Sublimation of Benzoic Acid

Dr. Nathan J. Malmberg

September 25, 2006

This procedure follows closely a similar procedure by Dr. Mark Hemric, and utilizes principles from Technique 9 of Microscale Organic Laboratory Fourth Edition by Mayo, Pike and Trumper. As you follow the directions, refer to your lab textbook to clarify any difficulties with the procedure.

  1. Transfer the contents of your bicarbonate extractions from the previous week into a 30 mL beaker. Dissolve 200 mg NaCl in the solution, heating if necessary.
  2. Add 6 M hydrochloric acid to the beaker, gently swirling the beaker to mix the acid and the solution. As you add the acid to the solution, a white precipitate should form, and the solution should bubble or foam. What is precipitating out of solution? Why is the solution bubbling? Continue to add acid until the bubbling stops or until no more precipitate forms on addition of acid.
  3. Filter your precipitate using your Hirsch funnel and vacuum filtration flask as described in your textbook under filtration techniques. Continue the vacuum on the flask until it is reasonably dry (covering with Saran Wrap if necessary).
  4. Scrape the precipitate onto a watchglass and set the watchglass on the warm sandbath. Continue heating the solid at about 100C until it is completely dry. Determine the mass of the solid. What is the percent yield of the benzoic acid from the extraction?
  5. Reserve a small amount of the crude benzoic acid for use in a melting point determination. If the recovered amount of benzoic acid is more than 100 mg, place 100 mg of your benzoic acid into a round-bottom flask (either size will do). If you have recovered less than 100 mg but more than 50 mg, place all of your crude benzoic acid (less your melting point sample) into a round-bottom flask. If your benzoic acid weighs less than 50 mg, place the solid into a 5 mL reaction vial.
  6. Attach your sublimator to the top of the vial or flask, using vacuum grease to seal the joint. Attach the aspirator to the other end of the sublimator. Submerge the vial or flask into the sandbath. Fill the tube in the top of the sublimator (the cold finger) with ice water. Start water running through the aspirator in order to create a vacuum in the vial or flask.
  7. Slowly increase the temperature of the sandbath, stirring the sand to distribute heat. Frequently exchange the water in the cold finger for cooler water, taking care not to spill any water onto the joint between the sublimator and the flask.
  8. When the temperature of the flask reaches a certain point, the sample in the bottom of the flask will sublime from the flask and deposit on the cold finger. When this process begins, turn off the heat to the sandbath and continue the deposition for another 10 minutes, making sure not to disturb the sublimator and knock your crystals from the cold finger.
  9. Remove the flask and sublimator from the sandbath. Carefully disconnect the aspirator from the sublimator and turn off the water. Carefully remove the sublimator from the flask and scrape the crystals from the cold finger into a tared beaker. Determine the mass of the sublimed benzoic acid, and calculate a percent yield for the purification.
  10. Measure a melting point for the crude benzoic acid, the sublimed benzoic acid, and the residue left in the sublimation flask. Are the melting points consistent with a purification of benzoic acid?