Chem 2LA3
Analgesic Drug Preparation
Experiment 2. Analysis of an Analgesic Drug Preparation
* adapted from J.W. Lehman, “Multiscale Operational Organic Chemistry”, Prentice-Hall (2002), pp. 28-39 *
Techniques
Gravity filtration [TCD-16]
Extraction [TCD-18]
Recrystallization [TCD-24]
Melting point [TCD-33]
Vacuum filtration [TCD-17]
Evaporation [TCD-19]
Heating [TCD-9]
Weighing [TCD-4/5]
Drying solids [TCD-22]
Scenario. A client has submitted a sample of two commercial analgesic drug preparations
called “Aspophen I” and “Aspophen II” to our laboratories for testing. The labels indicate that the
two formulations each consist of the same three components, but in different relative amounts.
Each tablet of Aspophen I is supposed to contain aspirin (200 mg), acetaminophen (250 mg)
and sucrose (50 mg), while each tablet of Aspophen II is supposed to contain aspirin (300 mg),
acetaminophen (150 mg), and sucrose (50 mg). Our client believes that both Aspophen I and II
do indeed contain aspirin and sucrose, but suspects that the third component is not
acetaminophen, but rather a cheap chemical relative of acetaminophen, either acetanilide or
phenacetin. They further suspect that the amounts of each component listed on the labels are
incorrect, and furthermore that the identity of the third component may be different in the two
formulations. Both acetanilide and phenacetin are banned in Canada because of their toxicity,
so if either of them are present in these formulations then they need to be removed from the
market as soon as possible.
OCOCH3
COOH
NHCOCH3
NHCOCH3
NHCOCH3
HO
HO
OH
aspirin
(acetylsalicylic acid)
H OH
acetanilide
HO
H OH
H
O
OCH2CH3
acetaminophen
phenacetin
sucrose
H OH
OH
O
CH2OH
CH2OH
Figure 1
You are to analyze one of the two drug preparations to determine the approximate percentages
of aspirin, sucrose, and the unknown component it contains, and determine the identity of the
unknown component.
Experiment # 2
Page 1 of 7
Chem 2LA3
Analgesic Drug Preparation
Suggested Procedure. The three components of the mixture should be separable by taking
advantage of solubility differences and the different acid-base properties of the various possible
structures. Sucrose, a disaccharide, is just common table sugar and is the only compound in
the mixture that is soluble in water and insoluble in organic solvents. Since the four analgesic
compounds are all soluble in organic solvents such as dichloromethane, we should be able to
isolate the sucrose by gravity filtration, after first mixing the preparation with dichloromethane
to dissolve the aspirin and the unknown component. The aspirin can then be isolated from the
dichloromethane solution by extraction with an aqueous solution of sodium hydroxide. Aspirin
is a carboxylic acid, so treating it with base will convert it to its sodium salt, which is soluble in
water. The unknown component, if it is acetanilide or phenacetin, will remain behind in the
dichloromethane. After separating the aqueous phase from the organic phase, the aspirin can
be precipitated from the aqueous solution by acidifying it with hydrochloric acid, and then
isolated by vacuum filtration. The unknown can then be isolated from the remaining
dichloromethane solution by simply evaporating the solvent.
The percentage composition of the mixture can be determined from the masses of the individual
components, after drying them. Note that the actual composition may not be the same as that
given in the scenario. Careful work is required to obtain accurate results; errors can arise from
incomplete mixing with dichloromethane, incomplete extraction or precipitation of the aspirin,
incomplete drying of the recovered components, losses in transferring substances from one
container to another, or errors in weighing.
The unknown component will be purified by recrystallization and then analyzed to establish its
identity and degree of purity. Recrystallization is the most commonly used method for purification
of solids; others include chromatography and sublimation, which you will experience in later
experiments. Liquids are usually purified by distillation or chromatography. The solubility
information given in the Table below indicates that while acetanilide and phenacetin are both
insoluble in cold water, they are both relatively soluble in boiling water. This means that it should
be possible to purify both compounds by recrystallization using water as the solvent.
Table 1 Physical properties of acetanilide and phenacetin
M.W.
m.p.
Acetanilide
135.2
o
Solubility, cold water
Solubility, hot water
114 C
0.54
5.0
Phenacetin
179.2
135 oC
0.076
1.22
a. In grams of solute per 100 mL solvent
Based on the mass of unknown that you recover from the extraction procedure, you can estimate
the volume of boiling water needed to dissolve it depending on whether it is acetanilide or
phenacetin. For example, if the unknown is acetanilide and you recovered 1.50 g of the crude
solid from the extraction, then the amount of boiling water needed to dissolve it is calculable from
the solubility in boiling water (usually given in g / 100 mL) and the mass of the sample:
Volume water needed = 1.50 g acetanilide x (100 mL water / 5.0 g acetanilide) = 30 mL
Phenacetin is less soluble in boiling water than acetanilide, and will hence require more water to
dissolve the same amount. You should begin the recrystallization process using the smaller
volume of water (don’t add it all at once, and add more only if your compound doesn’t dissolve in
that amount of water at its boiling point.
Experiment # 2
Page 2 of 7
Chem 2LA3
Analgesic Drug Preparation
Because the properties of both acetanilide and phenacetin are already reported in the chemical
literature, we can identify the unknown and assess its purity by its melting point range. If your
unknown is acetanilide then it should melt sharply near 114 oC, while if it is phenacetin it should
do so near 135 oC.
Pre-lab
-
Purpose of the experiment
Brief outline of procedure to be used
table of physical properties of aspirin, acetanilide, phenacetin, and sucrose (e.g. see
Table above)
table of physical properties of solvents to be used (M.W., density, boiling point)
flow chart illustrating the extraction procedure to be followed (see Appendix A)
Directions.
The experiment will be completed over two lab periods, with all operations except melting point
determinations done in the first period. This is to allow time for adequate drying of solids prior to
determining their melting points.
(a) Separation of sucrose. Accurately weigh about 3.00 g of Aspophen I or II on a creased
piece of weighing paper and transfer it to a clean, dry 125-mL Erlenmeyer flask. Add 50 mL of
dichloromethane to the flask. Stir the mixture thoroughly with a stirring rod to dissolve as much of
the solid as possible, using the rod to break up any lumps or granules that may be present.
Using a pre-weighed fluted filter paper, filter the solution by gravity into a small flask, saving the
filtrate for the next step. It is crucial that the contents of the flask be poured quickly into the filter
paper; if it is done slowly, the solvent will evaporate and cause the dissolved solid to crystallize in
the filter paper and funnel. This will slow the filtration considerably, and lead (among other
things) to poor recovery. Ask a TA for advice if you think your filtration is going too slowly.
Rinse the original flask with about 10 mL of fresh dichloromethane, and use this solution to rinse
the filter paper and funnel, collecting it in the filter flask along with the bulk of the filtrate. After
repeating the rinsing procedure a second time, set the filter paper aside in a small beaker, taking
care not to lose any of the sucrose, and reweigh it when it is completely dry. Record the mass
of the sucrose in your laboratory notebook. After you have shown your sample of recovered
sucrose to a TA, it should be placed in the jar labeled “Sucrose” that is located in the waste hood.
(b) Separation of Aspirin. Transfer the filtrate to a separatory funnel and extract it with two
separate 25-mL portions of aqueous 1 M sodium hydroxide. Because the density of
dichloromethane is greater than that of water, the organic layer will be on the bottom, and you
will thus have to transfer each layer to a different (labeled) container – return the
dichloromethane layer to the separatory funnel before the second extraction. Combine the two
aqueous extracts in the same container and save the dichloromethane layer, which should now
contain only the unknown component, for the next step.
Slowly add 10 mL of 6 M hydrochloric acid to the combined aqueous extracts while stirring with
a glass rod. Test the pH of the solution with pH paper (to do this, use your stirring rod to transfer
a drop of the solution to a strip of pH paper – do not dip the pH paper in the solution), and if
necessary add more acid to bring the pH down to 2 or lower. Cool the mixture in an ice/water
bath for at least 10 minutes, collect the aspirin by vacuum filtration, and wash it on the filter with
cold water. Let the aspirin dry on the filter for a few minutes with the aspirator running, then dry
it to constant mass. Weigh the aspirin and record its mass in your lab notebook. After you have
Experiment # 2
Page 3 of 7
Chem 2LA3
Analgesic Drug Preparation
shown your sample of recovered aspirin to a TA, it should be placed in the jar labeled “Aspirin”
that is located in the waste hood.
(c) Isolation of the Unknown Component. Dry the dichloromethane solution containing the
unknown from (b) over anhydrous sodium sulfate, gravity-filter it into a clean, dry 100-mL roundbottom flask, and use the rotary evaporator to evaporate the solvent. Transfer the solid that
remains to a pre-weighed vial and let it dry to constant mass.
(d) Compositional Analysis. Calculate your percent recovery by dividing the sum of the
masses of all components by the mass of Aspophen that you started with. Calculate the
approximate percentage composition of Aspophen, based on the total mass of components
recovered (these percentages should add up to 100%).
(e) Purification of the Unknown Component. Recrystallize the unknown component by
boiling it with just enough water to dissolve it completely, then letting it cool slowly to room
temperature. Use an Erlenmeyer flask large enough to accommodate the maximum amount of
recrystallization solvent that might be required. If necessary, induce crystallization by scratching
the sides of the flask with a glass stirring rod (consult a TA for advice on how to do this, if
necessary). Then cool the flask further in an ice/water bath to increase the yield of product.
Collect the solid by vacuum filtration, washing it with a small amount of ice-cold water. Dry the
product to constant mass, transfer it to a pre-weighed vial, and determine the amount recovered
from the recrystallization.
(f) Analysis & Identification of the Unknown Component. Grind a small amount of the
purified, dry compound to a fine powder on a watch glass using a spatula or a flat-bottomed
stirring rod. Measure the melting point range of the purified unknown, recording the temperature
at which you see the first trace of liquid and the temperature at which the sample becomes
completely liquid. Carry out at least two measurements with the sample. Turn in the remaining
product to your TA.
Waste Disposal. Place recovered dichloromethane in the “Halogenated Waste” container.
Conclusions.
These should begin with the statement…
“Analysis of Aspophen I or II indicates it to consist of:
-
sucrose (x% by weight)
acetylsalicylic acid (y% by weight)
identity of unknown (z% by weight)”
Indicate the percent recovery of material from the extraction procedure, and give your opinions
on the most likely reasons for material loss. Also indicate the most likely sources of other errors
in the compositional analysis.
Experiment # 2
Page 4 of 7
Chem 2LA3
Analgesic Drug Preparation
Appendix A. Planning and executing extraction procedures
Planning and keeping track of an extraction procedure always takes some thought, and it’s easy
to get things mixed up. Even seasoned veterans find it useful to follow two basic rules in
planning and executing an extraction procedure: (1) organize yourself before beginning by
drawing up a flow chart outlining the procedure to be followed; and (2) never throw anything
away until you’re sure it is really waste.
A flow chart illustrating the separation of the analgesic drug mixture of this experiment by solvent
extraction should look something like this:
ASA
+ acetanilide/phenacetin
+ sucrose
(solid mixture)
1. dichloromethane
2. filter
organic phase
solid phase
sucrose
(solid)
ASA
+ acetanilide/phenacetin
in
CH2Cl2
1M NaOH
(aqueous)
aq. phase
(UPPER)
org. phase
(LOWER)
acetanlide/phenacetin
in
CH2Cl2
OCOCH3
CO2-Na+
dry and
evaporate
sodium acetylsalicylate
in
water
acetanilide
or
phenacetin
(solid)
1. 6M HCl
2. filter
acetylsalicylic acid
(solid)
Figure 2
Experiment # 2
Page 5 of 7
Chem 2LA3
Analgesic Drug Preparation
Note that in this example, there is only one fraction for which rule (2) above would apply: the
(aqueous) filtrate from the last step. Throw it away only once you’re satisfied that you’ve
recovered more or less the expected amount of solid (ASA) in the filtration, and that the solid
recovered is indeed what you expect it to be. In the meantime, keep it in a loosely stoppered,
labeled Erlenmeyer flask in your fume hood.
A somewhat more complicated example is given by the separation of a mixture of 4-chloroaniline
(a solid amine; soluble in aq. acid & organic solvents), benzoic acid (a solid carboxylic acid;
soluble in aq. base & organic solvents), and toluene (a neutral liquid hydrocarbon; soluble in
organic solvents). This mixture is a viscous liquid at room temperature; none of the components
are soluble in neutral water, and all of them are soluble in organic solvents.
We again begin the procedure by taking up the mixture in an organic solvent, this time diethyl
ether (d25C = 0.706 g/mL; bp = 34.6 oC).
p-chloroaniline
+ benzoic acid
+ toluene
in
ether
2M HCl
(aqueous)
org. phase
(UPPER)
benzoic acid
+ toluene
in
ether
Cl
toluene
in
ether
dry
and
distill
toluene
(liquid)
NH3 Cl
p-chloroanilinium chloride
in
water
5% NaHCO3
(aqueous)
org. phase
(UPPER)
aq. phase
aq. phase
CO2-Na+
1. 10% NaOH
2. filter
p-chloroaniline
(solid)
sodium benzoate
in
water
1. 6M HCl
2. filter
benzoic acid
(solid)
Figure 3
Note that in this example there are two fractions for which rule (2) above applies.
Experiment # 2
Page 6 of 7
Chem 2LA3
Analgesic Drug Preparation
Experiment # 2
Page 7 of 7