Chemistry 242 Experiment #5
The Synthesis of Ferrocene
Introduction:
Read Bruice, Chapt 14.1 –14.3 (p640-644) and 14.5 (p647-648)
Review Bruice Chapt 7.12 (p326-334)
See also http://www.ncl.ox.ac.uk/mom/ferrocene/ferrocene.html
The reactions involved are the “cracking” of cyclopentadiene dimer (a dimer is two
identical molecules joined together and “cracking” is opening to separate, like cracking a
nut).
This reaction is followed by the removal of an acidic hydrogen to make a stable aromatic
ring that happens to have a negative charge. Finally, Iron (II) is brought in and we have
all the pieces to make our ionic ferrocene molecule.
Synthesis of ferrocene
CAUTION: KOH is extremely corrosive and highly hygroscopic. Wear Gloves. 6 M
HCl is also corrosive. Be careful when handling.
Reaction
1. Grind 0.750 g of KOH in a mortar with a pestle as rapidly as possible.
2. Quickly transfer the finely ground powder to a small round bottom flask equipped
with a magnetic stir bar and add 1.3 mL of 1,2-dimethoxyethane (Glyme). The
chromatography column funnel may be very useful for this transfer.
3. Cap the flask with a good septum, then pass nitrogen through the solution for
about 1 minute. This is done by connecting a tank of nitrogen via a rubber tube to
a 22-gauge needle and adjusting the nitrogen flow to a few milliliters per minute.
When the flow rate has been correctly set, insert an empty syringe needle through
the septum as an outlet for the pressure, then insert the nitrogen needle through
the septum into the solution.
4. Remove the needles after a minute (any longer is a waste of nitrogen), shake the
flask to dislodge any solid stuck to the bottom, then turn on the stir bar. Don’t
forget to turn off the nitrogen flow if no one else is waiting to use it.
5. To a separate 10 mL vial, add 0.350 g of finely powdered green iron (II) chloride
tetrahydrate and 1.5 mL of dimethyl sulfoxide (DMSO).
6. Cap this vial with a good septum, insert an empty syringe needle to bleed off
excess pressure, then insert the nitrogen inlet needle.
7. After one minute remove both needles (don’t forget to turn off the nitrogen flow if
no one else is waiting to use it), and shake the vial vigorously to dissolve all the
iron chloride. Some GENTLE warming may be necessary. Use the warmth of
your hands first before reaching for a heating mantle.
8. Using an accurate syringe, add 0.300 mL of the freshly distilled cyclopentadiene
directly to the mixture of KOH in Glyme. Do not grasp the body of the syringe
(what was the boiling point of cyclopentadiene again?).
9. Stir the flask vigorously. The solution should turn brown because of the
formation of potassium cyclopendadienide salt.
10. After waiting five minutes for the anion to form, pierce the septum with an empty
needle to relieve pressure and then inject the iron (II) chloride solution in six 0.25
mL portions over a ten minute period with a separate needle. Continue stirring.
If the bar becomes immobile, remove both needles and shake the flask rabidly.
11. After all the iron (II) chloride has been added, rinse the reaction tube with 0.25
mL more of DMSO and add this to your reaction. Continue stirring for 15
minutes to complete the reaction.
Isolation
12. Prepare a mixture of 5.0 g of ice and 4.5 mL of 6 M HCl in a 30 or 50 mL beaker.
13. To isolate the ferrocene, pour the dark reaction slurry (heterogeneous mix of
liquid and solid) into the beaker with the ice and HCl.
14. Stir the mixture thoroughly to neutralize any leftover KOH (check with pH paper
and add more HCl if necessary).
15. Filter the orange crystals and wash the crystals with two 1 mL portions of cold
water.
16. Continue to draw air through the funnel to air dry for an additional five minutes,
then dry the crystals further between the folds of a piece of filter paper.
17. Weigh the crude product and set aside a small amount to do a crude product mp.
18. Purification will take place by sublimation. Sublimation is the process of going
right from a solid phase to a gas phase, as dry ice (frozen carbon dioxide) does. A
material sublimes when the local pressure is below the substance’s triple point.
Add the crude product to a 25-mL filter flask equipped with a neoprene fliter
adapter (Pluro stopper) and a 15 mL centrifuge tube that is pushed to within 5 mm
of the bottom of the flask.
19. Place a rubber bulb over the side arm of the flask, add ice to the centrifuge tube,
and then gently heat the flask on the sand bath to sublime the product.
20. Tilting and rolling the filter flask in the hot sand will help sublime the ferrocene
onto the centrifuge tube. Be sure to occasionally remove the bulb on the side arm
so pressure does not build up too much.
21. When sublimation is complete, cool the filter flask, remove the ice water from the
centrifuge tube, and replace it with room temperature water (to prevent
condensation from forming on the tube).
22. Transfer the pure ferrocene to a tared stoppered vial, determine the mass, and
calculate the percent yield.
Characterization
23. Determine the mp of the final product as well as the crude product. Since
ferrocene sublimes, you will have to do this in a sealed capillary tube.
24. Obtain a KBr pellet IR of your product.
25. Save your ferrocene for a later experiment (acetylation of ferrocene) in an airtight
jar.
Clean Up
26. the filtrate should be slightly acidic. Neutralize with sodium carbonate, dilute
with tap water, and then flush it down the drain. Place any unused
cyclopentadiene in the recovered cyclopentadiene container in the hood.
27. Add 0.4 mL of concentrated nitric acid to the sublimation flask to clean the
residue. Next week, neutralize with sodium bicarbonate and flush that down the
drain.
Questions
1. Cyclopentadiene dimer cracks slowly to give monomeric cyclopentadiene, but on
standing, even at room temperature, the monomer reverts to the dimer. Explain
why. I want more of an explanation than “The reaction is reversible.”
2. It is observed that the addition of hydrogen bonding (protic) solvents like water or
alcohol to DMSO solutions of potassium hydroxide sharply reduce the effective
base strength. Explain why.