Experiment #4 - Recrystallization; Microscale vs Macroscale Purification of Benzoic Acid

Reading Assignment in Williamson: Chap 3, Expt 2

Overview - You will be carrying out the recrystallization of an impure sample of benzoic acid. Because the procedures used for recrystallization are markedly different depending on the amount of material to be purified, and because it is important for a beginning organic experimentalist to be capable of purifying any amount of an organic solid, the procedures will be carried out on both a microscale (around 50 mg of impure solid) and a macroscale (around 1 g of solid). The procedures below are divided into the seven basic steps of any recrystallization procedure as delineated in the Williamson text.

Microscale Procedures

Refer to the microscale apparatus list provided during check-in (also on p 10 of Williamson) for identification of glassware - letters given in parentheses below refer to this list. You may also refer to the course web site for a web version of this handout with hyperlinks to photos and diagrams of the required apparatuses. Here is the report sheet for this experiment. (The report sheet is in Microsoft Word 97 format.)

(1) Getting Ready. You will be recrystallizing 50 mg (0.050 g) of impure benzoic acid from distilled water. At the start of the lab, set up a sand bath using a Thermowell half-filled with sand connected to a Powermite power control. (Set the Powermite to about 30% of maximum).

(2) Dissolving the Solute. Add the weighed impure solid to a 10 x 100 mm reaction tube (p) (the plastic funnel makes the addition easier) and then add 2 mL water and a boiling chip. Attach the 100-mm column (o) to the reaction tube using the Viton connector with support rod (m). Clamp up the apparatus to a ring stand using the support rod. (The 100-mm column serves as an air cooled reflux condenser in this microscale set up.) Note the total volume of solution in the tube and gently heat the solution to boiling. (Do not immerse the tube too deep in the sand bath. The deeper it is immersed the hotter it is!) Once the solution begins boiling any undissolved material is probably not benzoic acid (see solubility table below).

(3) Decolorizing the Solution. Allow the solution to cool by about 10-15° (i.e., for 15-20 sec) before adding 10-15 mg pelletized Norit (decolorizing charcoal in pellet form) and then gently reheat the solution to boiling and allow the Norit to work for 5-10 min. Make sure to monitor the level of the solution in the tube; if any solvent boils off (not likely with this set up) it should be replaced with more water.

(4) Filtering Suspended Solids - Hot Filtration. Filter the solution to remove the Norit and any other undissolved solids using a short-stem Pasteur Pipette with cotton plug and 3-4 mm of Celite as in procedure 4B on p 45 of Williamson. Both Pasteur pipettes used for this step should be warmed in a beaker on a steam cone prior to use and the liquid should be transferred all at once and filtered as quickly as possible. The filtration is completed by immediately attaching a pipette bulb to the filter pipette and squeezing to force the liquid through. In order to allow the filtered solution to cool as slowly as possible, it is a good idea to filter the hot solution into a reaction tube which is held in a 50-mL Erlenmeyer flask containing hot water. (See Figure 1).

(5) Crystallizing the Solute. Set the Erlenmeyer flask containing the reaction tube aside on the bench top and allow the tube to cool; it will only cool as fast as the water in the flask. If crystals do not form as the solution reaches near room temperature, see the instructor.

(6) Collecting and Washing the Crystals. Cool the solution on an ice bath before removing the solvent using a pipette placed flush against the bottom of the tube (procedure 6F p 52, also see procedure 4A on p 45). After removing as much of the water as possible with the pipette add approx 0.25 mL ice-cold H₂O and then remove this wash solvent with the Pasteur pipette as above. Allow the purified acetanilide to air-dry in the reaction tube until next week.

Macroscale Procedures

(1) Getting Ready. You will be recrystallizing 1.0 g of benzoic acid from the minimum amount plus 10% of distilled water needed. Calculate the minimum amount using the data in the following table. You will need an appropriate size Erlenmeyer flask and a boiling stick.

(2) Dissolving the Solute. Add the weighed solid and the water to the Erlenmeyer flask. Add the boiling stick and heat gently to boiling on a sand bath. Swirl the flask periodically in order to promote dissolution.

(3) Decolorizing the Solution. After the liquid has started to boil and most of the solid appears to have dissolved, allow the solution to cool to below the boiling point and then, add pelletized Norit (again approx. 5%) in order to effect decolorization. Heat once again to boiling and allow 5-10 min for the Norit to work. Add more hot water, if necessary, to maintain the original volume of the solution.

(4) Filtering Suspended Solids - Hot Filtration. Once decolorization is deemed complete a hot, gravity filtration should be done (part B, p 37-38). Use a powder funnel (wide-stem) instead of the stemless funnel mentioned in the text. Make sure to use a fluted filter paper as shown in the text (fig 3.7, p 38).

(5) Crystallizing the Solute. Allow the filtered solution to cool slowly to room temperature on the bench top before cooling further on an ice bath.

(6) Collecting and Washing the Crystals. Collect the resulting benzoic acid crystals by vacuum filtration on a small Büchner funnel.  Because a water aspirator will not be used as a vacuum source (the lab is equipped with vacuum lines), the water trap shown in Fig 3.21 will not be necessary. Do not use water to rinse left over solid from the crystallization flask as this would be counterproductive. Instead scrape as much of the left-over solid as possible out with a spatula and/or rubber policeman. Wash the crystals with one or two milliliters of ice-cold water. After pressing the crystals as dry as possible on the Büchner funnel, weigh them into a small beaker. Allow the recrystallized solid to air-dry in your drawer until next week.

Post-Lab

10. On your own time sometime before next week's lab come in to the lab and obtain melting points on the impure and recrystallized samples of benzoic acid. Transfer the crystals to small preweighed zip-loc bags. Record the weights of the filled baggies and subtract to get the yield for each recrystallization. Label each baggie completely and staple them to your lab report before turning it in.