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Experiment #6 Synthesis of Ferrocene
Prior reading: Bruice, Chapter 14: Aromaticity, p.640-649, Bruice, Ch 7:Dienes, p.301-334
Introduction
Ferrocene is a stable, yellow organometallic compound, C10H10Fe. When it was first
discovered in 1951 there was a considerable amount of debate about the structure of this
compound. An early proposal was that the divalent iron makes two Fe-C bonds to
cyclopentadienyl rings. However, it was soon shown to consist of two 5-membered rings
sandwiched around an iron atom. One way to represent this compound is as a two
cyclopentadienylanions, C5H5-, and a ferrous ion, Fe2+. The five carbons of each ring interact
equally with the iron atom. The cyclopentadienyl anion is particularly stable because it is
aromatic: it has a cycle of 5 p orbitals containing a total of 6 electrons, therefore obeying
Hückel's 4n+2 rule for aromaticity. This structure explains a number of observations about
ferrocene. In particular, ferrocene undergoes electrophilic aromatic substitution reactions
(i.e., nitration, halogenation, Friedel Crafts alkylations and acylations) common for
derivatives of benzene.
Among simple hydrocarbons, cyclopentadiene is relatively acidic (pKa= 15.5). Strong bases
such as potassium hydroxide will react with cyclopentadiene to produce the
cyclopentadienide ion. The choice of solvent is important if sufficiently strong base
solutions with potassium hydroxide the are to be obtained. Two suitable solvents are 1,2dimethoxyethane (Glyme) and dimethyl sulfoxide (DMSO). These two solvents have
moderate polarity, which favors dissociation of the potassium hydroxide into potassium and
hydroxide ions.
In today’s experiment cyclopentadiene is our starting material for the preparation of the
cyclopentadienyl ion. Potassium cyclopentadienyl is then reacted with iron (II) chloride to
produce a metallocene, ferrocene, and the resulting ferrocene slurry purified by sublimation.
The reactions involved are the “cracking” of cyclopentadiene dimer (a dimer is two identical
molecules joined together).
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Cyclopentadiene can be synthesized from dicyclopentadiene (the Diels-Alder self adduct) by
"cracking" it, which just means heating it above the boiling point of cyclopentadiene.
Because the Diels-Alder reaction is a reversible one and at equilibrium, the cyclopentadiene
is removed by boiling and the equilibrium is driven to the right.
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.
Experimental Procedure
There are two parts to this experiment: cracking of dicyclopentadiene to obtain fresh
cyclopentadiene followed by the preparation of ferrocene. In your notebook, write the
balanced chemical reaction that shows what is happening in this distillation apparatus as one
of your prelab balanced equations. You will obtain freshly distilled cyclopentadiene from the
still in the fume hood. This will be used to prepare the ferrocene.
Cracking of Dicyclopentadiene.
The cracking apparatus is a simple fractional distillation set-up. Freshly prepared
cyclopentadiene is necessary to accomplish this synthesis. A source of dicyclopentadiene
will be available in one of the hoods in the laboratory.
Set up the apparatus for cracking the dicylopentadiene (this should already be in fume hood).
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The bulb of the thermometer in the 5-mL flask should extend into the mineral oil. The heater
controller should be set to "5." In about 10 min the sand bath and reaction flask will be
equilibrated to about 250 °C, and the heater control should be turned back to a setting of "34" before proceeding. At this point, you may start adding the dicyclopentadiene dropwise.
The 1,3-cyclopentadiene will be formed very rapidly and will begin to reach the thermometer
in the distillation head within 2-3 min
Continue adding the dimer dropwise from the addition funnel (1 drop every 5-10 s) once you
are collecting 1,3-cyclopentadiene in the receiving flask. Add the dimer slowly enough so
the volume in the hot round-bottomed flask does not change appreciably. The condenser
should be cool and the receiving flask should be submerged in ice since 1,3-cyclopentadiene
is low boiling and volatile. It has a pungent, characteristic odor. Collect enough 1,3cyclopentadiene (34 mL) to perform the assigned reactions
The dimer cracks slowly on heating and the monomer begins to distill steadily in the 40 42C range.
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.
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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.
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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.
http://www.ncl.ox.ac.uk/mom/ferrocene/ferrocene.html
http://courses.cm.utexas.edu/archive/Fall2001/CH431/expt_5.pdf
http://www.usm.maine.edu/~tracy/chy374/chy374labs/PrepFerrocene.pdf
CH242L
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CH 242
Student Name
Maazouz
Date
Grading Guidelines for Synthesis Lab
Check
List
Description of Grading Criteria
Name, Date, Experiment Title
2
Purpose
3
Table of Physical Properties
Chemical structures
Appropriate entries (see handout)
Table format (neatness
Organization Outline/Procedure
Neatness (done before lab)
Waste Disposal Procedure
8
8
3
Experimental Set-up
Sloppiness: spilling, breaking, handling of chemicals, etc.
Proper use of hood for ventilation
7
Experiment completed
5
Observation and Data
(completeness)
Yield: show all calculations
% yield
Spectral Data: Quality, Interpretation of Quality
12
Discussion/Analysis of ALL Data
m.p, IR, purification methods….
Conclusion of experimental Data
16
Questions (2 questions @ 3pts each)
6
Overall organization, readability, completeness
4
Total for Lab Report
Comments:
Percent
(%)
4
8
7
7
100% = 20
points
Yours