Isopentyl Acetate [Banana Oil]
CH3
CH3
O
+
CHCH2CH2OH
C OH
CH3
Acetic acid
excess
H+
Isopentyl alcohol
CH3
O
CH3
C OCH2CH2CH + H2O
CH3
Isopentyl acetate
In this experiment, we prepare an ester, isopentyl acetate (isoamyl acetate). This ester is
often referred to as banana oil, since it has the familiar odor of this fruit.
The following experiment has been adapted from an experiment in Introduction to
Organic Laboratory Techniques, a Microscale Approach, by Pavia, Lampman, Kriz, and
Engel.
Isopentyl acetate is prepared by the direct esterification of acetic acid with isopentyl
alcohol. Since the equilibrium does not favor the formation of the ester, it must be shifted to the
right, in favor of the product, by using an excess of one of the starting materials. Acetic acid is
used in excess because it is less expensive than isopentyl alcohol and more easily removed from
the reaction mixture.
In the isolation procedure, much of the excess acetic acid and the remaining isopentyl
alcohol are removed by extraction with sodium bicarbonate and water. After drying with
anhydrous sodium sulfate, the ester is purified by distillation.
REQUIRED READING from Zubrick, 6th Ed.
Reflux
Extraction
Drying
Products of Reactions
Simple Distillation
Chapter 23
Chapter 15
Chapter 10
Chapter 11
Chapter 20
SPECIAL INSTRUCTIONS
Since a 30 minute reflux is involved, this experiment should be started at the very beginning of
the laboratory period. During the reflux period, other experimental work may be performed. Be
careful in handling concentrated sulfuric acid. It will cause extreme burns if it is spilled on
the skin.
PROCEDURE
Pour 15 mL (12.2 g, 0.138 mole) of isopentyl alcohol (also called isoamyl alcohol or 3-methyl-1butanol) and 20 mL (21 g, 0.35 mole) of glacial acetic acid into a 100-mL round-bottomed flask.
Carefully add 4 mL of concentrated sulfuric acid to the contents of the flask, with swirling. Add
several boiling stones to the mixture.
Assemble a reflux apparatus as shown on p. 202. Bring the mixture to a boil with a
suitable heating source, such as a heating mantle or an oil bath. Heat the mixture at reflux for 30
minutes. Remove the heating source and allow the mixture to cool to room temperature. Pour
the cooled mixture into a separatory funnel and carefully add 50 mL of cold water. Rinse the
reaction flask with 10 mL of cold water and pour the water into the separatory funnel. Stopper
the separatory funnel and shake it several times (vent early, often). Separate the lower aqueous
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layer from the upper organic layer (density 0.87 g/mL). Discard the aqueous layer after making
certain that the correct layer has been saved.
The crude ester in the organic layer contains some acetic acid, which can be removed by
extraction with 5% aqueous sodium bicarbonate solution. Carefully add 25 mL of 5% base to the
organic layer contained in the separatory funnel. Swirl the separatory funnel gently until carbon
dioxide gas is no longer evolved. Caution: Vent early, vent often after you stopper the funnel.
Stopper and gently shake the funnel once or twice, and then vent the vapors. Shake the funnel
until no vapors are evolved when the separatory funnel is vented. Remove the lower layer, and
repeat the above extraction with 25 mL of 5% sodium bicarbonate solution. Remove the lower
layer and check to see whether it is basic to litmus. If it not basic, repeat the procedure with
additional 25-mL portions of 5% base until the aqueous layer is basic. Discard the basic
washings and extract the organic layer with one 25-mL portion of water. Add 5 mL of saturated
aqueous sodium chloride to aid in layer separation. Shake the mixture gently (not too hard).
Carefully separate the lower aqueous layer and discard it. When the water has been removed,
pour the ester from the top of the separatory funnel into a flask. Add about 2 g of anhydrous
sodium sulfate to dry the ester. Swirl the flask. If the solution is still cloudy, add a bit more
sodium sulfate until the liquid is clear. If the solution is still cloudy after this decant the solution
into a clean erhlenmeyer, and add a small amount of drying agent.
Assemble a simple distillation apparatus using the supplied still head and round bottom
flask (make sure all glassware is dry before use). Carefully decant the ester into the distilling
flask so that the drying agent is excluded. Add several boiling stones and distill the ester. The
receiver can be cooled in an ice bath should you desire. Collect the fraction boiling between 134
and 143oC in a dry flask, but pay attention that you do not distill to dryness. Weigh the product
and calculate the percent yield.
At your instructor’s option, obtain an infrared spectrum and a proton NMR spectrum.
Interpret and include the spectra with your report to the instructor. Submit the prepared sample
to the instructor in a labeled vial.
QUESTIONS
1. What would happen if you added a little bit of water to your reaction during the reflux?
Explain your answer.
2. What was the purpose of each extraction step that you performed during this experiment?
3. Suppose that 1.00 g of 3-methyl-1-butanol was reacted with 1.00 g of acetic acid and that 0.37
mL of isoamyl acetate was obtained as product. Calculate the percent yield for this reaction.
Show your calculations.
4. Write a mechanism for the synthesis of ethyl acetate.
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