Topics and Techniques
i)
identification of solvent layers of two immiscible solvents
ii)
partioning of a c
ompound between two immiscible solvents and determination of K
D
iii)
liquid
-
liquid extraction with aqueous acids and bases with organic solvents.
iv)
use of drying agents
Introduction
Liquid
-
liquid extraction is a method used for the separation of a mixt
ure using two immiscible
solvents. The ability to separate compounds in a mixture using the technique of liquid
-
liquid extraction
depends upon how differently the compounds of the sample mixture partition themselves between the
two immiscible solvents. T
he technique of liquid
-
liquid extraction often involves a number of
processes. First, the component mixture is dissolved in a suitable solvent and a second solvent that is
immiscible with the first solvent is added (e.g., two solvents that are immiscible
are diethyl ether and
water). Next, the contents are thoroughly mixed (shaking) and the two immiscible solvents allowed
separating into layers. The less dense solvent will be the upper layer, while the more dense solvent will
be the lower layer. The com
ponents of the initial mixture will be distributed amongst the two
immiscible solvents as determined by their partition coefficient. The relative solubility that a
compound has in two given solvents can provide an estimation of the extent to which a compo
und will
be partitioned between them. A compound that is more soluble in the less dense solvent will
preferentially reside in the upper layer. Conversely, a compound more soluble in the more dense
solvent will preferentially reside in the lower layer. L
astly, the two immiscible layers are separated,
transferred and the component in that solvent is isolated by solvent evaporation and/or crystallization.
The technique of liquid
-
liquid extraction is frequently used for the separation of an organic product
from a reaction mixture after an aqueous work up, or for the isolation of naturally occurring substances.
Organic solvents frequently employed for liquid
-
liquid extraction procedures include petroleum
ether (which is a mixture of low molecular weight alk
anes), ethyl acetate, methylene chloride
(dichloromethane) and toluene. Diethyl ether (ether) is an effective extraction solvent and with its low
boiling point (37 °C) is widely used. However, the high volatility and extreme flammability of diethyl
ether
is a deterrent in employing ether as a solvent for extraction. In the experiment below, methyl
tertiary
-
butyl ether (MTBE) is used as a replacement for diethyl ether since MTBE is less of a fire
hazard.
Extraction under basic and acidic conditions
As
mentioned above, the ability to separate compounds of a mixture using liquid
-
liquid
extraction procedures depends upon the relative solubility that each compound has in the two
immiscible solvents. A change in the pH of the solvent (the addition of acid o
r base) can change the
solubility of an organic compound in a solvent considerably. For example, the solubility of a
carboxylic acid (RCO
2
H) in water is increased considerably by the addition of a base e.g., NaOH(aq) or
NaHCO
3
(aq). The increase in water
solubility is due to an acid base reaction that converts the less
water soluble carboxylic acid (RCO
2
H) into the more water soluble sodium carboxylate (RCO
2
Na)
equation (a). The water solubility of other organic compounds having acidic hydrogens e.g., Phe
nols
can also be increased by addition of base equation (b).
2
Noteworthy is that an aqueous solution of NaHCO
3
converts carboxylic acids to their sodium
carboxylates equation (c), but NaHCO
3
(aq)
is not a strong enough base to form sodium salts of phenolic
compounds equation (d). Thus two weak organic acids can be partitioned or separated providing their
pKa values differ by five of so pKa units and the aqueous base used in the deprotonation is not basic
enough to remove the hydrogen from the weaker organi
c acid. Note, if aqueous NaOH were used
instead of NaHCO
3
(aq)
in the extraction of a mixture containing a carboxylic acid and a phenol, aqueous
NaOH would deprotonate both the carboxylic acid and the phenol (eq a and b) and subsequently, both
the carboxyl
ate and phenolate would reside in the aqueous base layer and the separation would not be
feasible using NaOH
(aq)
as a base.
The water solubility of organic compounds having basic groups e.g., amino, can be increased by
the addition aqueous acid. The le
ss water soluble organic amine in the presence of acid is converted to
the more water soluble organo ammonium ion. Dilute hydrochloric acid is often used for extraction of
basic organic substances or for removal of basic impurities from an organic phase.
Liquid
-
liquid extraction can be used to separate a weak organic acid from a weak organic base.
The extraction can be performed using aqueous acid or aqueous base as described above along with an
immiscible organic solvent such as ether. The process of
extraction and separation can be best
understood if a flow chart is used as an "outline" to detail the process. The flow chart will allow you to
"know where you are in the process" and as well, keep track of the reagents, solvent layers and
products.
Likewise, an organic compound that is not a weak acid nor weak base (alkanes, alkenes, ketones
etc.) can be separated from weak organic acids or bases. In short, weak organic acids, bases or neutral
organic compounds whether they be the desired compound
or impurities with in a mixture can be
separated by extraction using the appropriate aqueous acid, aqueous base and a suitable organic solvent.
To assure yourself that you understand these principals, you are required to
generate a flow chart as
part of
the prelab preparation for experiment IIB
.
3
Part A
:
Determination of the partition coefficient of benzoic acid between MTBA and H
2
O.
Equipment and supplies
You need a centrifuge tube fitted with a cap and a couple Pasteur filter pipettes. Syringe
pi
pettes will be provided for each solvent used in this part of the experiment.
Your TA will instruct
you on how to use the syringe pipettes
. The chemicals needed for experiment IIA are benzoic acid,
MTBE, water and anhydrous sodium sulfate.
Procedure
A
dd 50 mg of benzoic acid followed by the addition of 1 ml of water and 1 ml of MTBE to a
centrifuge tube. A syringe is supplied for each transfer (a syringe is attached to each solvent bottle).
Cap the centrifuge tube and carefully shake the tube for 30
seconds by hand or a Vortex mixer.
Remove the cap and allow the two layers to separate. Which solvent is the top layer? Which solvent is
the bottom layer? What is a quick and simple technique/way to identify either layer?
Carefully remove the aqueous
phase using a Pasteur pipette. Transfer the MTBE layer to a dry
tube and add about 50 mg of anhydrous sodium sulfate. Sodium sulfate is a drying agent (absorbs
water) and removes only trace of moist. You might need to add some extra anhydrous sodium sul
fate in
order to dry the organic phase completely. Recap the tube and let the sodium sulfate dry the organic
phase for 5 minutes.
Transfer the dried organic phase (the MTBE layer which was treated with the drying agent) via
a dry Pasteur pipette to a ta
red and dry conical vial containing a boiling chip. Rinse the sodium sulfate
that was left in the tube by adding ~600
μ
l of MTBE. Transfer the MTBE to the conical vial (combine
with the first extraction). Evaporate the organic solvent in the fume hood using a warm sand bath until
a constant weight of the solid is obtained. Turn in your dried product in a properly labele
d plastic bag.
Determine the amount of benzoic acid recovered and calculate a value for the distribution coefficient
(K
D
).
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