Lab 2: Purification of Organic Compounds
Reading
Part I: The Recrystallization Pre-Lab Video on the orgolabs website. Zubrick Chapters
12 (The Melting Point Experiment; we use the ‘Mel-Temp’ variety) and 13
(Recrystallization). Appendix 5 of this manual.
Part II: The Distillation Pre-Lab Video on the orgolabs website. Zubrick Chapters 17
(Boiling Stones), 18 (Sources of Heat, especially heating mantle and variac), 20
(Distillation), 36 (Theory of Distillation). Yes, you must read all of them!
Summary
In this lab, you will recrystallize the organic solid that you isolated from your
unknown mixture. After recrystallizing your solid, you will identify it based on melting
point. You will also purify your organic neutral by distillation and subsequently identify
it based on its boiling point.
Introduction
Often even after organic compounds are separated from starting materials and byproducts, some impurities remain and must be removed. Two of the most common
methods of purification in organic chemistry are recrystallization (for solids) and
distillation (for liquids), both of which you will be performing today.
Part I: Recrystallization
Recrystallization is a commonly employed method for the purification of organic
solids. In the proper solvent, an organic compound can be dissolved to form a hot,
saturated solution that, upon cooling, generates crystals in a pure form. The key to a
good recrystallization is to find the proper solvent. The solvent should have the
following properties:
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It should completely dissolve the desired compound when hot (usually at the boiling
point of the solvent).
It should dissolve very little of the desired compound when cold (usually after fifteen
minutes in an ice/water bath).
It should dissolve impurities at all temperatures or not at all.
It should be unreactive toward the compound.
It should dissolve the compound in a reasonable amount of solvent. Too much solvent
will make recrystallization extremely slow and the yield is likely to be low. Too little
solvent will make it difficult to filter off all of the dissolved impurities when collecting
crystals. As a rule of thumb, you might aim to find a solvent that will dissolve a gram
of compound in ~5-10 mL of solvent.
Now, how can you predict which solvent will work the best? You can’t – not with
any degree of certainty, anyway. But chemists (and chemistry students) must understand
how the structure of compounds influences their solubility. General guidelines for
predicting acid/base solubility were given in the last chapter, but while solvent extraction
relied upon those principles (you were actually carrying out acid-base reactions),
recrystallization relies upon principles of structure, polarity, and solubility.
Understanding these principles is extremely important in designing pharmaceuticals, as
solubility will determine not only the proper route of administration but also the kinetics
and metabolic fate of the drug as well. Furthermore, as water solubility decreases for a
given drug, in general its ability to cross the blood-brain barrier increases, which can be
either essential to the drug’s action or detrimental to the patient. Although it is usually
impossible to predict the exact solubility of any organic compound on sight, below we
have provided some general principles to guide your predictions:
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Only organic compounds that contain polar functional groups (e.g., hydroxyl,
carbonyl, amino) can be dissolved in water to any appreciable extent. Like dissolves
like, and the only way to make an organic compound like water is to make it
extremely polar.
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It is commonly observed that only five or fewer carbons can be rendered soluble by a
polar functional group; any more than that and the compound behaves like a nonpolar
compound.
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Branched chains or rings can increase water solubility due to a decreased surface area
– remember oil and water – hydrocarbon chains would prefer to decrease their contact
with water and vice versa.
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Compounds with multiple polar functional groups can display behavior more like
polar compounds than hydrocarbons, and they can even prevent a compound from
dissolving in organic solvents!
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Although by principles of electronegativity halogen atoms are quite “polar,” they
actually make a molecule repel water (they lack significant hydrogen-bonding
interactions) and actually decrease its water solubility.
Here are some examples illustrating the guidelines above:
A recrystallization involves the following steps:
1) Dissolve your compound in a minimum of hot solvent.
2) Decolorize if necessary, and remove insoluble impurities by hot gravity filtration.
3) Cool slowly so your compound crystallizes but impurities remain dissolved.
4) Collect the crystals by vacuum filtration.
Let’s now consider each step in further detail.
1) Dissolve the compound.
A wide variety of common solvents can be used for recrystallizations. As discussed
above, the solubility of an organic solid in a solvent depends largely on relative polarity.
A highly polar solvent (like water) will dissolve polar or ionic compounds like acetic acid
(vinegar) or sodium chloride (table salt), while a relatively nonpolar solvent like hexane
will dissolve nonpolar compounds like olive oil. In general, “like dissolves like.”
Ideally, the solvent that you choose will dissolve the compound when hot but not when
cold. What follows are some of the more common laboratory solvents arranged from the
most nonpolar to the most polar.
Polarity
Solvent
bp (°C)
nonpolar
hexane
diethyl ether
ethyl acetate
methylene chloride
acetone
ethanol
methanol
water
69
35
77
40
56
78
65
100
↓
↓
↓
polar
Notice that all of the solvents listed above (except for water) are fairly low boiling;
this makes it relatively easy to evaporate off residual solvent after the crystals have been
collected by filtration. When using water in a recrystallization, it is often difficult to
“dry” the crystals, which can lead to melting point depression.
All of these solvents are flammable except for water, so a steam bath is the
appropriate heating device — never an open flame or a hot plate! The latter devices can
cause solvents to boil over and ignite, causing a big fireball. Use a hot plate only for
heating water.
Even when you know the structure of your compound, it can sometimes be difficult to
find a solvent that works well for recrystallizing your compound. This problem can often
be solved by using a binary mixture of solvents: if your compound dissolves quite well in
one solvent at all temperatures (hot and cold) and doesn’t dissolve in another solvent at
any temperature (hot or cold), then a mix of these solvents may provide the necessary
properties for a recrystallization (provided that the solvents are miscible). In this lab, you
will learn two methods of determining an appropriate mixed solvent system for
recrystallization.
After a solvent is selected, the compound to be recrystallized is dissolved in a
minimal amount of hot solvent, in an Erlenmeyer flask. It is crucial that you use a
minimum amount, or your compound may remain in solution when you cool it.
2) Remove any insoluble or colored impurities.
At this stage, it may be necessary to decolorize the solution using activated charcoal.
If the solution is strongly colored (but your product isn’t supposed to be), then you have a
colored impurity. Because colored impurities are often polar, they usually will adsorb
onto activated charcoal (also called “decolorizing carbon”), which is then removed by hot
gravity filtration. This step is not required.
A hot gravity filtration is the removal of any insoluble impurities (dirt, lint, any
activated charcoal you may have added, etc.) by filtering the solution (while it is still hot)
through fluted filter paper into another clean Erlenmeyer flask. Be careful — agitating
the hot saturated solution and putting it in contact with the sidewalls of a relatively cold
funnel can cause the compound to “crash out” as crystals form very quickly, clogging the
funnel and lowering your recovery. You may wish to heat the funnel and receiving flask a
bit with steam beforehand.
This step removes many impurities that are much less soluble than our compound in
the chosen solvent. But there are also impurities that are much more soluble; these are
not removed at this stage. We will deal with them in steps 3 and 4.
3) Crystallize your compound from solution.
After the solid and colored impurities have been removed, the solution is cooled
slowly to induce crystallization. The slower the cooling process, the purer the crystals.
Let the hot solution cool to room temperature without disturbing it, then put it in an
ice/water bath. If crystals fail to form, there are a number of ways to induce
crystallization.
You can try scraping the bottom of the Erlenmeyer slowly and gently with a glass rod
(it sounds crazy but it often works well). Also, if you are lucky enough to have some of
the pure crystallized product, you can use a few of these crystals as seed crystals to
induce crystallization. You should check with your TF before resorting to these
techniques; the crystals will be purer if they can form without being disturbed.
4) Collect the crystals.
The crystals should be collected by vacuum (suction) filtration on a piece of filter
paper in a Büchner funnel. The vacuum will pull the cold mother-liquor through, leaving
your insoluble crystals on the filter paper. Impurities that are soluble in your cold
solution will pass through the filter paper into the flask.
The crystals are often then washed once or twice with small portions of ice-cold
solvent to further rinse away these impurities – you don’t want your product to dissolve
and wash away.
The vacuum also serves to dry the crystals by pulling air over them. Air drying can
be accelerated by scraping the cake of crystals from the filter paper with a spatula,
making a fluffy pile (be careful not to tear the filter paper). Residual solvent can also be
removed by pressing the crystals between pieces of dry filter paper followed, if
necessary, by drying in an oven.
Once the product is completely dry, a melting point and percent recovery can be
determined. Residual solvent will lower the melting point of the crystals and artificially
increase recovery. In a number of cases (including this lab), the product can be identified
by comparing the experimental melting point with the values reported in the literature.
Safety Precautions
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All chemical work should be carried out in the hood.
Wear the required personal protective equipment at all times – appropriate shoes,
adequate leg coverage, lab coat, and goggles!
This lab involves the heating of solvents and requires caution when handling hot
solvents and hot glassware. Also be careful of hot steam from the steam bath.
Ethanol is flammable! Use a steam bath or hot water bath.
Do not breathe in hot solvent vapors!
Remember to turn off the Meltemp when it is not in use!
Odds and ends
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Run the steam tap for 30 seconds or so before attaching it to the bath; this allows the
corrosion inhibitors and other nastiness in the pipes to clear out instead of blowing it
all into the bath.
Let steam baths and hot plates cool down before putting them back.
Leave hood sashes at the appropriate level. There are height locks on them for a
reason.
Use the steam baths with only a gentle supply of steam, or it will start raining in your
hood.
Turn off the steam when you aren’t using it.
Procedure
Small-scale test recrystallizations with different solvents
Get the vials containing your
unknown compounds from your TF.
You
will
determine
a
good
recrystallization
solvent
by
experimenting with pure water and
pure ethanol. This preliminary test will
give you some idea of the behavior of
your unknown in solution; most often
the solvent used in practice is actually
a mixture of solvents.
Get two small test tubes, place
them in a test tube rack or small
beaker, and put about 0.1 g (a spatula
tip’s-worth) of your unknown into each
tube. Put about 0.5-1 mL of ethanol in
one tube and the same amount of water
in the other (be sure to label each one). Observe the solubility of your unknown in the
two solvent systems at room temperature by gently shaking each test tube and seeing if
the solid dissolves. If you look carefully, you may be able to see tiny clear “tendrils”
rising from the dissolving solids. Now heat the test tubes on a steam bath as shown on
the left. Again, observe the solubility of the unknown in the solvents, now at an elevated
temperature. Be sure you distinguish between melting and dissolving; an organic
chemical that is insoluble in water may melt and float on top of (or in) the water as a
near-invisible globule.
Keep in mind that it may take a while for your compound to dissolve, so be patient.
If an unknown solid is insoluble in a solvent at room temperature but soluble in the hot
solvent, this is a good candidate solvent to use for your recrystallization. This may be the
case with more than one solvent system, so put your solvents to the test. Let the test
tubes cool to see if crystals form as they’re supposed to.
An ice/water bath may be handy to have here. An ice/water bath is a thick slush
made from ice and water. Metal ice containers are located above the rotovaps. The ice
machines are at the end of the center bench. Don’t fill up your container with ice; you
only need it about 1/3 to 1/2 full. Remember, the ice will melt so don’t stick a test tube in
the ice and just walk away; you may return to find your sample has fallen over sideways
and you have to start over.
Based on your observations, decide which of your systems is the best candidate for
recrystallizing each of your unknown. At this point, you could use that solvent to
recrystallize the rest of your products, but you won’t. Most often an organic solid is most
effectively dissolved in a mixture of solvents. This method is described in the next
section.
Recrystallization using a “doused” solvent system
Testing many recrystallization solvents can take a long time and is usually more
trouble than it’s worth. A quicker method is to dissolve your compound in a hot solvent
in which it is very soluble and then “douse” or “bait” it with another solvent to reduce the
solubility. You end up recrystallizing from a mixed solvent system, but you create the
right mixture “on the fly.”
Weigh the rest of your unknown, and place it in an appropriately sized Erlenmeyer
flask (a 125 mL flask will do, unless we pass out something smaller). Based on your
results from small-scale recrystallizations, dissolve the unknown in the solvent in which
it is most soluble when hot. It is essential that you dissolve it in a minimal amount of hot
solvent. The best way to do this is to heat the solvent in a separate flask and then slowly
add the solvent to your unknown as you stir and heat it.
Ethanol
is
flammable!
Perform
the
recrystallization using a steam bath (previous page)
or hot water bath (to the left)—not solely a hot
plate.
Now heat up some of the solvent in which
your compound was less soluble, and add it to your
solution dropwise. Perhaps right away or maybe
after many drops have been added, you should see
cloudiness appear. Wait for the cloudiness to
disappear while heating and swirling. Keep adding
this solvent dropwise to reduce the solubility until
it requires a lot of heating and swirling to make the
cloudiness disappear. If you’ve added too much,
you may need to increase the solubility with a bit
of the first solvent. Ideally, the solution is now hot and saturated. Filter the hot solution
through fluted filter paper supported by a stemless glass funnel. If crystals crash out onto
the filter paper, add a small amount of hot solvent or re-heat the solution on the steam
bath before continuing addition.
Let the flask cool slowly to room temperature. Crystals should form when you
cool the solution down because you have essentially optimized for the ideal solvent
system. After the flask has reached room temperature, cool the flask additionally in an
ice/water bath for 10-15 minutes; this maximizes crystal formation.
Collection of purified crystals
Collect the crystals by vacuum filtration using a Büchner funnel (don’t forget the
filter paper!) and an aspirator trap, and wash the crystals with a small amount of cold
solvent. Keep the aspirator on, draw air through the crystals for a few minutes, and fluff
them up with a spatula to accelerate the drying process as discussed before. Be patient.
Clean up your bench while you are waiting for the crystals to dry. Transfer the crystals to
a clean, dry piece of filter paper, and press them dry. Dry them in an oven if necessary.
Note the appearance of the crystals in your notebook (color, shape, etc.)
Weigh the amount of recovered purified compound, and determine a percent
recovery. Also, record the melting point of the product. Show your crystals to your TF
(make sure he/she sees them!) and then dispose of your product properly. If you are
unsure where to put it, see the “Cleanup” section below.
Identification of unknown solids
Part II: Distillation
Organic liquids can often be separated by the process of distillation – in other
words, boiling a liquid and allowing it to recondense in a pure form. This way, impurities
are either left unchanged in the original vessel, deposited before the liquid’s vapor
reaches the receiving vessel, or sometimes boiled away completely before the pure liquid
boils. In order to understand more about this procedure, be absolutely certain to read
chapters 20 and 36 in Zubrick, as directed above.
Safety Precautions
All chemical work should be carried out in the hood.
Wear the required personal protective equipment at all times – appropriate shoes,
adequate leg coverage, lab coat, and goggles!
• This lab involves the heating of solvents and requires caution when handling hot
solvents and hot glassware.
• Never heat a closed system. You should be able to trace an invisible path from the air
in your flask out to the air in the laboratory
• Never distill to dryness. Leave ~1 mL of liquid in the bottom of the distilling flask.
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Odds and Ends
If you have too little product to distill, simply push the thermometer down into the
flask and record the temperature at which the liquid condenses onto the thermometer.
• Use gloves when handling glass wool or else you’ll itch for a week. If you happen to
touch glass wool and your hands start to itch, go to the sink right away and scrub your
hands with cold water.
• Immediately recap anhydrous reagents (like magnesium sulfate) when finished with
them! Anhydrous means “no water” and the air alone contains enough water to ruin
many dry reagents.
•
Procedure
The simple distillation set up is as shown. Note that you can set up and begin this
distillation while recrystallizing (i.e. while waiting for your hot solution of pure
compound to cool). Then, once the distillation is running smoothly, you can filter and
dry your crystals and take a melting point if you have time. Multitask, but always keep
an eye on your distillation!!!
Set up a heating mantle as the heat source to distill your compound. Use a lab jack to
support it so that you can lower the heat source quickly if you need to. The heating
mantle should be plugged into a variac (never directly into an electrical outlet!) The
variac (the heavy thing with the dial) should be plugged into an electrical outlet. The
variac has an on/off switch and a control dial. The numbers on the dial measure voltage,
not temperature.
You should distill the liquid into a pre-weighed round-bottom flask. This will enable
you to obtain a yield without removing the liquid from the flask by simply weighing the
flask with the product and subtracting the mass of the flask. This is a good general policy
whether doing a distillation or removing solvent from a purified product on a rotovap. If
you remember to always do this, it will save you the pain of scraping your product out of
a round bottom after rotovapping in order to get a yield.
Place a boiling stone into the flask you will be distilling from. Be sure that your
apparatus is clamped properly before heating; the joint of the round bottom flask and the
water condenser should each be secured with a clamp. Wiggle the apparatus a bit to test
it. You’ll probably need to insulate your apparatus as shown. Glass wool underneath
aluminum foil works well. Insulate the round bottom flask neck and the distillation head,
but not the bottom of the flask. Be sure to recycle the glass wool back to the Glassware
Storage room or the white shelves.
Do not turn on the variac until your TF has checked your distillation apparatus and
given you the thumbs up to start heating. Start the variac on a low setting (like 40), and
turn up the voltage as needed (be patient, the heating mantles take a while to heat up and
you don’t want to overdo it!) A setting between 30 and 60 is usually used, though a
setting of 90 may be necessary for this lab. Never turn up a variac all the way.
In this experiment, where we have a very small amount of liquid, you will have to
shove the thermometer down to just above the boiling liquid to get an accurate boiling
point. This is not normal (usually you just leave the bulb just below the intersection of
the distillation head). Assume the highest temperature reading is the most accurate.
Once you have obtained a reading, pull the thermometer up far enough that it does not
obstruct the path of the distilling vapor (to the level of the sidearm).
Now identify your organic neutral liquid from the following:
Cleanup
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Wash all of your individual glassware, and return it to the dirty glassware bins.
Return glass wool and aluminum foil to their containers.
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Remove the Tygon tubing from the condensers and return them to the common
glassware room, not the dirty glassware bins (are they made of glass?)
Dispose of test tubes, pipets, melting point capillaries, and glass vials in the glass
waste containers.
Dispose of leftover solids and filter paper in the solid waste buckets.
Dispose of paper towels and gloves in the trash.
All acetone rinses must go into recyclable acetone waste.
Aqueous (water) and ethanolic solutions of organic chemicals go in the aqueous waste
bucket.
Wipe off your benchtop with paper towels (water and acetone can be used if needed).
Wash your hands well before you leave.
Pre-Lab Write-Up
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No mechanism is necessary this time.
Summarize the procedure for using the Meltemp for quick reference.
Summarize the procedure for doing a recrystallization for quick reference.
Summarize the procedure for doing a distillation for quick reference. Include a picture
of the setup that you will use.
Lab Write-Up
Be sure to include in your Results and Conclusion section:
• Identification of your unknowns with explanation. Remember to include your original
unknown sample letter!
• Melting point and other physical characteristics (color, appearance, odor, etc.)
• Comparison of experimental MP with literature MP.
• Recovery in grams.
• Calculation of your percent recovery.
• Brief explanation of your solid compounds’ solubility behavior in water and ethanol.
(Base your explanation on the structure of the compound!)
• Explanation of sources of error and why you obtained any unexpected results, e.g., the
melting point was lower than the literature MP, you only obtained 20% recovery, etc.
Possible Quiz Questions
1) Why is fluted filter paper more efficient than filter paper folded into a smooth cone for
gravity filtrations?
2) Why should you use a stemless funnel for hot gravity filtrations during a
recrystallization?
3) Circle all of the following that you should not heat on a hot plate:
a. diethyl ether
b. ethanol
c. water
d. ethyl acetate
4) Why is slow cooling important to the purification?
5) Match the following compounds with their expected solubility behaviors.
6) How would the following affect the percent recovery and melting point of your
unknowns?
a) too much recrystallization solvent was used.
b) the hot solution was not gravity-filtered before cooling.
c) the hot solution was gravity-filtered and immediately plunged into an ice bath.
7) How would the following affect the percent recovery and melting point of your
unknowns?
a) the crystals (after suction filtration) were washed with hot solvent.
b) the crystals were not washed at all.
c) the crystals were not allowed to fully dry during vacuum filtration.
8) Name three characteristics of a perfect recrystallization solvent.
9) How does the melting point of an impure compound compare to the literature melting
point? Briefly explain the chemical principle (in terms of structure and intermolecular
forces) behind this observation.
10) The boiling point of a liquid can likewise be affected by several factors. What effect
would each of the following changes have upon a distillation?
a) lowering atmospheric pressure (as in a vacuum distillation)
b) the presence of significant dissolved impurities
c) the presence of water sufficient to form an azeotrope
11) List three possible safety risks specific to this experiment. Briefly describe
precautions to address these risks.