Experiment 7 Recrystallization: Purification of Crude Benzoic Acid and Phenanthrene Reading: Handbook for Organic Chemistry Lab, section on Recrystallization (Chapter 12).
Recrystallization is the primary technique for the purification of compounds that are solids at room temperature. In the process of recrystallization, molecules are deposited from a saturated solution and are selected, according to their shapes, to fit into growing crystal lattices. The technique can be carried out by dissolving a compound to be purified in a hot solvent (or solvent mixture) and then allowing the solution to cool. If the solvent or solvent mixture is properly chosen, the compound has a decreased solubility at lower temperatures, and it will form crystals in the solution. In this experiment, you will recrystallize both the benzoic acid and the phenanthrene that you separated during the previous experiment. The benzoic acid will be recrystallized from hexanes and the phenanthrene will be recrystallized from the solvent pair of ethanol and water. Summary of the Steps in Recrystallization:
Step 1: Dissolve the impure solid in a minimum amount of hot solvent. o If the solution is highly colored but you know that your product is not highly colored, go to Step 2. o If insoluble impurities are present, skip to Step 3. o Otherwise, skip to step 4. Step 2: Treat with decolorizing charcoal pellets (Norite) – about a quarter of a spatula tip’s worth. o If color goes away, go to Step 3. o If color persists, add more Norite and wait five minutes. If color still persists, disregard it and go to Step 3. Step 3: If insoluble impurities are present, decant or gravity filter the hot solution, then proceed to Step 4. Step 4: Allow solution to cool slowly to room temperature. Step 5: Chill cooled solution by immersion in an ice bath – crystals should appear. If not, try scratching the bottom of the flask with a glass rod. Once crystals have formed, filter to isolate the product.
Safety Precautions Ethanol and hexanes are highly flammable. Procedure You will work with a partner for this experiment. Before you begin, you should each clean an NMR tube and let it dry so it is ready to use later. If you did not isolate enough benzoic acid or phenanthrene during the previous experiment, you may take some from the recovery jars in the hood. Recrystallization of Benzoic Acid Weigh out 0.10 g of the benzoic acid that you isolated in the previous lab period. Place the benzoic acid in a small Erlenmeyer flask and add about 3 mL of hexanes. Swirl the contents of the flask and heat the 1