Extraction
The coca plant
Vin Mariani – ethanol
and coca leaves
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Separation Techniques
1. Recrystallization
2. Extraction
3. Distillation
4. Chromatography
Advantages of Extraction
Very Efficient
Simple Equipment
A method called
continuous extraction is
used for large scale
separations industrially.
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Disadvantages of Extraction
Extraction requires use of an organic
solvent that requires disposal (today’s
lab) or recycling by distillation
(increases complexity and costs
energy).
When desired materials must be
isolated from the organic solvent, this
requires distillation (time and energy
costs) or evaporation of solvent
(expense of solvent loss and pollution).
Basis of Extraction
Extraction is a purification
technique that makes use of
the unequal distribution of a
compound between two
immiscible liquids (e.g.
water and hexane).
The distribution between the
two liquids is related to the
compound’s solubility in
each liquid.
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Basis of Extraction - 2
In order to hasten the
distribution between phases to
reach equilibrium, we place the
two solvents in a separatory
funnel and shake the funnel
vigorously to mix the solvents.
We allow the layers to separate,
and the lower phase can be
drained from the funnel by
opening the stopcock
Basis of Extraction - 3
Extraction will lead to
separation of compounds
only if they differ
significantly in their
distribution between the
two solvents. Ideally,
each compound should
prefer a different solvent.
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Uses for Simple Extraction -1
Separation of organic products from a reaction,
particularly in removing inorganic salts that are
reagents or by-products of the reaction.
Inorganic salts are very soluble in water and
mostly insoluble in organic solvents. This
technique is called an aqueous work-up of a
reaction.
The organic products (as well as starting
materials and organic by-products) will remain
in the organic phase.
Aqueous Work-up
For example, a mixture of sodium chloride
and benzoic acid would be separated easily
by extraction.
Solubility
Compound
in CH2Cl2
in H2O
Benzoic Acid
5g/100 mL
0.3g/100 mL
NaCl
~ insol.
35g/100 mL
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Aqueous Work-up
NaCl in
aqueous layer
Shake with
Benzoic acid
dichloromethane
+
and water
Sodium Chloride
Note: The water phase is the
upper layer because
dichloromethane has a larger
density than water.
Benzoic acid in
CH2Cl2 layer
Uses for Simple Extraction - 2
A second type of extraction involves
separating organic compounds.
Separation of organic compounds from
each other by extraction requires
a) compounds with greatly differing
solubilities in water, or
b) compounds which can be converted to
forms with greatly differing water
solubilities.
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Separation of organic compounds
An example of separation of organic
compounds with greatly differing solubility
in water would be a mixture of ethanol
(miscible with water) and decanol (insoluble
in water).
A solution of these two alcohols in an
organic solvent that was shaken with water
would result in most of the ethanol in the
water layer and most of the decanol in the
organic layer.
Aqueous Work-up
Ethanol in
aqueous layer
Ethanol
+
1-decanol
Shake with
dichloromethane
and water
Note: The water phase is the
upper layer because
dichloromethane is more
dense than water.
1-decanol in
CH2Cl2 layer
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Changing Solubility Properties
In Today’s Experiment, we will examine the
use of acid-base reactions to change the
solubility properties of organic compounds
in order to allow separation by extraction.
The acid-base reactions convert water
insoluble neutral organic compounds into
ionic salts that are much more water soluble.
This procedure is possible only with
compounds that are reasonably acidic or
basic and react with dilute aqueous acid or
base.
Changing Solubility Properties
The ionic salts then will distribute
differently between water and an organic
solvent. For example, benzoic acid is
almost insoluble in water, but sodium
benzoate is quite soluble in water.
CO2H
0.3g/100 mL H2O
dil. NaOH
CO2
Na
+
H2O
65 g/100 mL H2O
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Extraction Mixture
Your unknown sample today is a mixture of
three compounds. All three compounds are
quite soluble in organic solvents, but are
much less soluble in water.
CO2CH2CH3
CO2H
CONH2
H2N
Benzoic Acid
0.3g/100 mL H2O
Benzocaine
0.04g/2500 mL H2O
Benzamide
1.3g/100 mL H2O
Distribution between solvents
If these three compounds are
shaken in a separatory funnel
with dichloromethane
(CH2Cl2) and water, all three
compounds will remain
almost totally in the organic
phase upon equilibration.
All three compounds
will be in the lower
dichloromethane
layer (phase).
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Reaction with dilute HCl
However, if the dichloromethane layer is shaken
with dilute hydrochloric acid, the amine group of
benzocaine will be protonated to form an amine
hydrochloride salt that will be water soluble, but
insoluble in dichloromethane!
CO2CH2CH3
CO2CH2CH3
H3O+
H 2N
H3N
Cl
insoluble in dichloromethane
highly soluble in water
Distribution with acidic layer
Now most of the
benzocaine will be a
salt in the upper acidic
aqueous layer. The
other two organics are
unchanged and remain
in the lower organic
layer.
Hydrochloride
salt of
benzocaine
Benzoic acid and
benzamide
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Separation of benzocaine salt
The organic layer can be drained from the
separatory funnel and the aqueous layer
poured into a beaker. The organic layer is
returned to the separatory funnel and
extracted a second time with dilute HCl to
complete the removal of benzocaine from
the organic layer. The second aqueous
extract is combined with the first.
The organic layer now contains only
benzoic acid and benzamide.
Isolation of Benzocaine
Since benzocaine can be slowly destroyed
by acid, we want to isolate the benzocaine
before finishing the extraction. If the
hydrochloride salt of benzocaine is treated
with dilute base, the proton is removed to
regenerate benzocaine. The benzocaine will
be insoluble in the aqueous layer and
precipitate out of solution.
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Isolation of Benzocaine
Then the crystals of benzocaine can be
isolated by filtration after cooling the
solution to maximize yield.
CO2CH2CH3
CO2CH2CH3
NaOH
H3N
H2 N
Cl
insoluble in water;
forms crystals
Reaction with dilute base
Now the organic layer containing benzoic acid
and benzamide is placed in the separatory
funnel and treated with dilute sodium
hydroxide. This deprotonates benzoic acid to
form water soluble sodium benzoate.
CO2H
NaOH
CO2
Na
+
H2O
soluble in water
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Distribution with basic layer
Now most of the benzoic
acid will be a sodium
salt in the upper aqueous
layer. The benzamide is
unchanged and remains
in the organic layer.
(Benzamide does not
react with either weak
acid or weak base
solutions.)
Sodium salt of
benzoic acid
Benzamide
Separation of sodium benzoate
The organic layer can be drained from the
separatory funnel and the aqueous layer poured
into a beaker. The organic layer is returned to
the separatory funnel and extracted a second
time with dilute sodium hydroxide to complete
the removal of benzoic acid from the organic
layer. The second aqueous extract is combined
with the first.
The organic layer now contains only
benzamide.
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Isolation of Benzoic Acid
Now solid benzoic acid can be isolated by
acidification of the aqueous extract.
Protonation give benzoic acid which
precipitates from the solution and can be
isolated by filtration.
CO2
soluble in water
Na
H3O+
CO2H
insoluble in water;
crystals form
Characterization of Crystals
You will need to store your crystals of
benzoic acid and benzocaine until next week
to allow them to dry before you obtain a
weight to calculate a recovery yield and
obtain melting points on your products.
Fold a filter paper around each of your
crystals and use purple tape to label the
‘pad’ with your name. The TA will show
you where to store your samples.
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Use of the Separatory Funnel
Closed
stopcock
Be sure the
stopcock is closed
before pouring
anything into the
separatory funnel!
Keep an empty
beaker under the
funnel as a
precaution.
Use of the Separatory Funnel
When shaking the
separatory funnel,
vent through the
stopcock frequently!
Open the stopcock
slowly with the
Hold stopper in place
funnel inverted and
with fingers over
NOT pointed at
stopper before
anyone.
inverting or shaking.
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Warnings
Make sure you know which layer you need
to be saving after every extraction. HINT: It
is always the same layer in this experiment!
DO NOT throw away any of your layers
until you have isolated your products.
Keep each of your various extracts labeled!
Adding acid to an acid layer won’t lead to
neutralization!
You CANNOT start the experiment over!
Warnings
During the lab period, DO NOT use
acetone to rinse your GRADUATED
CYLINDER between measuring your
various solvents.
RINSE WITH WATER ONLY.
Acetone reacts with strong acid and
strong base!
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Safety
Avoid contact with all compounds and
solutions in today’s experiment. Wash
hands if you get either aqueous or organic
solutions on them.
All halogenated solvents are considered
hazardous, and exposure should be
minimized. Keep solvents near minihood intakes.
Clean-up
Dispose of your aqueous phases (water
solutions) from the filtrations by pouring
them down the sink – DO NOT pour these
in waste bottles.
HOWEVER, you should pour your
unused 3M and 6M NaOH and 6M HCl
solutions into WASTE BOTTLES in the
hood.
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Clean-up
Dispose of the methylene chloride
solution in the appropriately labeled
bottle in the hood.
Kim Wipes, pH paper, filter paper,
and paper towels contaminated with
chemicals should be placed in the
container in the hood labeled for
such waste.
Clean-up
Rinse all glassware with acetone; pour the
rinse solution into the container in the sink.
Your work area should be clean and free
from spilled chemicals before you leave the
lab.
Your TA will be asking those of you who
finish early to clean up the bench area and
balances before you leave.
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Aggie Honor Code
An Aggie does not lie, cheat or steal,
or tolerate those who do.
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