Experiment 2
Recrystallization: The Purification of
Crystalline Organic Compounds
The method selected for purifying a given organic compound depends on a number of factors
such as the physical state of the compound (solid, liquid, or gas), the properties of the compound,
and the nature of the impurities. Many crystalline organic solids can be purified by the technique
known as recrystallization. This method may be applied to small quantities found in research
laboratories (less than a gram) to the large quantities (tons) manufactured in the chemical
industry.
In principle, the procedure consists of (1) slowly dissolving the solid in an appropriate solvent
while heating until a nearly saturated solution is reached, (2) cooling the solution until crystals
form, and (3) recovering the crystals by filtration or centrifugation.
Solubility Principles
Understanding the relationship between solubility and temperature of an organic compound is
essential to understanding recrystallization. The solubility of an organic compound in a given
solvent depends primarily on the relationship between the polarities of the solvent and organic
compound and the crystal lattice energy of the organic compound.
Generally speaking, nonpolar solvents such as the hydrocarbons like benzene, toluene, and
hexane, dissolve nonpolar solids such as naphthalene, dichlorobenzene, etc. On the other hand,
polar hydroxylic solvents, like methanol and ethanol, dissolve polar compounds which contain
polar groups such as -OH, -NH, and -CO2H. This behavior is sometimes called "like dissolves
like". This is a useful principle but it should not be used as the answer to all solubility questions.
The energy required to break up the lattice structure of the crystal also affects the solubility. The
solubility of a compound may be approximated from its melting point. For example, a
compound with a m.p. of 287 oC would have different solubility characteristics than a compound
with a m.p. of 42oC. High melting solids have strong intermolecular forces between the
molecules in the crystal lattice. Strong solvent-solute interactions must overcome these forces in
order for the compound to dissolve. Assuming a higher melting solid can be dissolved in a hot
solvent it is frequently easier to recrystallize because there is a greater tendency for the
molecules to form the crystal lattice.
If the polarities of a solvent and an organic solid are similar, the compound will tend to be
soluble. If the polarities are dissimilar, the compound will tend to be insoluble. For example,
naphthalene (m.p. 80-82 oC) is soluble in toluene (nonpolar solid/nonpolar solvent) but it is
insoluble in water (nonpolar solid/polar solvent).
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CH3
O
insoluble in
soluble in
H
naphthalene
naphthalene
toluene
H
water
Between the two extremes of soluble and insoluble, many organic compounds are partially
soluble in solvents of intermediate polarity. Ethanol is a polar compound because of its
hydroxyl group, -OH, but it also contains the nonpolar ethyl group, CH3CH2-. In ethanol, the
hydroxyl group makes a larger contribution to the overall polarity and ethanol is therefore
considered to be a polar compound. When naphthalene is mixed with ethanol a limited amount
dissolves. Nonpolar interactions between naphthalene and the CH3CH2- portion of ethanol are
favorable but the interaction of naphthalene and the OH- of ethanol are not. Therefore
naphthalene is only partially soluble in ethanol. Compounds that are partially soluble usually
show an increase in solubility as the temperature is raised, as shown in the following
Temperature-Solubility plot.
80
60
cool
40
o
C
heat
20
0
-20
0
0.5
2.5
3.0
1.5
2.0
Solubility (grams/10 mL)
1.0
3.5
Purification by recrystallization is based on this temperature-solubility relationship and consists
of the following operations: compound A (containing impurities) is placed in a flask and heated
while slowly adding a solvent until it just dissolves (usually at the solvent's boiling point). In the
preceding figure the solubility of compound A at 80oC, the boiling point of the solvent, is about
3g per 10 mL. If you begin with 3 g of A, 10 mL of solvent is required to dissolve it at the
boiling point. At this point, insoluble impurities, if present, may be removed by filtering the hot
solution. The solution is then cooled. The solubility of A decreases. At some point, if the
conditions are right, compound A begins to crystallize.
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After placing the flask in an ice bath the solubility of A is approximately 0.5 g per 10 mL. This
means that at 0oC 0.5g of compound A is still dissolved and the remaining 2.5g (3g-0.5g=2.5g)
has crystallized. This corresponds to an 83% (2.5/3 X 100%) recovery. Soluble impurities,
present in a lower concentration than compound A, will usually remain in solution. Purified
compound A is then obtained by suction filtering the crystals and rinsing with a minimal amount
of pure, cold solvent. The rinsing with cold solvent removes soluble impurities. If too much
solvent is used in the rinse the % recovery will decrease, since even at a low temperature
compound A is still slightly soluble. Residual solvent adhering to the crystals is finally removed
by evaporation in air or in a vacuum desiccator, with or without heat. Most of the material that
you recrystallize will be dried by evaporation of the residual solvent in air.
Several recrystallizations, from different solvents, may be required to reach the desired level of
purity. In each recrystallization some of the compound is lost because of its inherent solubility,
even at ice temperature. These losses can be minimized by careful technique but not totally
eliminated. Second crops of crystals may be obtained by evaporating the filtrate and
recrystallizing the residue.
Choice of Solvent
The choice of solvent for recrystallization depends on several factors:
1.
2.
3.
4.
The solvent should dissolve a relatively large amount of organic compound at high
temperatures and a small amount at low temperatures.
The solvent should dissolve the soluble impurities even at low temperatures.
The solvent should not chemically react with the compound being crystallized.
The solvent should be easily removed (usually by evaporation) from the recrystallized
product. A lower boiling solvent is sometimes used to rinse the crystals to remove a
higher boiling solvent.
The choice of solvent in a recrystallization is determined experimentally. This is done by
observing the outcome of several small-scale trial recrystallizations with a variety of solvents,
usually done in test tubes. This point cannot be overemphasized! If you try to recrystallize all
of a substance with the “wrong” solvent or use too much of it there may be no going back. Once
the appropriate solvent is found the remainder of the material can be recrystallized.
Mixed Solvents
Mixed solvent systems are sometimes used in recrystallizations to adjust the solvent polarity.
For example, a mixture of toluene (relatively non-polar) and ethyl acetate (relatively polar) may
have better characteristics than either alone. In this instance the compound is usually dissolved in
the solvent of greater solubility and the solvent of lesser solubility is added slowly until the hot
solution is near the cloudy point (saturation point). The solution is then cooled and crystals
form. A similar result can be obtained by experimenting with samples of the solvent mixed in
various proportions. Finding the best solvent mixture can require a considerable amount of
effort.
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General Experimental Techniques of Recrystallization
Heating
The heat source most commonly used in a recrystallization is the steam bath since it provides a
constant temperature and will not ignite the highly flammable solvents frequently used in
recrystallizations. If steam is not available, an electrically heated hotplate may be used. In some
situations the hot plate is preferred over the steam bath because it can reach higher temperatures.
A gas burner should only be used when water is the solvent since most organic solvents are
very flammable. When heating to dissolve the compound the solvent should not boil at a rate
which would boil it off. If solvent is lost during heating more may be added. In some situations
a condenser may be used. On the steam bath, solvents with boiling points less than 100oC are in
danger of boiling over. For example, when a test tube containing hexanes is placed on a steam
bath the contents will boil vigorously and shoot out the tube! For many recrystallizations it is
necessary to add boiling stones to promote smooth boiling. The stones are later removed.
The steam bath should be connected as shown above. Steam enters the
top hole of the steam bath and the condensate (hot water) drains out the
lower hole into the sink. Rings should be removed to provide a large
opening for efficient heat transfer to the flask. The steam bath should
not be turned up to full force except initially to drive out the condensed water in the line. Just enough steam to heat a solution
should be used. The surface of the steam bath is not smooth so care
must be taken or the flask may tip over. Caution: Steam and the
steam valve are hot!
steam in
steam/water out
Hot Filtration
If the hot solution has insoluble material or if decolorizing carbon is used
for the removal of colored impurities, the solution must be filtered to
remove the insoluble material. This step is done as rapidly as possible to
Fluted filter paper
minimize crystallization on the funnel. A stemless funnel, heated with
steam and dried before use, is fitted with a fluted filter paper (your
instructor will demonstrate how to "flute" a piece of filter paper) and
stemless funnel
(hot)
supported on a ring. The hot solution is directed toward the middle of the
filter paper with a glass rod, as shown in the diagram at the right. A
collection flask is placed under the funnel and heated during the filtration.
It is not always necessary to place the collection flask on the steam bath but
for those compounds that crystallize out rapidly, the rising warm vapors of
solvent help keep the funnel hot. It is good practice, in most cases, to place a few mL of the
recrystallizing solvent in the collection flask beforehand. The heat of the steam bath will
vaporize it and warm the filter paper and funnel. After the solution has been filtered, the funnel
is rinsed with small portions of hot solvent. If crystallization has occurred in the collection flask
during the filtration, the flask may be reheated to dissolve the crystals. Excess solvent may also
be used so that crystallization is less likely to occur on the funnel. After filtration, the solvent
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can be evaporated to near saturation by passing a stream of air or nitrogen (not burner gas!) into
the heated flask until the desired volume is reached.
Crystallization
The hot solution should then be placed on an insulated surface (wood, folded towel etc.) for
crystallization to take place. Once crystals start to form the flask should be left undisturbed.
A slow rate of crystallization usually affords larger and purer crystals although extremely slow
crystallization may trap solvent and impurities in the crystals. In some instances large crystals
may be difficult to rinse and a purer product is obtained with smaller crystals. This is
particularly true when the impurity is colored. For example, if slow crystallization gives large
brown crystals a faster crystallization may give small white crystals simply because rinsing was
more effective with the smaller crystals. With some compounds the initiation of crystal
formation is very slow and scratching the inside of the flask under the liquid level with a non
fire-polished glass rod or adding a seed crystal may be necessary to initiate crystallization. Each
compound/solvent system seems to have its own personality when it comes to crystallization.
Some compounds give spectacular displays of large needles, prisms, rosettes, etc. The shape of
the crystals depends on the compound, the solvent, and the rate of crystallization. Some
compounds may form different crystal shapes depending on the solvent. This may lead to
different melting points for the same compound as mentioned in Experiment 1. After the
solution reaches room temperature and it appears that no more crystals are forming the mixture
should be cooled in an ice bath (ice and water should be used) to reach a minimum solubility and
maximum recovery of the crystals. The solvent for rinsing the crystals, in a flask, should also be
cooled. The solvent for rinsing is the same as that used in the recrystallization.
Filtration
Many organic compounds crystallize as
fine crystals so that gravity filtration is
very slow. With a slow filtration rate the
mixture would warm up and dissolve some
of the crystals. To facilitate a rapid
filtration and effective rinsing, the crystals
are filtered by suction on a Büchner or
Hirsch funnel. The Hirsch funnel is
preferred with smaller quantities (about 1
g or less) since rinsing is more effective
and the product is more easily recovered.
The Büchner and Hirsch funnels require a
piece of properly fitting filter paper that
just covers the holes but does not go up
the side.
The steps to follow in a filtration are: (1) Set up the apparatus using clamps (a filtration flask
that is not clamped is easily upset and Büchner funnels are expensive). A three-prong clamp is
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ideal for securing the side arm flask. (2) Place a properly fitted piece of filter paper on the funnel
and place the funnel on the rubber adapter on the filter flask. (3) Turn the vacuum on all the
way. (4) Add a few milliliters of cold solvent (the same solvent used in the recrystallization) to
seat the filter paper. (5) Loosen the crystals in the flask by vigorous swirling or with a glass rod.
Pour the resulting slurry onto the filter all at once and push the funnel down on the adapter to
ensure a tight fit. To make stubborn crystals come out of the flask it may be necessary to swirl
the flask and turn it upside down quickly while tapping (care is needed here!). Avoid decanting
the liquid. (6) Transfer any crystals adhering to the flask onto the filter with a minimum amount
of cold solvent (the same solvent used in the recrystallization). (7) Rinse the crystals with a
minimal amount of cold solvent using a Pasteur pipette. Avoid using too much solvent in
attempting to transfer every crystal.
Decolorizing Carbon
Colored impurities are common unwanted by-products in many organic reactions. They are
frequently removed by adsorption on decolorizing carbon (also called activated charcoal)
which is added to the hot solution just before the hot filtration. The hot solution should be
cooled slightly before adding the carbon since the fine particles of carbon can promote rapid
boiling, causing the contents to boil out of the flask! The right amount of carbon to add is
determined by experiment but a guide might be 5-10% of the solute weight. Too little carbon
will result in a less than white product whereas too much will give a lower yield of product since
the carbon also absorbs the compound to be recrystallized. If the filtrate has a gray cast, fine
particles of carbon have passed through or around the filter paper. Residual carbon can be
removed by refiltration of the heated solution through a clean filter paper or through Celite®, a
filter agent (see instructor for this eventuality).
The Experiment
The object of the experiment is to purify an unknown by recrystallization. The experiment
consists of two parts: Part (1) selection of appropriate solvent for recrystallization, Part (2)
recrystallization of the unknown.
NOTE: All waste that contains the unknown should be poured into the waste container in
hood Z. This includes the filtrates after performing the recrystallizations.
Only take a minimal of solvent (5-10 mL) to your workstation. Do not return solvents to the
original bottles.
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Part I. Solubility Tests and Choice of Solvent for Recrystallization.
Exchange a clean and dry vial for one containing a ~ 3 gram sample of unknown. Immediately
write down the unknown number in your lab notebook. Save a small portion (0.1g) for the
melting point by placing it in a vial.
In this part of the experiment the best solvent to recrystallize your unknown will be determined.
The solvent in which the organic compound is most soluble when hot and least soluble when
cold is usually a good solvent for recrystallization. Note that if a solid is soluble in the cold
solvent it is also soluble when hot!
Place a small amount of the unknown in a test tube and just cover it with solvent. Mix well and
observe. If the material dissolves it is soluble at room temperature and clearly the solvent it not
suitable. If the material does not dissolve or appears to be partially soluble, heat with the steam
bath, mixing well. If it appears to dissolve, even partially, add more solvent and continue to
heat. If it dissolves allow the solution to cool. Keep in mind that the unknown contains insoluble
impurities so that not everything will dissolve. Be careful not to heat too long and evaporate the
solvent. Allow the solution to cool and note the type of crystals. Finding the best solvent and
watching the crystals form can be fun. Be aware that an insoluble solid may appear to dissolve
when heated if its melting point is below the temperature of the heated solvent. In this case the
solid has simply melted and on cooling solidifies. This is not a recrystallization! It is simply
melting and solidification. A true recrystallization takes place when crystals are formed directly
from solution. They will usually be seen to form from the entire solution-sides/top/bottom.
Tests for recrystallization should be done with: water, 95% ethanol, 95% ethanol-water (use in a
ratio of ~8:2), methanol, toluene, and heptane. Observations should be recorded for all tests.
Some substances may be recrystallized from ethanol but recoveries are poor. This is because
these substances are too soluble in ethanol even at cold temperatures. In this case a better
recovery may be obtained by using an ethanol-water mixture where water serves to decrease the
substance’s solubility. Typically 10-30% water is effective. Higher %’s can increase the recovery
but will lower the purity since the solubility of impurities may also decrease.
This may be carried out as follows. Add ethanol gradually to the substance to be recrystallized.
A slight excess of ethanol is desired so that there will be a lesser tendency to crystallize during
the hot filtration. Perform the hot filtration as usual. Then reheat the filtered solution and
gradually add water allowing it time to stay hot. Approximately 10-20% water may be added or
until the solution is near saturated as evidenced by a little cloudiness. Then allow the solution to
cool and material to crystallize.
Part 2. Recrystallization of the Unknown
Review the general procedure for recrystallization. The skill of recrystallization requires
practice. Since you will recrystallize substances throughout the year you should give attention to
52
understanding the principles and operation of this technique.
recrystallization, the steps of the technique should be reviewed.
Before beginning the
Weigh the remaining amount of the unknown, after performing the solubility tests, (weight to the
nearest 0.001 g) into a 50 mL Erlenmeyer flask. Add the solvent selected in Part 1 to the flask to
just cover the material. Heat on the steam bath with sufficient time for the solvent to reach the
steam bath temperature (exercise care with hexanes since steam bath temperature is above its
boiling point). Then slowly add additional solvent until the material just dissolves. In most cases
it is important to use a minimum amount of solvent in dissolving the compound otherwise it may
not crystallize or may partially crystallize giving a poor % recovery. However, in this case you
are doing a hot filtration and it is better to err on the side of a little too much solvent otherwise
the solution may crystallize during the hot filtration.
The next steps are not always done but can be useful to provide information about the behavior
of the solid. Cool the flask by placing it in an ice bath. Stir with a glass rod to break up the
crystals. If the flask contents are too thick add more solvent to obtain a slurry of pourable
consistency.
Setup the equipment for a hot filtration and review the procedure. Reheat the flask on the steam
bath, add more solvent if necessary, and perform the hot filtration. It is important to carryout the
hot filtration rapidly with the fluted filter so that the solution does not cool and crystallize in the
funnel. It may be necessary to begin with a hot solution that has a slight excess of the solvent to
minimization crystallization in the funnel during the hot filtration. After filtration allow the
filtrate to cool. Crystals should form at this time. If crystals form immediately the flask may be
reheated to dissolve them though this is not always necessary. Once the flask has cooled to room
temperature, cool it further in an ice-water bath (cool the solvent you will rinse the crystals with
at the same time in a 25 mL flask). Break up the solid with a glass rod before the final filtration.
If the mixture is too thick at this point add a small amount of solvent and continue to cool; the
flask may also be reheated and allowed to cool.
Filter on the Büchner or Hirsh funnel rinsing with cold solvent. Remove the "cake" of partially
dried crystals and filter paper by sliding a spatula under the filter paper. Place them on a clean,
tared piece of paper or watch glass. Peel off the filter paper and allow the solvent to evaporate
over the week. Volatile solids may sublime during this time period. A solid is considered
volatile if you can smell it! Determine the weight and transfer it to a properly labeled vial.
Submit the vial to the instructor by placing the plastic tray on the desk.
The % recovery = g purified material/g unpurified material X 100
Be aware that some unknowns now and in the future may require recrystallization from water, for
instance, benzoic acid. The water will require a burner to heat it up. When the ventilation system is
operating the smell or sound of an open gas jet cannot be detected. When the lab is over the ventilation
system is turned off. A gas jet that is left on will eventually fill the room with gas, which could result in
a serious explosion.
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The notebook report should contain:
1.
2.
3.
4.
Title
Purpose of the experiment
The procedure (reference and outline)
Data and Observations (organized on your notebook page as shown below)
Unknown number: _________________
Observations
Data
A. Solubility test table
B. Data for the recrystallization
solvent used in recrystallization
approximate volume of solvent
mass before
recrystallization
mass after
recrystallization
% recovery
mp range
before
recrystallization
mp range
after
recrystallization
5. Conclusions
ASA Questions:
1.
What is the purpose of recrystallization?
2.
What solvent should be used to rinse crystals in a recrystallization?
3.
What precautions should be taken when using hexanes as a recrystallizing solvent?
4.
What is the relationship between solubility and temperature for most organic
compounds?
54
5.
The solubility of compound A is 2 g /100 mL at the temperature of a steam bath. What
is the minimal amount of solvent required to dissolve 5.0 g of the material?
6.
If the solubility of compound A is 0.5 g/100 mL at ice temperature how many grams of
compound A (problem 5) may be isolated in a recrystallization?
7.
Why might it be useful to recrystallize a substance from two different solvents in
sequence?
8.
Circle the solvents that are miscible with water: ethanol, hexanes, toluene.
9.
Which is more soluble in hexanes, methanol or ethanol? Why?
10.
How can you determine if a substance is dissolved in a hot solvent or has melted without
actually dissolving?
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