EXPERIMENT 2
PART 1: RECRYSTALLIZATION OF AN ORGANIC SOLID
THEORY
Recrystallization is a purification technique that is very frequently used for organic compounds.
When solid reaction products are isolated, they are seldom "pure" but are usually contaminated with
small amounts of impurities such as reaction by-products, unreacted starting material, etc..
The
process of recrystallization is used to "purify", or separate, the desired compound from the impurities.
If you want to dissolve an ionic (salt-type) or very polar material, you use a polar, ionizing
solvent such as water. If you want to dissolve a non-polar material (many organic compounds) you use
a non- or relatively non-polar (organic) solvent.
For intermediate cases where the material is
moderately polar, water and/or organic solvents such as the alcohols, ethers, etc., may be suitable.
In most cases the solubility of solids in liquids increases with increasing temperature.
In
crystallizations (or recrystallizations) we take advantage of this phenomenon by dissolving the solid in
the minimum amount of hot solvent, rapidly filtering the hot solution to remove any insoluble impurities,
and then allowing the filtrate to cool slowly and undisturbed so that large, well-formed crystals are
obtained with the minimum number of imperfections and minimum surface area to trap and hold
impurities.
After cooling slowly to near room temperature, the material is chilled, in ice, to get
maximum recovery of crystals. The solid is then collected by suction filtration and dried. If desired or
necessary, the filtrate can be concentrated to give a second crop of crystals.
DEFINITIONS AND TERMS:
Pure: All molecules of the sample are identical (an impossible goal!)
Impurities: Other undesired compounds present in the sample of desired compound. The
impurities are usually unreacted starting materials or by-products from the reaction.
Solvent: A liquid, either a single compound or a mixture, which will dissolve the solid to be
purified.
Solute: The material (impure) that is dissolved in the solvent.
-2Mother liquor: The solution remaining after the "pure" product has been removed by suction
filtration. This solution contains some desired product that did not crystallize but now has a higher
proportion of impurities.
Saturated solution: A solution that has the maximum possible concentration of solute that can
exist at equilibrium with the solid substance, at a given temperature.
Solubility: The concentration (in g/100 mL, mol L-1, etc.) of solute in a saturated solution.
Solubility generally increases with increasing temperature, though there are exceptions.
Crystallization: The process of cooling, without the loss of solvent, a hot, nearly saturated
solution of a compound so that as the temperature falls, the solubility of the solute decreases and
crystalline material is deposited from the solution. These crystals, that can be collected by filtration, will
be of higher purity than the starting solute.
Recrystallization: Repetition of the above process using the crystalline product that came out
of solution, to achieve further purification.
GENERAL
Unless an impurity is very insoluble in the chosen solvent, it may remain in solution through the
whole process. Since it is only a minor component, there may not be enough of it to form a saturated
solution, even after cooling. A very soluble component in small proportions will certainly not come out
of solution on cooling. The crystallized sample will only contain any of the soluble impurities if any
remaining mother liquor evaporates from the crystals at the end of the cold suction filtration step. The
purified product will probably still contain some of the impurities, but at a much lower level.
1. Solvent: The choice of a solvent is an art! In this course, you will be told what solvents to
use. In general, one follows the adage "like dissolves like" (e.g. polar solids tend to dissolve
in polar solvents, non-polar solids in non-polar solvents), but the choice of a good solvent is
much more complex.
Some factors to consider for a solvent follow:
It must have a
reasonably low boiling point, to aid in easy removal from the isolated "pure" product. There
should be a wide range between the boiling point of the solvent and the melting point of the
product. The solvent should boil well below the melting point of the solute, otherwise the
product will not crystallize but separate as an oil as the saturated solution cools. The
solvent must not react with the product and must be able to dissolve the product when hot,
but not dissolve much of it at 0°C. Frequently, mixtures of miscible solvents (e.g. ethanol
and water) are employed. If a solid is very soluble in hot ethanol and still rather soluble in
cold ethanol, most of the desired product will remain in solution on cooling and yields will be
low. If this solid is insoluble in water, an ethanol-water mixture can be found that has the
-3desired properties, e.g. high solute solubility when hot, low solubility when cold. Mixed
solvents are used in two ways: Either the mixture is made up before the crystallization then
used as if it were a pure solvent, or, more frequently, the solute is dissolved in the hot
solvent in which it is very soluble and then the second solvent is added, slowly, to the hot
solution (after hot filtration) to force the solute out of solution. This process requires skill;
check with your instructor the first time it is attempted.
2. Dissolving the solute: The solute is placed in an Erlenmeyer flask, boiling stones are
added and the MINIMUM amount of boiling solvent is added to dissolve the sample. The
solution is boiled and if all the solid does not dissolve, more solvent is added and the
solution boiled again. This procedure is continued carefully until all the solute is dissolved
or until no more remaining solid will dissolve.
(The remaining solid could be insoluble
impurities.) Use of too much solvent will cause the desired product to remain in solution on
cooling, rather than crystallizing. Use of too little solvent will cause premature crystallization
in the flask or in the filter paper during the hot gravity filtration. When all of the solute has
been dissolved, a few mL of extra solvent (maybe 10% of the solution volume) should be
added to compensate for solvent loss that occurs during subsequent heating/boiling.
3. Charcoal: If the solution (solvent + solute) is coloured by high molecular weight impurities,
activated charcoal may be added, as it tends to adsorb these impurities and thus remove
the colour. The charcoal is removed during the hot gravity filtration. Charcoal particles will
cause boiling solvents to foam, so remove the solution from the heat and when the boiling
stops (~20–30 seconds), add the charcoal (~0.2g). The solution should be boiled gently for
a minute or two before filtering.
4. Hot filtration: This procedure removes insoluble impurities from the solution. Hot gravity
filtration employs a fluted #4 (615) filter paper in a glass funnel. The funnel and flask are
preheated on the steam bath and are supported in or on the steam bath (in the bath for
large flasks or high boiling solvents, on the bath for small flasks or low boiling solvents)
during the filtration to avoid cooling the solution and thereby preventing premature
crystallization in the filter paper, funnel or flask. When the solvent is non-aqueous, it is
advisable to dry the glass funnel, with a towel, just before use. The filter paper should also
be kept as full as possible during the filtration by continually adding hot (boiling) solution.
This filtration is not done with suction as the vacuum will cause the hot solution, which is to
be retained, to boil vigorously which could cause loss of or contamination of the product.
5. Cooling: The hot filtered solution is cooled to cause the crystallization of the product. First
the solution is cooled slowly to room temperature and then chilled in an ice bath for 10–15
-4minutes. Allowing the solution to stand undisturbed, so that slow steady cooling occurs,
allows the formation of large crystals, while swirling the solution causes more rapid cooling,
resulting in smaller crystals.
6. Ice bath: For efficient cooling of a flask and its contents, an ice bath should contain enough
water to form a slurry of ice cold (0°C) water and ice around the flask. Using only ice results
in inefficient cooling as the flask is cooled only at the points of ice contact rather than over
the whole immersed surface.
7. Suction filtration: This is used to collect the purified crystals on a flat filter paper (#5 or
610, hardened) in a Buchner funnel placed on a thick walled suction flask that is connected
to a water aspirator. The water aspirator, when turned on full, achieves a partial vacuum
that pulls the solvent from the crystals quickly. Before pouring the cold crystals/solvent
mixture into the funnel, the filter paper, wetted with solvent (read note 8 and/or consult a
demonstrator), is sealed onto the funnel by turning on the vacuum. The crystals/solvent
mixture should be swirled and poured carefully, but quickly, into the funnel.
The "mother
liquor" may be used to rinse any remaining crystals out of the crystallizing flask and it may
be used to obtain a second crop of product so the suction flask must be clean. It is a good
practice to clean out the suction flask before each filtration.
If necessary, pressing the
crystals with a clean spatula will speed up removal of solvent.
8. Sealing the filter paper: Before the solution is filtered, the filter paper must be sealed onto
the funnel. The paper must be sealed by pouring some of the solvent through the paper,
placed in the funnel, and applying the suction so that the paper is sealed. If the solvent
used is water, the paper will easily be sealed, but if the solvent used is a more volatile
organic solvent, such as ethanol, problems can arise.
If the solvent used is ethanol, the
paper may not seal very easily because the ethanol may evaporate from the paper very
rapidly and the seal will be broken. If filtration is attempted after the seal has been broken,
the crystals will be sucked through the funnel and will be lost. To prevent this, one of the
following methods can be employed.
Either
Pour a little water through the funnel , apply the suction to seal the paper and then pour
some ethanol through to wash the water from the paper.
The filtration must then be
performed fairly rapidly, before the paper becomes unsealed. Also note that if all of the
water was not rinsed from the paper, the paper will likely become clogged with product as
the solution is filtered and the filtration will proceed slowly.
Or
-5Pour a little cold ethanol through the paper, apply the suction until all of the ethanol has
been pulled through the paper (and the paper is sealed) and immediately filter your crystals
before the paper can become unsealed.
9. Washing crystals: Washing is necessary to remove mother liquor from the solid product
after suction filtration.
Washing is best done with small volumes of fresh, ice-cold
crystallization solvent, but note that this will cause some loss of crystals as the product will
still have some solubility in the cold solvent. It is useless simply to pour solvent through the
crystals under suction as the solvent goes through so quickly that little is achieved. The
proper procedure is to break the vacuum either at the aspirator or by lifting the funnel, add
the ice-cold, fresh solvent to the crystals in the funnel, carefully swirl so the solvent mixes
with the crystals but does not disturb the filter paper, then reapply the vacuum to remove
the washing solvent. Consult an instructor before washing crystals.
NOTE: Never turn off the aspirator without breaking the vacuum or water may suck back
through the aspirator into the suction flask.
10. Product: The product crystals can be dried on the Buchner funnel by suction for 15–20
minutes if the solvent is volatile. If not, it is best to scrape the crystals onto a clean dry
watch glass and allow them to dry in your locker over the week. Note: Some solid products
will sublime if left open in this way, so if you are unsure, check with an instructor. When the
crystals are dry, weights and melting points can be determined.
-6-
RECRYSTALLIZATION OF TRIMYRISTIN
The crude trimyristin (from lab 1) should be in your locker in a 250 mL round bottom flask.
Before proceeding, make sure that you know the mass of the crude product from experiment one. Boil
a portion of absolute ethanol. To the crude sample, add hot absolute ethanol (about 10 - 15 mL)
and heat the solution to boiling (boiling stones!) using a steam bath.
If the crude lipid is not
completely dissolved, add more ethanol, a few mL at a time, until no more solid will dissolve. (The
amount of ethanol needed will vary, depending on the yield of crude material you have obtained, but
you should not need more than 15 mL). Remove from the steam, let stand for 20–30 seconds, then
slowly add 0.2 g activated charcoal. Boil the solution gently for 2 minutes on the steam bath. Heat your
short stemmed funnel on the steam bath while boiling the sample.
Filter the hot solution by gravity through a fluted filter paper into a clean dry Erlenmeyer flask
(placed on the steam bath to prevent premature cooling and crystallization during filtration).
The flask containing the filtered solution should be set aside to cool to room temperature. As the
solution cools, a white crystalline solid should form. When crystals are present you may cool the flask
quickly (e.g. in an ice bath) to complete the recrystallization. Chill a small sample of absolute ethanol
(10 - 15 mL in a sample vial) for use in the washing step.
Set up the apparatus for a suction filtration, using a clean, dry suction flask and a Buchner
funnel (#5, hardened filter paper). Before filtration, ensure that the filter paper is sealed using ethanol
(See notes 7 & 8 on page 22). Filter the cold solution by suction through the Buchner funnel. Use
some ice-cold absolute ethanol to transfer the remaining crystals out of the Erlenmeyer flask into the
funnel and wash them with a small volume of ice-cold ethanol. Use a small clean spatula to squeeze
excess solvent from the product and allow the aspirator to draw air through the solid for a few minutes.
This will have the effect of drying the crystals. Spread the crystals on to a small pre-weighed watch
glass and leave them in your locker for a week to complete drying.
At this point, proceed to PART 2 of this experiment.
After the product has been drying for a week, weigh the product and determine the melting
point of the sample. Crush a small sample of trimyristin on a watch glass and place it into a melting
point capillary. You should look up the expected melting point to help determine the appropriate power level.
Place the capillary in the melting point apparatus and determine the melting range for the sample and
add this to the raw data for lab #2. Compare the experimental melting point to the literature value.
Comment on the purity of the sample. After the melting point has been measured, transfer the purified
trimyristin to sample vial, label it and store it in your locker until it is needed in lab period 10 and 11.
-7-
PART 2:
MELTING POINTS
THEORY
The melting point is the temperature at which the solid and liquid phases of a compound are in
equilibrium. The term "melting point" should be more appropriately be referred to as a "melting range"
because even for very pure substances, there is a small temperature range between the start and
completion of melting. Pure substances will melt over a narrow range (1 - 3 °C). Impure substances
tend to melt at a lower temperature and over a broader range. Both the size of the range and the
actual melting point are thus useful indicators of the purity of a compound.
PROCEDURE
Before coming to the lab, look up the expected melting points for trans cinnamic acid and for
urea and read appendix 1 for details on the procedure for determining a melting point.
Crush a small sample of trans cinnamic acid on a watch glass and place it into a melting point
capillary. Repeat this procedure for a small sample of urea. Combine a sample of your crushed
cinnamic acid with a sample of crushed urea on a watch glass and thoroughly mix them together.
Place this sample into a third capillary. Place all three capillaries in the melting point apparatus and
determine the melting ranges for the three samples. You can melt all three samples during one trial,
since the melting point device can accommodate up to three samples at one time.
In the Discussion section, compare the experimental melting points to the literature values and
comment on the purity of the samples.
REPORT
In the “purpose section”, explain (briefly) how the recrystallization procedure removes impurities
from the sample. Your explanation should explain how impurities are removed by each of the following
steps: the use of charcoal, the hot gravity filtration and the cold suction filtration.
Calculate the percentage recovery for the recrystallized product and comment on sources of error.
Comment on the melting points of the recrystallized sample.
-8-
QUESTIONS (FALL 2010)
1. A sample of benzoic acid contaminated with borax and sand is to be recrystallized.
Compound
Solubility in water @ 100°C
Solubility in water @ 0°C
in g / 100 mL
in g / 100 mL
Benzoic acid
6.8
0.2
Borax
170.0
2.0
Sand
insoluble
insoluble
The sample initially contained 3.0 g of benzoic acid, 1.5 g of Borax and 1.0 g of sand and
was treated with 50 mL of boiling water, filtered by gravity and then cooled in ice to obtain a
precipitate which was isolated by suction filtration. How much benzoic acid should be
formed and will it be pure (in theory)? Use calculations to aid in your explanation.
2. After the suction filtration, the filtrate in the suction flask contains (among other things)
some dissolved product, which had failed to crystallize. Could you process this filtrate to
obtain a sample of the product? Explain. Would the second crop of crystals be as pure as
the first crop? Explain.
3. Why is 2-methyl-2- propanol rarely used as a solvent for recrystallization? (Look up some
of it's physical properties such as boiling point, melting point)
QUESTIONS (WINTER 2011)
1. A sample of benzoic acid contaminated with NaCl and sand is to be recrystallized.
Compound
Solubility in water @ 100°C
Solubility in water @ 0°C
in g / 100 mL
in g / 100 mL
Benzoic acid
6.8
0.2
NaCl
39.1
35.7
Sand
insoluble
insoluble
The sample initially contained 3.0 g of benzoic acid, 2.0 g of NaCl and 1.0 g of sand and
was treated with 50 mL of boiling water, filtered by gravity and then cooled in ice to obtain a
precipitate which was isolated by suction filtration. How much benzoic acid should be
formed and will it be pure (in theory)? Use calculations to aid in your explanation.
-92. You are given a mixture of solids A,B and C. In the solvent you are using, compound A is
slightly more soluble than compound B, while C is essentially insoluble. Could you use the
technique of recrystallization to obtain a sample of pure A? Explain.
3. During the suction filtration, the recrystallized product is washed with ice cold ethanol.
Explain why this “washing solution” must be ice-cold.
Since the recrystallized sample is not soluble in water, why not use ice-cold water as the
“washing solvent”?