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50
Operational Organic Chemistry
EXPERIMENT
Preparation of Synthetic Banana Oil
Simple Distillation, Gas
Chromatography
2
Preparation, Purification, and Analysis of Liquids. Gas Chromatography.
Operations
OP-10
OP-24
OP-25
OP-30a
OP-37
OP-2
OP-4
OP-7
Mixing (optional)
Washing Liquids
Drying Liquids
Distillation of Liquids
Gas Chromatography
Using Specialized Glassware
Weighing
Heating
Before You Begin
1. Read the experiment and operations OP-10 (if you’ll be using a magnetic
stirrer), OP-24, OP-25, OP-30a, and OP-37. Read or review the other operations as necessary. (Reading OP-30a and OP-37 may be deferred until the
second lab period for this experiment.)
2. Calculate the mass and volume of 150 mmol of isopentyl alcohol and the
theoretical yield of isopentyl acetate from this amount of the alcohol.
3. Read Appendix V, “Planning an Experiment,” and then write an experimental plan like the one you used in Experiment 1.
Scenario
Your supervisor has just received the following message from the
Cavendish Distilling Company.
banana tree
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Greetings:
We have a problem. Cavendish Distilling Company markets the
popular liqueur Banana Elixir, which is flavored with a natural banana
extract from fruit grown on our Caribbean banana plantations. Last
June, Hurricane Floyd blew down all of our banana trees. Our stock
of banana extract is running low, and we have no alternative source of
bananas at this time.As a temporary solution, we have decided to add
a synthetic banana flavoring to our remaining stock of the natural
extract until our plantations start producing again.
Synthetic banana flavorings are formulated mainly from
isopentyl acetate, with smaller amounts of other esters. I understand
that esters can be prepared economically by a process called the
Fischer esterification, which involves the combination of an acid,
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Experiment 2
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Preparation of Synthetic Banana Oil
such as acetic acid, with an alcohol. According to our technical staff,
one problem with this method is that Fischer esterifications don’t go
to completion, leaving considerable amounts of starting material
in the product. We can tolerate up to 10% isopentyl alcohol in our
isopentyl acetate, since the alcohol is a component of natural banana
extract. But we cannot tolerate more than 2% acetic acid, because it
would tend to degrade the flavor of the liqueur. Will you have your
consulting chemists prepare some isopentyl acetate and analyze it to
see if it falls within our tolerances?
Amy Lester, CEO
The banana plant (Musa cavendishii
and other species) is actually a gigantic
herb and not a true tree. Each year, the
foliage-bearing part withers away and
is replaced by new growth from an underground stem.
Applying Scientific Methodology
By now, you should be familiar with the steps involved in applying scientific
methodology to the solution of a problem, so they won’t all be repeated in
subsequent experiments. In this experiment, the problem described in the
Scenario involves the purity of the product—not its identity—so your course
of action will include the use of an instrumental method, gas chromatography,
to determine the composition of the product.
Esters and Artificial Flavorings
The word flavor is used to describe the overall sensory effect of a substance
taken into the mouth. Flavor may involve tactile, temperature, and pain sensations, as well as smell and taste. Many fruits, flowers, and spices contain esters that contribute to their characteristic flavors—an ester is an organic
compound that contains the functional group (characteristic combination of
atoms) shown in the margin.
Most volatile esters have strong, pleasant odors that can best be described as “fruity.” Some esters with flavors characteristic of real and
“fantasy” fruits are shown in Table 2.1. The ester you will prepare in this
experiment, isopentyl acetate, has a strong banana odor when undiluted and
an odor reminiscent of pears in dilute solution. It is used as an ingredient in
artificial coffee, butterscotch, and honey flavorings, as well as in pear and
banana flavorings.
Many different esters are included in the basic repertoire of the flavor
chemist, who combines natural and synthetic ingredients to prepare artificial flavorings. These ingredients may include natural products, synthetic
organic compounds identical to those found in nature, and synthetic compounds not found in nature but accepted as safe for use in food. Each flavor
ingredient is characterized by one or more flavor notes that suggest the
predominant impact the ingredient makes on the senses of taste and smell.
Although the flavor note of a single ingredient may seem unrelated to the
overall character of a natural flavor, the combination of carefully selected
ingredients in the right proportions can often yield a good approximation of
that flavor. A high-boiling fixative, such as glycerine or benzyl benzoate, is
usually added to an artificial flavoring to retard vaporization of volatile
components, and the flavor notes of the individual components are blended
by dissolving them in a solvent called the vehicle. The most frequently used
vehicle is ethanol (ethyl alcohol).
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O
C
O
R
functional group
of an ester
O
CH3COCH2CH2CHCH3
CH3
isopentyl acetate
Some food products, such as cola beverages and Juicy Fruit gum, are characterized by fantasy flavors that have no
counterparts in nature, but most artificial flavorings are meant to resemble
natural flavors.
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Operational Organic Chemistry
52
Table 2.1 Flavor notes of some esters used in artificial flavorings
Name
Structure
Flavor note
O
propyl acetate
CH3C
OCH2CH2CH3
pears
O(CH2)7CH3
oranges
OCH2
peaches,
strawberries
O
octyl acetate
CH3C
O
benzyl acetate
isopentenyl
acetate
isobutyl
propionate
CH3C
O
CH3C
CH3
OCH2CH
O
CH3CH2C
C
CH3
“Juicy Fruit”
CH3
rum
CH3
OCH2CH
O
ethyl butyrate
Fixatives
OH OHOH
CH2CHCH2
glycerine
O
COCH2
benzyl benzoate
Vehicle
CH3CH2OH
ethyl alcohol
CH3CH2CH2C
OCH2CH3
pineapples
The formulation of artificial flavorings is perhaps as much an art as a
science. The components of a strawberry flavoring, for example, may vary
widely depending on the manufacturer and the specific application. Because natural flavors are usually very complex, a cheap artificial flavoring
may be a poor imitation of its natural counterpart, but advances in flavor
chemistry have made possible the production of superior flavorings that
reproduce natural flavors very closely. Superior flavorings may contain
natural oils or extracts that have been fortified with a few synthetic ingredients to enhance the overall effect and to replace flavor elements lost during the distillation or extraction process. Even the most experienced flavor
chemist can’t hope to do as well as a strawberry plant, which may combine
several hundred different flavor components in its berries. But a superior
strawberry flavoring with a few dozen ingredients may be hard to distinguish from the real thing—except by the most discriminating of strawberry
aficionados.
Understanding the Experiment
pear
In this experiment you will prepare synthetic banana oil, which is known
by several chemical names, including isopentyl acetate, isoamyl acetate,
isopropyl acetate
and 3-methylbutyl ethanoate. Esters are often prepared by the Fischer
esterification method, which involves heating a carboxylic acid with an
alcohol in the presence of an acid catalyst, as shown by the following
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Preparation of Synthetic Banana Oil
general equation:
O
RCOH
carboxylic
acid
O
+ HOR⬘
alcohol
+
H
RCOR⬘
+ H2O
ester
The acid catalyst is used to increase the rate of the reaction, which would
otherwise require a much longer reaction time.
isopropyl acetate
You will synthesize isopentyl acetate by combining isopentyl alcohol
(3-methyl-1-butanol) with acetic acid and sulfuric acid and then heating the
reaction mixture under reflux for an hour.The alcohol is the limiting reactant,
so it should be weighed; the acids can be measured by volume. The esterification reaction is reversible, and it has an equilibrium constant of approximately 4.2. If you were to start with equimolar amounts of acetic acid and
isopentyl alcohol, only about two-thirds of each reactant would be converted
to isopentyl acetate by the time equilibrium was reached.Your highest attainable yield in that case would be only 67% of the theoretical value. Thus,
despite having a high atom economy, the reaction efficiency of the synthesis
could be rather low. To increase the reaction efficiency, you will apply Le
Châtelier’s principle by using a 100% excess of acetic acid—the less expensive reactant—to shift the equilibrium toward the products. Even then, the
reaction will not be complete at equilibrium, so the reaction mixture will
contain some unreacted isopentyl alcohol as well as the excess acetic acid.
A desired substance can be obtained from a mixture by separating it
from all other components of the mixture, using procedures that take advantage of differences in solubility, boiling points, acid–base properties, and
other characteristics of the components. Because isopentyl acetate is a liquid,
the separation and purification operations will differ from those used previously for solid products.
At the end of the reflux period, the reaction mixture will contain (in addition to the ester) unreacted acetic acid, sulfuric acid, water, unreacted
isopentyl alcohol, and some unwanted by-products (see Figure 2.1). Isopentyl
acetate is quite insoluble in water, whereas both acetic acid and sulfuric acid
are water soluble and acidic. This makes it easy to separate the two acids from
the product by washing the reaction mixture with water and then with aqueous sodium bicarbonate.Water doesn’t remove the acids entirely, because they
are somewhat soluble in the ester as well, but it removes the bulk of them and
thus helps prevent a violent reaction with sodium bicarbonate in the second
washing step. The aqueous sodium bicarbonate converts the acids to their
salts, sodium acetate and sodium sulfate, which are insoluble in the ester but
very soluble in water; these salts migrate to the aqueous layer, where they can
be removed. The rule of thumb given in OP-24 will help you estimate the
quantity of aqueous sodium bicarbonate needed.
The water that forms during the reaction will be separated from the
ester along with the wash liquids. Any traces of water that remain are then
removed by a drying agent, either magnesium sulfate or sodium sulfate. The
rule of thumb given in OP-25 will help you estimate the amount of drying
agent needed.
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H2SO4 as acid catalyst
high atom economy, but low efficiency
Key Concept: For a reaction at equilibrium, adding more of a reactant or
removing a product will shift the equilibrium to favor the products.
aqueous: H2SO4, water, acetic acid, 2-propanol
organic: isopropyl acetate
Reactions carried out with acid catalysts often yield polymeric, tarlike byproducts that have high boiling points
and are insoluble in water.
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Operational Organic Chemistry
acetic acid
isopentyl alcohol
sulfuric acid
REACTION MIXTURE
Reflux
PRODUCT MIXTURE
isopentyl acetate
acetic acid*
isopentyl alcohol*
sulfuric acid
water
by-products
Wash
aqueous layers
acetic acid
sulfuric acid
water
as salts
organic layer
isopentyl acetate
isopentyl alcohol
water (trace)
by-products
solid residue
water—as hydrate
Dry
isopentyl acetate
isopentyl alcohol
by-products
forerun, residue
Distill
isopentyl acetate
isopentyl alcohol
by-products
*unreacted starting materials
Figure 2.1 Flow diagram for the synthesis of isopentyl acetate
Because isopentyl alcohol has a lower boiling point than that of
isopentyl acetate, and the by-products have higher boiling points, it should
be possible—in principle—to remove the alcohol and by-products from the
ester by distillation. Isopentyl alcohol should distill first, followed by the
ester, and any by-products should remain behind in the pot—the vessel in
which the reaction mixture is boiled. For the reasons described in OP-30a,
the separation is incomplete, so you will still have some isopentyl alcohol in
your isopentyl acetate after the purification step.
You will determine the composition of your distillate by injecting a
very small amount into an instrument called a gas chromatograph. (Your instructor will demonstrate the operation of this instrument.) Inside the gas
chromatograph, the liquid will vaporize, and the vapors of different components will travel through a packed column at different rates. As the vapors
exit the column, their presence will be detected and recorded on a graph
called a gas chromatogram, which should display several peaks of different
sizes. The area under the peak for each component will be proportional to
the amount of that component present, so by measuring the peak areas you
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Preparation of Synthetic Banana Oil
can estimate the percentage of each component in the distillate. This will
tell you how much (if any) isopentyl alcohol and acetic acid remain in your
product.
This should be a green synthesis because it has a high atom economy, no
organic solvents are used, and the catalyst reduces energy consumption and
improves the yield. Acetic acid is expected to be slightly toxic to aquatic life,
but it is a naturally occurring substance that readily breaks down to carbon
dioxide and water in the environment. Isopentyl alcohol and isopentyl
acetate aren’t considered to be serious environmental contaminants.
The procedure for the synthesis of isopentyl acetate is summarized in
the flow diagram in Figure 2.1, which illustrates the transformations or
separations that occur during each operation. Appendix V tells you how to
interpret such a flow diagram.
Isopentyl acetate is known to be an alarm pheromone of the honeybee.
A pheromone is a “molecular messenger” that produces a response, such
as mating behavior or aggression toward a perceived threat, in another
member of the same species. When a worker honeybee stings someone, it
releases a tiny amount (about 1 mg) of isopentyl acetate in its stinger, which
attracts more honeybees to the scene. Although isopentyl acetate alone
doesn’t cause the bees to sting (other pheromones in the stinger do that), it
agitates them and puts them on guard. So it might be wise to steer clear of
beehives on your way home from the lab!
Reactions and Properties
O
CH3C
CH3
OH
+ HOCH2CH2CHCH3
acetic acid
O
H2SO4
CH3C
isopentyl alcohol
CH3
OCH2CH2CHCH3
+ H2O
isopentyl acetate
Table 2.2 Physical properties
acetic acid
isopentyl alcohol
isopentyl acetate
sulfuric acid
mol wt
bp
d
Solubility
60.1
88.1
130.2
98.1
118
130
142
290
1.049
0.815
0.876
1.84
miscible
2.7
0.25
miscible
Note: Boiling points are in °C; densities are in g/mL; solubilities are in g/100 mL water.
DIRECTIONS
Acetic acid causes chemical burns that can seriously damage skin and eyes;
its vapors are highly irritating to the eyes and respiratory tract. Wear gloves,
dispense under a hood, avoid contact, and do not breathe its vapors.
Sulfuric acid causes chemical burns that can seriously damage skin and
eyes. Wear gloves and avoid contact.
Isopentyl alcohol and isopentyl acetate can irritate the skin, eyes, and
respiratory tract.
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Safety Notes
2
2
0
1
2
acetic acid
3
W
1
sulfuric acid
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Glacial acetic acid is a pure grade of
acetic acid that freezes at about 17°C.
Take Care! Wear gloves; avoid contact with acetic acid and sulfuric
acid; do not breathe their vapors.
Observe and Note: Look for and
record any evidence of a chemical
reaction.
Stop and Think: What is the density
of isopentyl acetate? Which layer
should be on top, the aqueous layer
or the organic layer?
Take Care! Pressure may build up
in the stoppered separatory funnel.
Waste Disposal: Unless your instructor directs otherwise, dissolve the
spent drying agent in the combined
wash solvents and flush the mixture
down the drain.
Stop and Think: What does the boiling range of the distillate tell you
about the purity of your product?
Waste Disposal: Put any forerun and
any residue left in the boiling flask
into a designated waste container.
Stop and Think: Can you guess
which peak corresponds to which
component based on the peak
areas?
Operational Organic Chemistry
Reaction. Accurately weigh 150 mmol of isopentyl alcohol into a roundbottom flask of appropriate size and add boiling chips [OP-7b] or a magnetic stir bar [OP-10]. Under a hood, add 17 mL ( ' 300 mmol) of glacial
acetic acid, and then carefully mix in 1.0 mL of concentrated sulfuric acid
while stirring or swirling. Connect a condenser to the reaction flask, turn on
the cooling water, start the stirrer (if you are using one), and heat the reaction mixture under reflux [OP-7c] for one hour after boiling begins.
Separation. When the reaction time is up, allow the reaction mixture to
cool nearly to room temperature. Turn off the cooling water and remove the
reflux condenser. Transfer the reaction mixture to a separatory funnel, leaving the stir bar or boiling chips behind, and wash [OP-24] the mixture with
50 mL of water. Drain the aqueous layer and leave the organic layer in the
separatory funnel. Then carefully wash the organic layer with two successive
portions of 5% aqueous sodium bicarbonate, draining the aqueous layer after
each washing. During the first washing, stir the layers until gas evolution
subsides before you stopper the separatory funnel, and vent it frequently
thereafter. Dry [OP-25] the crude isopentyl acetate with anhydrous sodium
sulfate or magnesium sulfate.
Purification and Analysis. Using standard-taper glassware [OP-2],
assemble an apparatus for simple distillation [OP-30a]. Be sure the thermometer bulb is positioned as shown in Figure E7, OP-30, and have your
instructor check your setup before you start. Distill the crude product, collecting any liquid that distills between 136°C and 143°C. Record the actual
boiling range you observe; wait until the entire thermometer bulb is moist
with condensing vapors, liquid is distilling into the receiver, and the temperature is stable before you record the initial temperature reading. Stop
the distillation when only a drop or so of liquid remains in the boiling flask
or when the temperature reaches 143°C. If the distillate is cloudy or contains water droplets, dry it [OP-25]. Weigh [OP-4] the distillate in a tared,
labeled vial. Using a Carbowax column or another suitable column, obtain
a gas chromatogram [OP-37] of the distillate and measure the peak areas
as directed by your instructor. Unless your instructor indicates otherwise,
assume that the components of the distillate appear on the gas chromatogram in the order (1) isopentyl alcohol, (2) isopentyl acetate, and
(3) acetic acid. From the peak areas, calculate the percentage of each
component in the distillate, then calculate your percent yield of isopentyl
acetate based on its percentage in the distillate.
Exercises
1. (a) Calculate the amount of isopentyl acetate that should be present in
the reaction mixture at equilibrium, based on the quantities of starting
materials you used and a value of 4.2 for the equilibrium constant. (Use
the quadratic equation; because volumes cancel out, moles can be used
in place of molar concentrations.) (b) Estimate the mass of isopentyl
acetate that was lost (1) as a result of incomplete reaction, (2) during
the washings, and (3) during the distillation (see OP-30a). Assume that
the ester’s solubility in aqueous NaHCO3 is about the same as in water.
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Experiment 2
2.
3.
4.
5.
6.
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Preparation of Synthetic Banana Oil
Compare the sum of these estimated losses with your actual product
loss and try to account for any significant differences.
What gas escaped during the sodium bicarbonate washing? Write balanced equations for two reactions that took place during this operation.
(a) Calculate the atom economy and reaction efficiency of your synthesis of isopentyl acetate. (b) Tell how the procedure for the preparation of
isopentyl acetate might be modified to increase the reaction efficiency.
(c) Describe some green features of your synthesis, and any that aren’t
so green.
Describe and explain how each of the following experimental errors or
variations might affect your results. (a) You failed to dry the reaction
flask after washing it with water. (b) You forgot to add the sulfuric acid.
(c) You used twice the amount of acetic acid specified in the procedure.
(d) You left out the sodium bicarbonate washing step. (e) Your thermometer bulb was 1 cm higher than it should have been.
(a) In the “Understanding the Experiment” section, it was stated that
the reaction of an equimolar mixture of isopentyl alcohol and acetic
acid will produce, at most, 67% of the theoretical amount of isopentyl
acetate. Verify this with an equilibrium constant calculation, using
K = 4.2. (b) Compare this with the corresponding percentage for the
conditions used in this experiment (see Exercise 1). Are your results
consistent with Le Châtelier’s principle? Explain.
Based on the procedure that you used in this experiment and using the
same molar quantities of reactants, develop a procedure that would be
suitable for the preparation of isobutyl propionate. Specify the amounts
of all materials required and a distillation range for the product. Obtain
the necessary physical properties from one of the reference books listed
in the Bibliography.
Isopentyl acetate equilibrium
i-PtOH
+ HOAc
i-PtOAc
K=
+ H2O
[i-PtOAc][H2O]
= 4.2
[i-PtOH][HOAc]
Ac = acetyl, CH3CO
i-Pt = isopentyl, CH3CHCH2CH2
CH3
O
CH3CH2C
CH3
OCH2CHCH3
isobutyl propionate
Other Things You Can Do
(Starred items require your instructor’s permission.)
*1. You and your coworkers can prepare a series of esters and compare
their odors.
*2. Prepare another ester of a primary alcohol, such as butyl acetate or
isobutyl propionate, by the general method described for isopentyl
acetate. Work out a procedure for the synthesis (see Exercise 6) and
have it approved by your instructor. In some cases, a longer reflux time
may be necessary for satisfactory results.
3. Read about the isolation of isopentyl acetate (isoamyl acetate) in the
alarm pheromone of the honeybee in Nature 1962, 195, 1018.
4. Read about various types of pheromones in Chemical Communication:
The Language of Pheromones [Bibliography, L3].
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