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EXTRACTION
Another technique of purification or separation is extraction, which involves separation
of a compound by means of its preferential solubility in a solvent. Several of the techniques you
have studied previously involve equilibrium. In melting point you measure the temperature at
which a solid and liquid are in equilibrium.
In distillation (and hence boiling point) the
temperature at which something distills (boils) is the temperature at which the equilibrium
vapour pressure equals the atmospheric pressure. In extraction an equilibrium is also involved.
For example, benzene and water are two solvents, which are insoluble in one another and
therefore will form a two phase system. If a third component, for example, propionic acid, is
added, it will dissolve and distribute itself between each phase, since it is miscible with either
benzene or water. At equilibrium in dilute solutions this distribution between benzene and water
can be described by a simple mathematical expression
where K is a constant called the distribution coefficient (or partition coefficient), [propionic
acid]benzeme is the equilibrium concentration of propionic acid in benzene and [propionic acid]water
is the equilibrium concentration of propionic acid in water. In general this expression can be
written
where the concentration in the first mentioned solvent (benzene in this example) is placed on top.
Therefore, a large K indicates that at equilibrium a larger proportion of the compound is
dissolved in the first mentioned solvent. In many synthetic procedures it is necessary to extract a
product or an impurity from solution. The problem arises concerning the kind of solvent to be
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used, the amount of solvent for the extraction and the number of extractions needed for
efficient separation.
Criteria for the choice of solvent:
The solvent must be immiscible with the solvent containing the compound to be
extracted. The value of the distribution coefficient should be as favourable as possible. That
means that K should either be fairly large or fairly small (indicating that the majority of the
component is in the first mentioned or the second mentioned solvent respectively). For example,
a K of 1 indicates that 50% of the component is in each solvent which is undesirable. The
material extracted must be readily separated from the solvent.
The amount of solvent to be used and the number of extractions are closely related. In
general, with a given amount of solvent, the use of several extractions in small portions is more
efficient than few extractions in large portions. For example, if the distribution coefficient of
material, X, between solvents M and N is 1.0, it is simple to calculate the recovery of X by
extraction from M with an equal volume of N:
Therefore [X]M = [X]N and if the volumes M and N are the same, the amount of X
removed will be exactly 1/2. If the solvent N were divided into two equal portions and these
were used for two successive extractions of X from solvent M, the amount removed could be
calculated as follows:
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Since there is twice as much M as N, V = 2V ; therefore, substituting in the above
M
N
expression:
Only 1/3 of X is removed by solvent N in this extraction and 2/3 of X remains in solvent M. If
this procedure is repeated, since there is again twice as much of M and N, then 1/3 of the X
remaining in M is removed by the solvent N and 2/3 remains. Therefore the solvent N would
now have removed 1/3 of the original X plus 1/3 of the remaining two-thirds. The total amount
removed would be 3/9 + 1/3 x 2/3 or 5/9. Consequently, this double extraction is more efficient
than a large single extraction.
In many instances, information is not available concerning distribution coefficients for
isolation of synthetic products or removal of the impurities from a compound but use can be
made of certain generalizations about the behaviour of compounds.
"Like dissolves like". This means that polar compounds tend to be more soluble in polar
solvents and non-polar more soluble in non-polar solvents (e.g. hydrocarbons in benzene).
Various materials may be added to one of the phases to increase or decrease the solubility
of a substance in that phase. Complexing reagents in one phase may lead to preferential
complexing of certain compounds in that phase. Neutral salts which make the aqueous phase
more polar, therefore decreasing the solubility of certain compounds in the aqueous phase, may
be added.
The distribution coefficient for acidic or basic compounds can be greatly altered by
altering the pH of one phase. Organic acids, RCOOH, of greater than 4 carbons are largely
insoluble in water but in the presence of base, a polar soluble salt can form
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O
O
NaOH
RCOH
H 2O
RCO Na
Which, is insoluble in the non-polar organic solvent. Similarly, higher molecular weight
organic bases, such as amines RNH , (derivatives of ammonia NH3), are largely insoluble in
2
water, but form soluble salts in acid solution which are insoluble in the non-polar organic
solvent:
This pH dependence of the distribution coefficient can be used to separate acidic and
basic compounds from neutral impurities and forms the basis of one of today's experiments.
Thus extraction is a versatile purification and separation technique.
Experimental Procedure
The extraction to be carried out today makes use of the fact that changes in the pH of one
solvent can affect the solubility of an acid or base component of a mixture in that solvent. You
will be given a mixture, which is soluble in dichloromethane and insoluble in water, of
naphthalene and benzoic acid.
O
OH
Napthalene
Benzoic acid
Benzoic acid is an organic acid which, is very soluble in dichloromethane and has a
solubility of only 3 g/L in water at 25°C. Since benzoic acid is an acid it will react with sodium
hydroxide to form a water-soluble sodium salt.
O
O
OH
NaOH
O
Na
H 2O
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This sodium salt is insoluble in the organic layer because it is so polar and hence it is
possible to remove the benzoic acid almost quantitatively from the naphthalene, which is in the
organic layer. You will be given a mixture of these two compounds and from the melting point
of the mixture (m.p. should have a large range and should be lower than that of pure benzoic
acid) and the m.p. of the final products, you will determine how efficient your separation
technique has been.
Carry out the following procedure to separate the 2 components.
Obtain a package of the naphthalene-benzoic acid mixture from the prep room. Mix
thoroughly by grinding in a mortar and determine its m.p. Dissolve the mixture in 20 mL of
dichloromethane then place it in a separatory funnel. Extract once with 10 mL of water. There
will be little or no benzoic acid in this water. Then extract with 10 mL of cold NaOH solution.
Shake carefully with frequent release of pressure, then allow to stand until the emulsion breaks
and a clear separation of layers is observed. Remove and retain the aqueous (upper) layer. Then
extract the organic layer once again with a further 10 mL of sodium hydroxide solution. Again
separate the layers (save both) and combine the two aqueous layers containing the sodium salt of
benzoic acid in solution. To recover the benzoic acid, carefully add concentrated hydrochloric
acid (about 10 mL) until no further benzoic acid precipitates. Filter by suction to collect the
benzoic acid and wash with two 5 mL portions of ice-cold water. Allow it to dry and then record
the melting point and weight of the recovered benzoic acid.
Hand the product to your
Demonstrator. The dichloromethane layer should contain only naphthalene. The naphthalene
can be recovered as follows. First, to remove any water from the dichloromethane, add 2.0 g of
anhydrous sodium sulfate (which will absorb the water) and allow to stand for 15 minutes. Filter
by gravity into a 125 mL Erlenmeyer flask to remove the sodium sulfate and then remove the
dichloromethane from the naphthalene by evaporation on the steam bath in the fume hood. The
naphthalene crystals should be weighed, the melting point taken, and the product handed in.