-weighed container. Record the mass of your product,
isopentyl acetate.
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3. Characterizing the Product
Your instructor may provide you with an IR spectrum and/or an NMR spectrum of isopentyl
acetate for analysis.
4. Cleaning Up
The "aqueous layers" you collected during the lab are pH neutral and non-toxic; they may be
washed down the sink with water. Do not discard the isopentyl acetate; although it is non-toxic,
your instructor may wish to keep it for further experiments. Clean your glassware by rinsing
thoroughly with water; it is not necessary to use soap or detergent.
Caution: Wash your hands thoroughly with soap or detergent before leaving the laboratory!
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Fischer Esterification
Lab Report and Post-Lab
Name ____________________________________________________________
Post-Lab Questions (please answer on a separate sheet):
On a separate sheet, you should provide a data table indicating the masses and/or volumes of
chemicals used in the experiment.
1. Based on the quantities of chemicals you used, determine the theoretical yield of isopentyl
acetate in grams, and the limiting reactant in this experiment. Calculate your percent yield of
isopentyl acetate. You MUST show your work, including calculation of theoretical yield, to
receive credit on this question.
2. List the distinctive features of the IR spectra of isopentyl acetate, as well as isopentyl alcohol
and acetic acid if provided. Assign the absorption bands to the appropriate functional groups.
What are the principal differences between the IR spectra for the reactant and product? (See
next pages).
3. Fully analyze the 1H-NMR and 13C-NMR spectra for your product, isopentyl acetate, which
are provided on the following pages. Provide a FULL discussion of each spectrum…. indicate
where each signal originates, and explain the number of signals, shifts, peak integrations and
multiplicities of the signals (as appropriate). For the 1H-NMR spectrum, the best way to do this
is to draw the structure of the compound, label each hydrogen type with a letter, and label the
corresponding peaks in the spectrum with the same letter (as with many examples in class and
in the book). You can do the same thing for the 13C-NMR spectrum, but labeling the carbons
instead.
4. a. Write the equation for the reaction of 5% NaHCO3 with acetic acid.
b. Why does the reaction mixture release CO2 when it is neutralized with NaHCO3 ?
Where does the CO2 come from?
5. What is the IUPAC name for isopentyl acetate?
7
IR spectrum for Isopentyl Alcohol
IR spectrum for Isopentyl Acetate
8
1
H-NMR spectrum for Isopentyl Acetate
ppm
13
C-NMR spectrum for Isopentyl Acetate
9
Pre-Laboratory Assignment - Fischer Esterification Reaction
Name ________________________________________________
1. What are the hazards you should be aware of when you work with the following reagents?
(a) concentrated H2SO4
(b) glacial acetic acid
2. (a) Write a detailed mechanism for the formation of ethyl acetate from ethanol and acetic acid
with H2SO4 as catalyst.
(b) How does concentrated H2SO4 catalyze the esterification reaction? Briefly explain.
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3. Describe two methods by which the Fischer esterification equilibrium described in this
experiment can be shifted to produce more of the ester.
4. Calculate the limiting reactant, and the theoretical yield of isopentyl acetate (in grams) for the
esterification reaction described in the Procedure. Note that the density of glacial acetic acid is
provided in the Reagents table as 1.049 g/mL. The overall balanced reaction is shown in
Equation 7 at the bottom of page 4. You must show your work to receive credit on this
question. (Note: you may also wish to record the result of this calculation, because you will also
need it for your post-lab questions).
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