Synthesis and Analysis of Banana Oil
Introduction:
This experiment will introduce you to several of the techniques
used in organic chemistry to prepare pure compounds. The compound
must be separated from any other substances used or produced in
preparing the compound. When the compound to be prepared is a liquid
(as in this experiment), and if it is mixed with another liquid in which it
is insoluble, it can be separated from that liquid by using a separatory
funnel (a funnel with a stopcock allowing the heavier liquid to be
separated from the lighter). The desired liquid compound can be
separated from remaining impurities by distillation.
In distillation the liquid is boiled, and resulting vapors are passed
through a condenser. The resulting liquid is collected. Low boiling
components will pass over first so a separation can be made.
The compound you will prepare is an ester. Esters are organic
compounds formed by the reaction of alcohols with carboxylic acids.
Though most carboxylic acids possess a sour taste and smell, esters
impart a pleasant odor and are often responsible for the flavor and scent
of fruits and flowers.
The following general reaction illustrates the preparation of an ester from
an acid and alcohol (with sulfuric acid as a catalyst).
O
R
C
OH
carboxylic acid
+
R'
OH
H2SO4
O
R
alcohol
C
O
R'
+
ester
H2O
water
(R represents a hydrocarbon group)
You will synthesize the ester found in banana oil; the ester's common
name is isoamyl acetate, and its boiling point is 142ºC.
The purity of your synthesized ester will be checked by the boiling point.
Part I. Preparation of Ester (1 week prior to lab)
1.
Esters are made from the reaction of carboxylic acids with
alcohols. Measure about 15 ml of acetic acid (ethanoic acid), and
transfer to a dry 125 ml Erlenmeyer flask.
2.
Slowly add about 20 drops of concentrated H2SO4 and mix
thoroughly.
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3.
Add about 13 ml isoamyl alcohol (3-methyl-1-butanol), stopper,
and mix thoroughly. (Avoid wetting the stopper – concentrated
H2SO4 reacts with it.) Label the flask with your name(s) and store
in your lab drawer for the next lab session.
Part II. Separation of Ester (Second Week)
1.
Pour the solution you prepared into 50 to 75 ml of ice cold H2O in
a 250 ml beaker and mix thoroughly. The beaker contains two
layers: (1) the bottom layer contains water and substances soluble
in H2O and (2) the upper layer contains organic compounds
insoluble in H2O (your ester and probably some unreacted acid or
alcohol (if they are not very water soluble).
To separate the lower aqueous layer from the organic layer
containing your ester, you will use a separatory funnel. (See Fig. 1
below.)
Figure 1:
Separatory funnel
a.
Transfer your mixture (in portions if necessary) to the
separatory funnel. Stopper. (Be sure the stopper fits snugly –
check to see if it leaks.) Shake vigorously for a minute to
insure thorough mixing of the layers, holding the stopper in,
and periodically inverting the funnel (stopcock up) and
cautiously opening the stopcock to release pressure.
b.
Support the funnel on a ring and allow the layers to separate
and clarify.
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c.
Remove the stopper and open the stopcock to draw off and
collect the bottom H2O layer. Pour it into the sink. Add 25
more ml of cold H2O to the funnel, mix again, let separate,
and discard the bottom H2O layer.
d.
Add 25 ml of 5% Na2CO3 solution and leave the stopper off
as gas is released. Swirl gently. After a minute, replace the
stopper and invert, releasing pressure by opening the
stopcock. (Be sure it's pointed away from other students!)
Mix thoroughly, being sure to release the pressure
frequently. It will react with any residual acid to give CO2
gas. Draw off and pour the bottom layer into the sink.
e.
Transfer the ester (top layer) through the mouth of the
separatory funnel to a dry Erlenmeyer flask containing about
0.5 g anhydrous sodium carbonate and about 0.5 g
anhydrous magnesium sulfate (both absorb water).
f.
Heat the mixture gently on a hot plate (don't let it boil) for 10
minutes, occasionally swirling well to allow the sodium
carbonate to neutralize traces of H2SO4 and to allow both
dehydrating agents to absorb traces of water. Carbon dioxide
evolution indicates that acid is being neutralized. While you
are waiting, proceed with Part III, No. 1, below.
Part III.
1.
Distillation of Ester
Set up the distillation apparatus shown on the below (Fig. 2). The
inside of all the glassware must be clean and dry. If it is necessary
to wash any, use acetone as a final rinse since it evaporates much
more quickly than H2O.
Figure 2:
Distillation apparatus
VWR Digital Thermometer
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 Caution: the pieces in the kit do not necessarily remain stuck
together! (Clamp them!)
 The thermometer must go up to at least 150ºC. Use a gentle
rotary motion and insert it in the grey rubber piece.
 The distilling pot should be no bigger than 100 ml. Use a wire
screen on a ring under the pot.
 Use an Erlenmeyer flask at first to collect the forerun, then a
graduated cylinder to collect purified ester.
 A hose should connect the lower condenser outlet to a faucet,
and another hose should drain the upper outlet into the trough
in the counter top.
2.
After the mixture has heated for 10 minutes, carefully pour the
ester away from the solids through a funnel into the 50 or 100 ml
distilling pot and add two boiling chips to make it boil more
smoothly.
3.
Adjust the thermometer so the bulb (or tip of the probe if digital) is
just below the sidearm of the distillation flask.
4.
Have the instructor check your setup.
5.
Read this entire part before proceeding. Gently turn on the
condenser water. Heat the distilling flask (pot) with a small flame.
When the liquid begins to boil, adjust the flame so the liquid drips
into the receiving Erlenmeyer flask at about 1 to 2 drops per
second (no faster!). (Remove the burner, if necessary, or increase
the flame to maintain the desired rate.) The point of distilling is to
separate your ester from impurities (which boil at other
temperatures)
The boiling point of your ester is 142ºC, while your acetic acid has
a boiling point of 118ºC, and the alcohol has a boiling point of
128.5ºC. You will save the liquid that distills within 4ºC of your
ester's boiling point, and discard the lower boiling liquid you are
collecting now. Therefore, stop heating when the temperature is
about 138ºC and replace the receiving Erlenmeyer flask with a dry
25 ml graduated cylinder. Discard the liquid in the Erlenmeyer
flask.
6.
7.
Continue distilling and collecting the ester until you only have a
milliliter or so of liquid left in the distilling flask (don't distill dry!)
or until the temperature reaches more than 146ºC.
Turn off the flame and water. Record the ester's volume.
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Name
Date
Report for Experiment: Banana Oil
Boiling range over which ester
was collected:
Volume of the ester collected:
Questions and calculations:
1.
2.
a.
Write an equation for the preparation of your ester from an
alcohol and an acid using structural formulas.
b.
Give the IUPAC name of your ester.
c.
Considering the structural formula of your ester, would you
expect it to be soluble or insoluble in H2O? Explain.
Look carefully at the condenser, (see p. 3). Explain what it does
and how it works.
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3.
The results of organic synthesis experiments are often reported in
terms of percent yield.
Percent yield =
actual amount of product
theoretical yield
 100
(where theoretical yield = amount of product you would have
obtained if the reaction had proceeded to completion (all reactants
converted to products). Since organic reactions rarely proceed to
completion – they are equilibrium reactions – do not expect to get
100% yields!
Calculate your percent yield (amounts of your reactants were
chosen so that your theoretical yield of ester was 40 ml).