Question

a. Draw figures over-viewing the apparatus/procedure for (a) vacuum filtration and (b) hot gravity filtration, two techniques used in this protocol. Briefly explain why the two methods are used in the specific steps of the procedure, contrasting the goals of each method.

b. What is the purpose of the CaCl2 drying tube? What chemical reaction is it preventing (please supply a mechanism).

c. Draw the product and detailed electron-pushing mechanism for a slightly different reaction: benzene (not ferrocene) being treated with chlorine and FeCl3 to generate chlorobenzene.

d. Based on the structures of the starting material and product, draw predicted TLCs of: (1) a complete reaction and (2) an incomplete reaction. (We’ll pretend the TLC is done at 10:1 H:E, but that is irrelevant for this question).

Reaction

1. Begin to heat a sand bath (to 80-90 0C) set on top of a stir plate while preparing the following reaction mixture. Proper temperature control is crucial in this reaction! Carefully try to avoid going over 90 0C. Only use thermometers that read up to 200 0C. (Thermometers reading only to 110 0C may explode!)

2. Place 1.5g ferrocene in a 25 mL round-bottom flask containing a magnetic stir bar. Cork-stopper the

flask. The magnetic stir bar should be ~1cm long for optimal stirring (2cm stirbars do not stir well).

3. In a fume hood, add 5.0 mL acetic anhydride and 1.0 mL of 85% phosphoric acid to the flask. The mixture should heat up and darken in color. Loosely cork the flask again temporarily.

4. Construct a calcium chloride drying tube: insert glass wool into a macroscale drying tube, add a ~1-2 inch layer of CaCl2 on top. Affix to the flask with a Keck clip.

5. Heat the reaction mixture, with stirring, in the 80-90 0C sand bath prepared in step 1. Heat for 15-20 minutes. A visual way to monitor the reaction is transformation from dark red to deep purple.

6. Remove flask from sand bath for TLC analysis.

7. Follow the reaction by TLC (10:1 H:E). (TLC: ferrocene SM on left, reaction on right, cospot in the

   middle). If the reaction is incomplete, you may continue heating. (but keep the temperature 80-90 0C)

   8. Pour the reaction mixture directly onto 25g (~60mL) ice in a 200 mL beaker, rinsing the flask with H2O (2 x 5 ml). (A black residue may remain in the flask, leave it behind). Stir the orange-brown mixture with a glass rod for a few minutes. Any insoluble black material present in your reaction mixture will be removed in the following steps.

      8. Add 37.5 mL of 3M aqueous NaOH solution. Check the pH – most students will observe pH 9-13 which is good.

      9. Stir the mixture well and crush any lumps, affording the product as a brown suspension.

      10. Allow the mixture to stand for ~10 min, then collect the crude product by vacuum filtration. Continue to

          pull air through the product for a few minutes to dry it. Finish the drying process by pressing the solid

          product between two sheets of filter paper or paper towels.

      11. Save a small amount of crude product for TLC analysis (perhaps in your ceramic tray for TLC samples)

      12. Transfer the crude product to a 50 ml ERLF and add 20 mL of hexanes. Gently boil for 3-5 minutes

          with gentle swirling to dissolve the product, then decant the dark-orange solution into another Erlenmeyer flask, leaving behind any black gummy substance. (If considerable solvent has evaporated, add more hexane to avoid product loss)

      13. To the hot solution, add a small spatula-full of decolorizing carbon (no more than 100mg), adding more will decrease your yield. Heat with swirling, (i.e. keep the mixture hot!), and then quickly perform a hot gravity filtration to remove the decolorizing carbon, into a new clean/dry 50 ml ERLF. (using a short- stem plastic funnel, not the long-stem glass funnel!)

      14. Set the flask aside to cool slowly. Red-brown needles of acetylferrocene should begin to form. Once the flask has reached room temperature, cool it in ice. Collect the crystalline product by vacuum filtration, washing with a small quantity of cold hexanes, and dry it by continuing to pull air through it for a few minutes.

      15. Re-analyze your crude and recrystallized products by TLC.
          Consider spotting left=SM (ferrocene), middle=crude product, right=purified product.

          (no co-spotting necessary in this experiment).

      16. Place your product in a labeled class sample (20 g vial) of this material.

      Only if your mixture is acidic (pH < 7), add small amounts of sodium bicarbonate until

      your pH is ~8-9. (Very few, if any students will not have to add sodium bicarbonate).

Answer 1

a. Vacuum filtration is performed to separate the crude product formed (a solid) from the liquid. It helps to wash the solid easily with the solvent without loosing any material and is time efficient as opposed to all liquid filtration.

A simple hot gravity filtration works when recrystallizing the crude solid product. This is a all liquid type of filtration which works efficiently with a simple filtration technique without having any trouble of clogging the pores of filter flask.

b. CaCl2 is used to absorb any water formed or present in the reaction.

CaCl2 + 2H2O <==> Ca(OH)2 + 2HCl

the reaction is very efficient and fast.

c. The mechanism of chlorination of benzene by Cl2 in presence of FeCl3 catalyst is given below.

d. A tlc plate possible for the given reaction in c. is also shown below.