While synthesis is fundamental to organic chemistry, this laboratory is not primarily concerned with the formation of the product. Organic chemists refer to the purification of their products after the actual reaction as "workup" for the synthesis. In this lab we are primarily interested in developing techniques for workup when the product is a solid. Our workup will involve decolorization of the impure product followed by recrystallization and isolation via vacuum filtration.
Decolorization is a process by which small amounts of deeply colored impurities (chromophores) can be removed from a product. Generally the material is dissolved in a suitable solvent and a quantity of activated charcoal is added to adsorb the chromophore. The activated charcoal is then filtered and discarded, leaving the decolorized product.
Recrystallization is a process by which crude (impure) crystals are purified by dissolving in the smallest possible volume of a hot solvent. The solvent must be chosen carefully. The main requirement is that the crystals are much more soluble when the solvent is hot than when it is cold. The saturated solution is then cooled and the purified product solidifies. These crystals are then filtered and set aside to dry.
Procedure
In a 125-mL Erlenmeyer flask, measure 2.0 mL of aniline (caution! toxic; avoid contact with skin) from a buret. Add 15 mL of water to the flask. In a small graduated cylinder, obtain 2.5 mL of acetic anhydride. While swirling the flask, add the acetic anhydride to the aniline. After the crude product forms remove a small sample (about 1 mL of wet solid) with a spatula and place on filter paper on a watch glass to dry. Label this sample "crude acetanilide."
At this point the synthesis is complete; now the workup begins. To the remaining flask of crude acetanilide, add 50 mL water and one boiling stone. Heat on a hot plate until all of the solid is totally dissolved in the water. Pour a few mL of the solution into a small labelled test tube and set aside. Label this test tube "partially purified acetanilide."
Add about 1 teaspoon (note the rough measurment) of activated carbon to the hot flask. Wait for all boiling to stop before adding the carbon or the flask might froth and boil over. Bring the flask back to a gentle boil and swirl occasionally for about 5 minutes. While waiting, prepare double thickness fluted filter paper as demonstrated by your instructor. Also arrange to have about 50 mL of boiling hot water. It would be convenient for several students to share a common flask of hot water.
Set up the hot filtration by placing a stemless funnel over a 250 mL beaker, using an iron ring to support the funnel. Place the fluted filter paper into the funnel and rinse with boiling hot water to warm the filter paper and funnel. Add the hot solution to the funnel in several small portions, swirling before each addition. Keep the solution hot by returning the flask to the heat as you wait for the funnel to empty. After all of the solution has been filtered, add 5-10 mL boiling hot water to the empty flask, rinse the flask and pour through the funnel to dissolve any crystals which might have formed on the filter paper. Dispose of the filter paper containing the carbon as directed by your instructor.
Allow the solution to cool by placing into an ice water bath. Do not allow water from the ice bath into the beaker. While waiting for crystals to form in the beaker, filter the partially purified acetanilide using the Hirsch funnel and vacuum filter flask. Set these crystals onto a labelled watch glass to dry until next lab. After cooling 15-20 minutes the main crop of purified crystals may be isolated on the large Büchner funnel by vacuum filtration. Place the wet filter paper onto a labelled watch glass and allow the crystals to dry until the next laboratory period. Dispose the waste liquid into the aqueous waste container.
Observe the appearance of all three crops of acetanilide. Note color, shape, and any other details which you can observe. Determine the purity of the samples by carefully measuring the melting point of each in duplicate. Comment on the relationship between appearance and apparent purity of the samples.
Your notebook conclusions should include a determination of limiting reagent in this synthesis, a calculation of your percentage yield (based only on the mass of the large final crop of crystals), tables of the melting points obtained for all samples and any relevant comments.
Literature Sources
Calculating limiting reagents and percentage yield
"A Chance Discovery: Acetanilide and the Invention of Tylenol"
Aldrich Chemical Company Search Page
ChemExper Chemical Directory
Acros Chemicals
NIST Chemistry Webbook