Identification of Unknowns

CH 227 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, identify the compound by its melting point, and find the other students in your lab section who have the same compound.

Potential Unknowns Acetanilide Benzoic acid Cinnamic acid p-Dibromobenzene N-Phenylsuccinimide Salicylic acid

Solvents Acetone bp 56 ˚ C Ethanol bp 78 ˚ C Toluene bp 110 ˚ C Water bp 100 ˚ C

Resources: SuperOrganicLab Web Page: http://chemed.ces.clemson.edu/SOL/SuperOrganicLab/ Click on “Lab Techniques,” then “Purification Procedures,” then double-click on “Recrystallization.”

“The Organic Laboratory Survival Manual” 8th Ed., 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 (weighed on the balance) of your unknown in each of four small test tubes. Your solvent choices for this experiment are acetone, ethanol, toluene and water. Label each test tube with one of the solvent’s names. Add 3 mL of solvent to each test tube and shake gently. 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 (side shelf) to help the solvent boil and 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.

If none (or very little) of the solid dissolved at room temperature, heat the test tube on a steam bath (if acetone, ethanol, or toluene is your solvent) or on the hot plate (if water is the solvent). 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 the table. Allow the hot solution to cool to room temperature. If crystals do not form spontaneously, scratch the inside of the test tube with a glass rod to induce crystal growth. If this fails, chill the tube in an ice-water bath. If the sample dissolved when hot and precipitates out at room temperature or lower (ice-water bath), record the solubility characteristics in your notebook. This is your recrystallization solvent.

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.

1 Part II Recrystallization of Impure Material (Macroscale)

From the data you obtained on the microscale recrystallization, predict the approximate amount of solvent you will need to recrystallize a 2 g sample of your unknown containing impurities that may be solvent-soluble or solvent-insoluble.

Accurately weigh about 2 g of impure sample into an Erlenmeyer flask of the size that will hold all of the solvent you have calculated will be required. A 125 mL Erlenmeyer usually works. Add approximately half of the 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 Erlenmeyer flask add ~25 mL of solvent and heat alongside the large flask.

When the solvent starts to boil, add small portions of solvent (with a disposable pipet) from the small flask until you cannot detect any further solution. If there are insoluble impurities in the sample, they will not dissolve, even in the presence of hot solvent. Filter the solution into a second Erlenmeyer flask through a fluted filter paper (from the side shelf) heated with hot solvent from your small flask through a stemless funnel. When you have a clear filtrate solution 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 induce crystallization. If no crystals form, try adding a seed crystal from Part I. If crystallization is still not happening, return your Erlenmeyer flask to the heated surface (either steam bath if using acetone, toluene or ethanol, or a hot plate if using water). Boil away the solvent until it is approximately one-half the original volume and allow it to cool and crystals to form.

When crystallization appears to be complete, vacuum filter the mixture using suction (side-arm flask, Buchner funnel, filter paper, and filter adapter) and wash the crystals with three small portions of the pure solvent that has been cooled, in another flask, in an ice-water bath. Dry the crystals on the filter by drawing air through the vacuum system. Put aside on a clean, dry watch glass to dry until the next lab period.

Next Week: Weigh your dry product. Calculate the percent yield. Record these values in your notebook.

Part III Identifying the unknown: finding other samples of 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 just to get the approximate range and then repeat the measurement but have the machine raise the temperature much more slowly 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.

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.