Solubility
Solubility is the property of a substance, which describes the ability of the substance to form mixtures
with other substances, usually liquids, which are chemically and physically homogeneous. In most cases,
the solubility of a substance in another substance increases with an increase in temperature. This property
allows for purification of compounds by recrystallization. The solubility of a substance in a liquid can be
used to classify it. It also can provide a method for separating a substance from other substances. This
separation technique is called extraction and will be used in many of your experiments to isolate the
organic product from a reaction mixture.
The solubility of a substance is a vital piece of information. Before starting to work with a compound,
it is necessary to know or to be able to predict whether the substance will be soluble in water, aqueous acid,
aqueous base, or organic solvents such as ether. If the molecular structure of a substance is unknown,
knowledge of its solubility characteristics can be the basis for certain deductions about its structure.
Extraction
Aqueous and Organic Phases
"Oil and water do not mix." This phrase describes the behavior of water and most organic liquids,
such as hexane, ether, and dichloromethane. When water and an organic liquid are shaken together and
than allowed to stand, the liquids separate into a two phase system with the more dense liquid on the
bottom.
Liquids which mix to form a homogeneous liquid phase are said to be miscible, meaning that the two
substances are mutually soluble in all proportions. Most organic liquid used as solvents for manipulations
in an organic chemistry laboratory are not soluble in water. Since the organic liquid is not soluble in water
it separates into a separate phase. Organic liquids and water are said to be immiscible in one another.
Because the commonly used solvents, such as hexane and ethyl ether, are less dense than water, it is a
common mistake for a laboratory novice to assume that the bottom layer is always water. You should be
aware that there are organic solvents, such as dichloromethane, which are more dense than water. When
working with solvents, you should know their densities so you will know which layer is the organic layer
and which one is the aqueous layer. If there is any doubt about the identity of a layer, there is a quick test
which can be performed to ascertain its identity. A drop of the layer in question is added to a small amount
(approx. 0.5 mL) of water in a small test tube. If the drop dissolves in the water, the layer in question is the
aqueous layer. If the drop does not dissolve, the layer is the organic layer.
Partition Coefficient
The process of extraction with solvents is generally employed either for the isolation of dissolved
substances from solutions or from solid mixtures or for the removal of undesired soluble impurities from
mixtures. The latter process is usually termed washing.
The theory of the process can best be illustrated by considering the operation, frequently carried out in
the laboratory, of extracting an organic compound from its aqueous solution with an immiscible solvent.
The distribution law or partition law states that if a substance is added to a system of two liquid layers,
made up of two immiscible or slightly miscible components, then the substance will distribute itself
between the two layers so that the ratio of the concentration in one solvent to the concentration in the
second solvent remains constant at constant temperature. At equilibrium, the partition coefficient is
described by the mathematical expression:
K = concentration in solvent A / concentration in solvent B
The constant K is termed the distribution or partition coefficient. As a very rough approximation,
the distribution coefficient may be assumed equal to the ratio of the solubilities in the two solvents.
Organic compounds are usually relatively more soluble in organic solvents than in water, hence they
may be extracted from aqueous solutions. If electrolytes, such as sodium chloride, are added to the
aqueous solution, the solubility of the organic substance is lowered. This is called salting out and can
assist in the extraction of organic compounds, which are partially soluble in water.
Technique of Extraction
The objective of a simple extraction is to partition one or more substances between two immiscible
solvents. This is usually accomplished with the use of a separatory funnel. If the separatory funnel has a
glass stopcock, prepare the funnel for use by making sure that the stopcock is lightly greased and will turn
without difficulty. Teflon stopcocks need not be greased.
With the separatory funnel supported in a ring, check to make sure that the stopcock is closed and pour
in the solution to be extracted. Then add the extracting solvent (the funnel should not be filled to more
than about three-fourths of its height), replace the stopper after wetting it with water (to keep the organic
solvent from creeping out around the stopper), and swirl or shake the contents to mix them. With vigorous
shaking, a total mixing period of ten to thirty seconds is usually considered adequate to establish
equilibrium. After allowing the mixture to stand in the separatory funnel until the two immiscible layers
have separated cleanly, remove the stopper at the top and draw off part of the lower layer through the
stopcock at the bottom. Wait a little while for the remainder of the lower layer to drain down (gentle
swirling of the separatory funnel can speed this up), and draw this off also. Then pour the upper layer out
the top.
PRECAUTION:
When a volatile solvent is involved in an extraction, the establishment of the equilibrium vapor
pressure of the solvent will cause the pressure to rise inside the stoppered separatory funnel. The pressure
is best released by turning the funnel upside down (with the stopper held in place with the palm of the
hand) and cautiously opening the stopcock. When a very volatile solvent such as ether is being used, the
first mixing should consist only of a slow inversion of the separatory funnel followed by release of the
pressure. After alternate cautious sloshing of the contents of the separatory funnel and then release of
pressure, the sound of the escaping vapors will indicate that the pressure is not being built up so fast and
the periods of mixing can be longer and more vigorous. If you neglect to release the pressure inside the
funnel, the stopper may be forced out.
It is dangerous to try to extract a solution if its temperature is near or above the boiling point of the
extracting solvent. Thus if you plan to extract with pentane, ether, or dichloromethane, you may need to
cool the solution. When these solvents are used, it is also a good idea to hold the separatory funnel by the
ends so that the contents will not be warmed by your hands.
If a strong acid is to be extracted with carbonate or bicarbonate solution, the carbon dioxide produced
can cause a large build-up in pressure unless mixing is done very cautiously with frequent release of
pressure. In cases where much carbon dioxide production is anticipated, it is best to do the mixing in a
flask or beaker and then to transfer the mixture to the separatory funnel for separation.
Choice of Solvent
If an organic product is to be purified by dissolution in an organic solvent followed by extraction of
the solution with two or more portions of aqueous solution, the whole process will be much faster and
easier, and will involve less loss, if the organic solution is less dense than water. In this case, the water
layer can be drawn off through the stopcock, and the organic solution is retained in the funnel, ready for
the next extraction. If the organic phase is heavier than water, it will have to be drawn off through the
stopcock, the aqueous layer poured out, and the organic layer returned to the separatory funnel for the next
extraction. Each such transfer will take time and may involve a loss of material. Conversely, if it is
necessary to extract an aqueous solution with several portions of solvent in order to achieve the maximum
recovery of a substance, it will be more convenient to extract with a solvent heavier than water so that the
solvent can simply be drawn off each time through the stopcock without removal of the water layer first.