CH 2270
What Did You Do?
What Did You Observe?
What Does It Mean?
Recrystallization & Identification of an Unknown
During this lab you will be given an impure sample of an unknown compound. It is your job to purify the sample by
recrystallization and identify the compound by its melting point.
Materials
From the Chemicals Hood:
Week 1:
Impure Unknowns A – F
Acetone
Ethanol
Toluene
Impure p-dibromobenzene
50/50 water/ethanol
From the Stockroom (Blue Bin):
Week 2 (Potential Unknowns):
Acetanilide
Benzoic acid
Cinnamic acid
Salicylic acid
N-Phenylsuccinimide
Naphthalene
Test tube holder
A video to review before lab is found on the Massachusetts Institute of Technology’s
“OpenCourseWare”: MIT Recrystallization Video
James W. Zubrik, The Organic Chem Lab Survival Manual; 9th Ed., Wiley: 2014; p. 104
Part I Finding a suitable recrystallization solvent. (Microscale)
Your unknown sample is impure, with some impurities which will dissolve and some impurities which will not
dissolve. You will have to closely observe to see if a good portion (but not all) of your sample dissolves.
Put about 100 mg (the tip of a spatula) of your unknown in a small test tube. Add 3 mL of solvent and shake gently.
Your solvent choices for this experiment are acetone, ethanol, toluene and water. If a good portion of your solid
dissolves at room temperature, then your unknown sample is soluble. Do not use this solvent as a recrystallization
solvent. (Make note of this in your notebook. See Post-lab I for table.) Use wooden sticks (available on the side
shelf) to break up chunks of your compound. But remember, even in the best solvent, not everything will dissolve.
This is because such impurities such as old boiling stones, sand, sawdust, floor sweepings, and so on are mixed in
with your unknown solid compound.
Repeat until you have four test tubes, each with your unknown and a different solvent.
If none (or very little) of the solid dissolved at room temperature, add a wooden stick and heat the test tube in a
steam bath. If the sample does not dissolve, do not use this solvent as a recrystallization solvent. Record the
solubility characteristics in your notebook. See Post-lab I for table. If the sample dissolved when hot and did not
dissolve at room temperature, record the solubility characteristics in your notebook. This is your recrystallization
solvent.
Allow the hot solution to cool to room temperature. If crystals do not form spontaneously, chill the tube in an icewater bath. If this fails, scratch the inside of the test tube with a glass rod to induce crystal growth.
To reiterate, the solvent should dissolve the solute (solid compound) when hot and give a high recovery of good
crystals when cold. Sufficient solvent should be used to keep impurities dissolved but not so much that it is
impossible to recover the solute in good yield. Check your decision with the TA before you proceed with the next
part. Save your product from this experiment. You can use these crystals, if you need, as seed crystals in the next
experiment.
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Part II Recrystallization of Impure Material (Macroscale)
Accurately weigh about 2 g (record the actual mass) of impure sample into a 125 mL Erlenmeyer flask. Add 40 mL
of your recrystallization solvent and a wooden stick (used to prevent superheating of the solvent that might result in
a violent eruption of the solution). Place the flask on a steam bath if acetone, ethanol, or toluene is your solvent.
Heat the flask directly on a hot plate if you are recrystallizing from water. Heat the contents to boiling. In a separate
50-mL beaker add ~25 mL of solvent and heat alongside the large flask.
When the solvent starts to boil, add small portions of solvent (with a glass pipet) from the small beaker until you
cannot detect any further dissolution. Record how much additional solvent is needed, if any. Do not exceed an
additional 20 mL. If there are insoluble impurities in the sample, they will not dissolve, even in the presence of hot
solvent. While the solution is still hot, filter the solution into a second Erlenmeyer flask through a fluted filter
paper (from the side shelf, see instructions posted on the wall for how to fold) heated with hot solvent from your
small flask through a stemless funnel. When you have a clear filtrate, set the flask aside to cool to room temperature.
If no crystals appear while the solution is cooling to room temperature, scratch the inside of the Erlenmeyer flask
with your glass rod. After the contents have cooled to room temperature, chill the flask in an ice-water bath to
encourage crystallization. If no crystals form, try adding a seed crystal from Part I. Also cool a small amount (30
mL) of the recrystallization solvent in the ice bath. When crystallization appears to be complete, vacuum filter the
mixture using suction (side-arm flask, Buchner funnel and filter adapter) and wash the crystals with three small (5 –
10 mL) portions of the cooled pure solvent. Dry the crystals on the filter by drawing air through the vacuum system.
Put aside on a clean, dry watch glass to dry until next week.
Weigh your dry product. Calculate the percent yield. Record these in your notebook.
Part III Identifying the unknown:
For now we will use one method to help identify your unknown: melting point.
Take a melting point of your compound, and compare it to the values for the available unknown compounds given.
Make sure that you record the melting point range for your compound in your notebook.
Melting Point – grind up a sample of the purified compound and place it in a melting point capillary. Take a quick
melting point (20 °C/min) just to get the approximate range and then repeat the measurement but have the
instrument raise the temperature much more slowly (1 °C/min) through the melting range.
Mixed melting point – When you think you have identified your compound you can confirm its identity by
performing a mixed melting point with an authentic sample of that compound. If the two samples are the same the
melting point will be unchanged, but if you mix two different samples together the mixture will have a melting point
that is lower than the pure compounds.
Grind up a small sample of each compound together on a watch glass or by rolling the samples with your glass
stirring rod between a folded over piece of weighing paper. Take the melting range as before.
Part IV Recrystallization Using A Mixed Solvent System
Recrystallization of p-Dibromobenzene – This part of the experiment illustrates the technique of using a mixed
solvent for recrystallization. p-Dibromobenzene is soluble in cold as well as hot ethanol. Therefore, ethanol alone is
not a good recrystallization solvent for this substance. On the other hand, the compound has a very low solubility in
hot or cold water. Therefore, water alone is also unsatisfactory as a recrystallization solvent for this substance. But
because ethanol and water are miscible, it is possible to find a mixture of the two solvents in which pdibromobenzene is soluble when hot but relatively insoluble when cold.
Procedure – Weigh out 1 g of impure (it has been contaminated with the orange dye, methyl orange) pdibromobenzene. Take the melting point of the impure sample. Transfer the solid to a 50-mL Erlenmeyer flask, add
10 mL of ethanol, a wooden stick, and heat the mixture on a steam bath. Swirl or stir the solution until the solid
dissolves. Add water in 0.5-mL portions to the hot solution until it becomes cloudy or just fails to clear when
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stirred. Then add a little ethanol (0.5 mL or less) until the turbidity just disappears. A plastic disposable pipet is
good for this purpose (measuring and dispensing small quantities of liquid).
Cool the flask in a pan of ice water for several minutes, and then collect the crystals by vacuum filtration. Rinse the
crystals on the Buchner funnel (with the vacuum off) with a few milliliters of ice-cold 50-50 ethanol-water. Stir the
contents on the funnel (carefully) for just a few seconds; then turn the vacuum on. Repeat this operation until a white
product is obtained or until most of the orange impurity is removed. Allow the pure crystals to air-dry on the
Buchner funnel (with the vacuum on) for five to ten minutes. After the crystals are dry*, weigh the product,
determine it melting point and the percentage recovery
Waste Disposal
Dispose of any excess solid in the black five-gallon buckets in the hood.
Dispose of the liquids in the liquid organic waste bottle.
Dispose of the melting point capillaries in the broken glass boxes at the end of your bench.
*p-Dibromobenzene sublimes if left uncovered at room temperature too long. Taking the yield and melting point
this week is recommended; otherwise transfer the solid to a clean, dry 50 mL Erlenmeyer flask and seal the top with
Parafilm. See TA if assistance with Parafilm is needed.
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