CHE205– General Chemistry Lab week #6
Crystallization
Objective statement: To purify the antibiotic sulfanilamide away from acetanilide. To learn the purification technique of
crystallization.
Prelab Assignment
1. For Lab #4 you need to write a mini prelab
In addition to the full prelab perform the following prelab calculations and answer the following questions
in your lab notebook:
1. Calculate how much 95% ethanol will be required to dissolve 0.3 g of sulfanilamide at 78 °C. Use data from the
background information in this document to perform this calculation.
*note: to dissolve the sulfanilamide in the minimum of hot solvent, you must keep the mixture at or near the boiling
point of 95% ethanol during the entire procedure, how do you think that will change the amount of ethanol you will
need for your procedure?
2. Using the volume of solvent calculated in step 1, calculate how much sulfanilamide will remain dissolved in the
mother liquor after the mixture is cooled to 0 °C
Background Information
In most organic experiments, the desired product is first isolated in an impure form. If this product is a solid, the most
common method of purification is by crystallization. The general technique involves dissolving the material to be
crystallized in the minimum amount of hot solvent and cooling the solution slowly. The dissolved material has a
decreased solubility at lower temperatures and will separate from the solution as it is cooled. This phenomenon is called
crystallization if the crystal growth is relatively slow and selective, or precipitation if the process is rapid and
nonselective. Because the impurities are usually present in much smaller amounts than the compound being
crystallized, most of the impurities will remain dissolved in the solvent even when it is cooled. The purified substance
can then be separated from the solvent and impurities by filtration.
In this experiment, you will use the technique of crystallization to purify the antibiotic sulfanilamide away from the
impurity acetanilide; you will use ethanol as the solvent.
Sulfanilamide is a sulfonamide antibiotic originally synthesized in 1908. It was credited with saving many lives during
WWII. While not used for systemic infections anymore, sulfanilamide is still commonly used in the treatment of yeast
infections.
“Sulfanilamide is an organic sulfur compound structurally similar to p-aminobenzoic acid
(PABA) with antibacterial property. Sulfanilamide competes with PABA for the bacterial
enzyme dihydropteroate synthase, thereby preventing the incorporation of PABA into
dihydrofolic acid, the immediate precursor of folic acid. This leads to an inhibition of
bacterial folic acid synthesis and de novo synthesis of purines and pyrimidines, ultimately
resulting in cell growth arrest and cell death.”
-from pubchem entry on sulfanilamide
There are a number of synthetic routes that pharmaceutical companies and research scientists use to produce
sulfanilamide. One of them is shown in figure 1. The starting material for this synthesis is acetanilide and upon
completion of the synthetic route, the sulfanilamide would need to be purified away from the leftover starting materials,
intermediates and side products.
It is instructive to look at the structure of sulfanilamide to compare the relative polarities of the compound with any
possible solvents. The benzene in sulfanilamide ring is non polar and the amine and sulfonate groups will lend some
polarity to the molecule. Therefore an intermediately polar solvent would likely be a good choice for this molecule.
Protocol adapted from Experiment #3; Crystallization. Introduction to Organic Laboratory Techniques 3rd edition by
Pavia, Lampaman, Kriz and Engel.
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CHE205– General Chemistry Lab week #6
Crystallization
While this analysis will in part
help a chemist determine
possible crystallization
solvents, experimental data
alone will allow one to
determine if a particular
solvent would act as an
effective crystallization
solvent. A solubility curve is
an analysis of a solute’s
solubility at various
temperatures in a given
solvent. A good
crystallization solvent is one
where the solute is highly
soluble at high temperature
and poorly soluble at low
temperatures.
The solubility profile of
sulfanilamide in 95% ethanol
has been thoroughly
characterized and is reported
in-part in table 1. While
sulfanilamide is only soluble
in ethanol at high
temperatures, acetanilide is
very soluble in ethanol even
Figure 1: A synthetic route for the production fo sulfanilamide. Figure obtained from:
at low temperatures. These
http://www.organicchem.org/oc2web/lab/exp/sulfa/exp4des.html
differing properties will allow
ethanol to serve nicely as a crystallization solvent as we purify sulfanilamide away from acetanilide.
Table 1: Solubility of Sulfanilamide in Ethanol as a function of solvent temperature
Temperature
0
20
40
60
78
Solubility (mg/mL)
14
24
46
88
210
Waste Disposal
Please place the mother liquor in the non-halogenated organic waste container at the completion of the lab and the
purified sulfanalimide into the labeled beaker in the hood.
Procedure
Preparing the solvent and sample
1. Weigh 0.30 g of impure sulfanilamide and transfer this solid to a 10-mL Erlenmeyer flask. To a second
Erlenmeyer flask, add about 10 mL of 95% ethyl alcohol.
2. Heat the solvent on a warm hot plat until it is boiling. Because 95% ethanol boils at a relatively low temperature,
it evaporates quite rapidly. Setting the temperature of the hot plate too high will result in too much loss of
solvent through evaporation.
Dissolving the Sulfanilamide
Protocol adapted from Experiment #3; Crystallization. Introduction to Organic Laboratory Techniques 3rd edition by
Pavia, Lampaman, Kriz and Engel.
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CHE205– General Chemistry Lab week #6
Crystallization
3. Before heating the flask containing the sulfanilamide, add enough hot solvent to the solid sample with a Pasteur
pipet to barely cover the crystals. Then heat the flask containing the sulfanilamide until the solvent is boiling.
Note: at first this may be difficult to see, because so little solvent is present.
4. Add another small portion of solvent (several drop or about 0.25 mL), continue to heat the flask and swirl the
flask frequently. You may swirl the flask while it is on the hot plate, or for more vigorous swirling, remove it rom
the hot plate for a few seconds while you swirl it.
5. Swirl for 10-15 seconds, then check to see if the sulfanilamide has dissolved. Continue the process of adding
small amounts of solvent, heating and swirling until all of the solid has dissolved.
6. Note that it is important to add enough solvent to dissolve the solid and not any extra. If you add too much extra
solvent, you will have a large loss of your sample
7. Because ethanol is volatile, you need to perform this entire process rapidly. Otherwise you may lose solvent
nearly as quickly as you are adding it and then the procedure will take a very long time. This whole process
should take 10-15 minutes.
Crystallization
8. Once the sulfanilamide has dissolved, remove the flask from the heat and allow the solution to cool slowly.
Cover the flask with a small watch glass or stopper the flask.
9. Crystallization should begin by the time the flask has cooled to room temperature. If it has not, scratch the
inside of the surface of the flask with a glass rod to induce crystallization (nucleation).
10. When it appears that no further crystallization is occurring at room temperature, place the flask in an ice-water
bath using a beaker. Be sure that both water and ice are present and that the beaker is small enough to prevent
the flask from tipping over.
Filtration
11. When crystallization is complete, vacuum filter the crystals using a Hirsh funnel.
12. Moisten the filter paper with a few drops of 95% ethanol and turn on the vacuum to the fullest extent.
13. Swirl the mixture in the flask and pour some of the mixture into the funnel, attempting to transfer both crystals
and solvent.
14. You need to pour the mixture quickly, before the crystals have completely resettled on the bottom of the flask
15. When the liquid has passed through the filter, repeat this procedure until you have poured all the liquid into the
Hirsch funnel.
16. There will likely be some crystals remaining in the bottom of the flask. Using your spatula, scrape out as many
of the crystals as possible from the flask.
17. Add about 1 mL ice-cold 95% ethanol (measured with a calibrated pasture pipette) to the Erlenmeyer flask, swirl
and pour onto the crystals in the funnel. Perform 2 more times even if you can’t observe crystals in the flask-this
serves the dual purpose of increasing your yield and cleaning the crystals.
18. Continue drawing air through the crystals on the Hirsch funnel by suction for about five minutes
19. Transfer the crystals onto a pre weighted watch glass for air-drying
20. Separate the crystals as much as possible with a spatula
21. The crystals should be completely dried within 10-15 minutes.
22. You can usually observe if the crystals are still wet whether or not they stick to a spatula or stay together in a
clump. Weigh the dry crystals and calculate percent recovery.
Special notes
Do note heat the crude sulfanilamide until you have added some solvent. The solid could melt and form an oil
which may be difficult to crystallize
When you are dissolving the solid, the hot solvent should be added in small portions with swirling and heating.
Use your observations to determine how much solvent you should add
One of the most common mistakes in this lab is to add too much solvent. This can occur if the solvent is not hot
enough or if the mixture is not stirred sufficiently. If this occurs, you can evaporate some solvent by heating, but
careful not to evaporate all the solvent and heat the solid alone
Sulfanilamide should crystallize as large, beautiful needles. However, this will not always happen, if the crystals
form too rapidly or if there is not enough solvent, they will tend to be smaller, perhaps even appearing as a
powder. Different compounds have different crystal lattice structures and will not have the same appearance as
sulfonamide
Protocol adapted from Experiment #3; Crystallization. Introduction to Organic Laboratory Techniques 3rd edition by
Pavia, Lampaman, Kriz and Engel.
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CHE205– General Chemistry Lab week #6
Crystallization
When the solvent is water or when the crystals form as a power, it will be necessary to dry the crystals longer
than 10-15 minutes. In that case, overnight drying could be required.
Lab Report
Write a formal lab report
In your results section, report the mass of your initial and final sulfanalimide and your observations of the color and
macro-scale structure of the sulfanalimide crystals. Calculate your percent recovery.
In your discussion, report on the difference between your percent recovery and the percent of sulfanalimide in the initial
sample. Where could you have lost sulfanalimide?
Protocol adapted from Experiment #3; Crystallization. Introduction to Organic Laboratory Techniques 3rd edition by
Pavia, Lampaman, Kriz and Engel.
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