CHM 226
Recrystallization of Acetanilide
Recrystallization is an important method used by chemists to purify solid compounds. Why is
purification so important? Well, from a real world perspective, when a chemist conducts a
chemical reaction as in (Scheme 1), it will rarely go to 100% completion, and will frequently
produce byproducts. These byproducts and some of the unreacted starting materials will be
present in the end as impurities. Chemists need ways to remove these unwanted impurities so
they can isolate their desired product in its pure form.
O
NH2
+
Aniline
H3C
H
N
O
O
+
CH3
Acetic Anhydride
O
CH3
O
Acetanilide
Scheme 1- Synthesis of Acetanilide
HO
CH3
Acetic Acid
Today you will be given an impure acetanilide sample and will be expected to purify the
acetanilide through recrystallization.
Recrystallization is just one of the many methods used to purify compounds. A few other
methods include chromatography and distillation, which you will learn later in the semester.
Purification is a huge part of organic synthesis, and an important technique to master during lab.
Recrystallization takes advantage of the fact that a given compound’s solubility increases with
temperature. If a crystalline compound is fairly insoluble in cold solvent, the solvent could be
heated, causing it to dissolve. If this hot solution is then cooled, the compound will gradually
become insoluble and will recrystallize.
Crystals that have been recrystallized tend to have fewer impurities trapped in them. There are
generally two reasons why impurities are excluded during recrystallization. First is that
impurities usually have a different solubility and will either not dissolve in the boiling solvent
and be filtered out, or they may stay dissolved in the cold solvent during recrystallization.
Second, they are excluded from the crystal growth because crystals often form by ordered
packing as shown below (Figure 1A), making a highly stable crystal structure. Crystals
incorporate an impurity while forming, would be less stable and are, consequently, less likely to
form (Figure 1B).
A
B
Figure 1- Crystal Structures: Contrast between a pure
crystal “A” and an impure crystal “B” with trapped impurities.
CHM 226
Recrystallization of Acetanilide
In order to make recrystallization occur, a few guidelines must be followed when choosing a
solvent to recrystallize with.
1. Generally like dissolves like. Polar solvents tend to dissolve polar compounds, etc.
2. A solvent (or mixture of solvents) should not dissolve the compound you are purifying at
room temperature. If the compound dissolves at room temperature, your compound will
not recrystallize back out of the solvent at room temperature.
3. A solvent should dissolve the compound you are purifying at the solvent’s boiling point.
If the compound does not go into solution when boiling, you will not be able to
recrystallize.
4. The boiling point of the solvent must be lower than the melting point of the compound
you are purifying. If boiling point of the solvent is greater than the melting point of the
compound, the compound will melt into an oil (oil out) and will likely not be miscible
with the solvent.
5. A solvent should at lower temperatures release a lot of crystals from solution.
Once a successful recrystallization has been carried out, there should be lots of crystals sitting in
a cold solvent. The last step is to isolate the purified crystals by using vacuum filtration.
In this experiment, you will have to choose the best recrystallizing solvent based on the above
guidelines. After recrystallizing your compound, you will vacuum filter the purified crystals, and
then calculate % recovery. .
A melting point of the recrystallized compound will be taken in order to determine the purity. A
melting point is taken by loading a small amount of solid crystals into a tiny hollow glass tube,
called a capillary. The capillary is then placed inside a melting point apparatus, which contains a
heat source and a magnifying glass on top for viewing. As the apparatus starts to heat, you can
view the crystals to see when they start to liquefy. A pure compound will exhibit a very sharp
melting point with a range spanning only a few degrees Celsius; however, if there are impurities
present, the impurities will cause the compound to start melting prematurely at a lower
temperature, and will often cause the range of melting to be broad (greater than 5 degrees
Celsius).
Purifying Acetanilide by Recrystallization
Reference: Wigal, C. T. Signature Labs Series: Chemistry Lab Experiments CHEM 226;
Jefferes, J., Ed.; Cengage Learning: Mason, OH, 2008; p 29-36
Data Table: Have a copy of this in your notebook prior to the beginning of lab. Your data table
should contain columns for the weight of your crude acetanilide, the weight of your filter paper,
the weight of your purified acetanilide + filter paper, the weight of pure acetanilide, and %
recovery, and the literature melting point of acetanilide.
CHM 226
Recrystallization of Acetanilide
Equipment:
50mL beaker
100mL beaker w/
boiling solvent
Sand Bath
Hot Plate
Figure 2- Recrystallization Apparatus
Eyepiece
Power Switch
Temperature
Readout
Stop Button
Set Button
Start Button
Figure 3- Melting Point Apparatus
CHM 226
Recrystallization of Acetanilide
Procedure:
To start, you will have four different recrystallizing solvents to choose from water, acetone,
ethanol, and petroleum ether. You will want to first test all four of these solvents to ascertain
which solvent is most suitable for your recrystallization. To do this, take four test tubes and label
them acetone, water, ethanol, and pet ether. Pipette about 2 mL of each solvent in the
corresponding test tubes and add a small scoop of acetanilide to each with your spatula.
Stir the solutions by shaking the tubes and record in your notebook for each solvent whether the
acetanilide is soluble or insoluble. Next take each test tube that the acetanilide did not dissolve in
and heat the test tubes by placing them in a sand bath, and turning the sand bath on high heat.
When the solvents start to boil, stir the solutions and record in your notebook whether the
acetanilide was soluble or insoluble. Let the test tubes cool to room temperature.
Finally, make an ice bath, by putting ice in a 250 mL beaker and filling the beaker up with water.
Place the tests tubes in the ice bath. Let them sit for 5 minutes and write in your notebook
whether recrystallization occurs.
Based on these quick tests select the solvent you will use to recrystallize acetanilide and record it
in your notebook. Check with your TA to make sure you select the right solvent.
Next, you can start your recrystallization! Weigh out 500 mg of crude acetanilide into a 50 mL
beaker. Record the exact weight in your notebook.
You should warm your sand bath again by turning the hot plate on medium-high. Measure out
20-30 mL of your solvent into a 100 mL beaker and place it on your hot plate. Once the solvent
starts boiling, pipette some of the hot solvent into the 50 mL beaker with your crude acetanilide
in it. The solvent cools down slightly while you are pipetting, so let it warm up to boiling again
by swirling the beaker gently in the warm sand. (Figure 2) DO NOT leave the beaker sitting in
the sand, or the acetanilide will oil out. Keep adding a pipette of hot solvent and warming in the
sand to boiling until all of the crystals have dissolved.
Next, remove the beaker of dissolved crystals from the heat and let it cool. Once it reaches room
temperature, cool it further by placing it in your ice bath for 5-10 minutes. White crystals should
start to fall out of solution as it cools, if no crystals fall out try scratching the bottom of the flask
with a glass stirring rod or metal spatula. While it is cooling, measure out 2 mL of your
recrystallizing solvent into a test tube and cool it in your ice bath. This will be used to rinse your
product when filtering.
Set up your vacuum filtration apparatus using a 125 mL filter flask, a Hirsch funnel, and a filter
paper. Weigh the filter paper and record it in your lab notebook.
Turn the vacuum pump on and filter the purified acetanlide. Use your metal spatula to scrape the
crystals out of the 50 mL beaker into the Hirsch funnel, and the cold solvent to rinse any
CHM 226
Recrystallization of Acetanilide
remaining crystals out of the beaker. Let the vacuum pump run for 10-15 minutes. Once the
crystals are dry on the filter paper, carefully weigh the filter paper and pure acetanilide.
Finally before leaving lab, you will need to obtain a melting point.
Taking a Melting Point (See Figure 3)
Start by spreading your crystals on a watch glass. Take a melting point capillary, and tap the
open end down into your crystals to load a few crystals inside the capillary. Turn the capillary
upside down and lightly tap it on the bench top to load the crystals into the bottom of the
capillary. Keep repeating this until 1-2 mm of crystals are in the bottom of your capillary.
Plug the melting point apparatus in. Turn the power on by pressing the switch on the left hand
side of the instrument.
If you do not know the literature melting point of your compound, start here.
Insert your sample and start by setting your plateau very high ~150C, you set the plateau by
holding down on the “Set” button and adjusting the temperature. Once you have set the plateau
temperature, insert your sample and press start. The instrument will rapidly heat up to the plateau
temperature. YOU MUST WATCH YOUR SAMPLE CLOSELY THE WHOLE TIME IT IS
HEATING, or you might miss the approximate melting point.
When your sample starts to liquefy look at what temperature the instrument is at, and record this
as your “orientation melting point”, just so you know the approximate range of melting during
rapid heating. Press the “Stop” button to let the instrument cool down again. To fine tune this
melting point you will use a second melting point capillary with your sample to slowly check the
melting point. Continue in the next step.
If your know the literature melting point of your sample or the “orientation melting point,”
start here.
Set the plateau temperature at 15 C below the literature or “orientation” melting point by holding
the “Set” button. Insert your sample in the instrument and press “Start”. Let the instrument heat
up until the plateau light comes one. Then press “Start” again and the ramping light will come
on. This will slowly ramp the temperature. Keep an eye on your sample and record the exact
range your compound melts.
Once your compound has finished melting press the “Stop” button, and then turn the instrument
off by flipping the power button.
CHM 226
Recrystallization of Acetanilide
Pre-lab Questions
1.) Let’s say you wanted to recrystallize compound “A”. Compound “A” is a semi-polar
compound with a melting point of 87 oC. There are four possible recrystallizing solvents
to choose from: acetone, water, dimethylsulfoxide, and hexane. Based on all of the
information given and the following flow chart, what ONE solvent would be best suited
for the recrystallization of “A”. Give a short explanation of why that solvent will work.
Impure Compound “A”
Room Temperature Solvent
Soluble in:
DMSO
Insoluble in:
DMSO, Hexane, Water
Boiling Solvent
Soluble in:
Acetone & Water
Insoluble in:
Hexane
2.) Compounds, even though they do not look visibly soluble, can still be slightly soluble in
solvents at room temperature. If acetanilide’s solubility is 4mg/L in your solvent at room
temperature how much acetanilide would you expect to lose if you use 9 mL of solvent to
dissolve 0.450 g of acetanilide, and rinse with 3 mL of solvent during filtration?
3.) Why is it bad to use too much solvent when dissolving the compound initially? Why is it
important to use ice-cold solvent when rinsing recrystallized product during filtration?
Post-lab Questions
1.) Why does an impure compound start melting at a lower temperature?
2.) Show your calculation for % recovery. Is this percentage what you were expecting?
Why?