Experiment 6
The Separation of Ferrocene, Acetylferrocene and
1, 1-Diacetylferrocene by Column Chromatography
Background
In 1951 Kealy and Pauson accidentally discovered ferrocene 3 by the attempted oxidation of
cyclopentadienylmagnesium bromide 1 with anhydrous ferric chloride. Instead of the expected
product, fulvene 2, a compound that was soluble in organic solvents and contained iron was isolated.
Almost simultaneously Miller, Tebboth, and Tremaine isolated the same compound from the
reaction of cyclopentadiene vapor with iron reduced at 300oC. The structure of ferrocene was shown
to consist of two cyclopentadiene rings with an iron atom sandwiched between. Within five years of
its discovery over 100 papers concerning the chemistry of ferrocene had been published.
MgBr
2
1
FeCl3
Fe
3
2
ferrocene
Of the many syntheses devised for ferrocene, the most satisfactory laboratory synthesis is the
reaction between ferrous chloride and cylcopentadienylsodium in THF or 1,2-dimethyoxyethane as
solvent.
FeCl2
+
2
Fe
Na
+ 2 NaCl
85-95%
Similar sandwich compounds with other metals (metallocenes) as well as substituted ferrocenes may
be prepared by this method.
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Ferrocene is unusually stable for an organometallic compound (stable to 500oC). The stability is
attributed to the sharing of the 12 -electrons of the cyclopentadienyl rings with the six outer shell
electrons of iron (II) to afford a stable 18-electron noble gas configuration around iron. When other
metals form similar compounds, known as metallocenes, they also react to achieve the 18-electron
configuration.
Because ferrocene is much more reactive than benzene (3.3 x 106 times in acetylation) it may be
acetylated under milder conditions using acetic anhydride and phosphoric acid as catalyst. When
acetic anhydride is used the major product is acetylferrocene with 1,1'-diacetylferrocene and
ferrocene as impurities.
O
O
CH3
Fe
O
O
O
CH3
H3PO4
CH3
Fe
+
Fe
CH3
heat
O
major
H3C
minor
1,1'-Diacetylferrocene may be prepared under more vigorous reaction conditions: acetyl
chloride/aluminum chloride; a mixture of mono- and diacetylferrocenes results.
O
O
Fe
2
CH3
Cl
Fe
CH3
O
+
Fe
CH3
AlCl3
CH2Cl2
O
minor
H3C
major
Thus, whichever procedure we use, there is always a mixture formed. We need to devise a way to
separate small amounts of the acetylated ferrocenes. Recrystallization is a useful technique in this
regard. However, we rarely ever isolate the minor components in a recrystallization. They
constitute impurities in the mother liquors and as such we discard them. Column chromatography,
on the other hand, is ideal for separating a small but macroscopic amount of a mixture into its
components. Provided our observation of the conditions for separating a mixture by TLC is correct,
we should obtain a comparable separation of a macroscopic sample using column chromatography.
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Column Chromatography
Column chromatography, as the name suggests, is carried out in a glass column, usually with a
stopcock like that of a buret. The column "packing" is usually silica gel which is of a lower mesh
size (i.e. more granular, providing less overall surface area) than that found on TLC plates. Hence,
separation of compounds with very similar Rf’s on a column is often less complete than that
achieved on a TLC plate.
Just as in TLC, the solvent "elutes", only now from top to
bottom. The objective is not only to separate the compounds on
the column of silica gel but also to collect each purified sample
as it exits the column through the stopcock. As long as fresh
solvent is applied, the column keeps "running", unlike TLC
where the eluting solvent is prevented from getting to the top of
the plate. Therefore, we can separate macroscopic samples of
solids (or sometimes liquids) from a mixture although not with
the same purity as exhibited by the spots on a TLC plate
The picture at the left shows a typical column. The column is
packed with silica gel, usually introduced as a slurry in the
eluting solvent. The solvent is allowed to run down to just
above the level of the silica gel. Gentle tapping on the glass
column while the solvent is running through the glass frit
causes the silica gel to pack more uniformly and more rapidly.
A concentrated solution of the mixture in the elution solvent is
carefully applied to the top of the silica gel column and allowed
to run down into the silica gel by opening the stopcock. Then
eluting solvent is, at first, CAREFULLY applied to the silica
gel, and afterwards filled up to the top of the column. The
solvent then drains through the stopcock. If the compounds are
colored the progress of the separation is evident as the bands separate, the higher Rf compounds
travelling faster down the column than the lower Rf ones. The analogy of an upside-down TLC plate
is evident from the progress of the "bands" of compounds. The solution containing the desired,
purified component is collected in a flask, and the solvent is removed on a rotary evaporator leaving
the purified solid behind.
Another good way of introducing the mixture to be separated to the top of the column is by
dissolving up the mixture in a round bottom flask in a minimum of a volatile solvent in which it is
very soluble. Then an amount of silica gel, enough to soak up the solution, is added. The solvent is
then removed to complete dryness on a rotary evaporator. This is really equivalent to applying a
spot of the mixture on a TLC plate. This method is quite effective for most mixtures.
Column chromatography is often used in place of recrystallization where complex mixtures are
involved. It is adaptable to milligram quantities all the way up to several tens of grams, the amount
of silica gel, and the size of the column being the only limitations. However, since the particle size
of the silica gel used in columns (usually 70 to 230 mesh) is much larger than that found on most
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TLC plates, the separations on columns are less resolved than those on TLC plates. This limitation
is resolved by decreasing the amount of the polar component in the eluting solvent. For example, if
a good separation on a TLC plate is achieved using 10% ethyl acetate in hexanes a good separation
on a column may be achieved with 5% ethyl acetate in hexanes. A good rule-of-thumb is: halve the
polar component of your TLC eluting solvent when you want a comparable separation on a column.
Pressure Applied to the Column
In flash chromatography pressure is applied to the column. This
speeds up the operation and gives a better resolution compare to the
slow elution observed without pressure. In the research lab
compressed air or nitrogen is used in conjunction with a pressure
regulator. In this lab balloons (double balloons) will be used to
provide pressure. The balloons may be filled with compressed air
from the dispensing hoods.
Often, applying pressure to the top of the column can speed along a
chromatography. When pressure is applied it is called flash
chromatography.
The Experiment
Column Chromatographic Separation of Ferrocene, Acetylferrocene and 1,1'Diacetylferrocene
WASTE: All used hexane and ethyl acetate should be poured into the waste collection bottle in hood
Z.
Check out a column from the organic prep room window-return it when finished.
Read the procedure carefully before beginning. Tare the round
bottomed flask in which you will collect the eluate.
Pouring the column: Weigh 10 g of silica gel into a 150 mL
beaker. Add approximately 30 mL 80:20 hexanes-ethyl acetate to
make a slurry. Mix well with a glass rod. Add the slurry to the
clamped chromatography column with the stopcock closed. Use
two clamps on the column and make sure it is vertical. After all
the silica gel has been added, let the solvent drain till the level is to
the silica gel. Tap the column gently but frequently to settle the
silica gel faster. Avoid air bubbles in the silica gel layer and do
not let the column go dry. The solvent dripping through the
column may be used to rinse the beaker containing the silica gel.
At this point, the silica gel layer should be between 10 and 15 cm
high.
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There are several methods to prepare a sample to place on the column. Generally, the sample is
dissolved in a small amount of the eluting solvent and the solution is applied to the column. In
this experiment, the ferrocene mixture is too insoluble in ethyl acetate-hexanes. Instead, the
ferrocene mixture will be dissolved in a methylene chloride, a more polar solvent. A small
amount of silica gel is then added to form a slurry. The methylene chloride is evaporated leaving
behind the silica gel containing the ferrocene mixture. The resulting silica gel is finally by
applied to the top of the column.
Place 0.2 g of the ferrocene mixture in a 25 mL round bottom flask. Add approximately 2 mL
(1/2 pipette) methylene chloride to dissolve the sample. Add approximately 0.5 g of silica gel
and mix well. Evaporate the methylene chloride on the rotary evaporator for about 5 minutes.
Allow the flask to rotate for 20-30 seconds before lowering it into the warm water. The water in
the bath should be slightly above room temperature otherwise the silica gel may shoot up into the
rotary evaporator.
Release the vacuum slowly and completely before removing the flask or air currents may enter
the flask when you remove it. The air currents will cause the silica gel to be scattered and lost.
Scrape the silica gel in the flask with a spatula to dislodge silica gel that may be adhering to the
glass and transfer it to a piece of creased weighing paper.
Applying the sample: Drain the solvent in the column so that it is level with the silica gel. Using
a paper funnel, as demonstrated in the prelab, add the fereocene-silica gel to the top of the
column distributing it evenly on the surface. Once the sample has been applied to the column the
following operations should be done in succession without delays. At no point should the
column go dry. Rinse the sides of the column with a small amount (1-2 mL) of the solvent using
a 9-inch pipette. Attach the balloon (shown below) and apply pressure with the stopcock open
until the solvent is level with the bed. Remove the balloon and add 2-3 mL solvent with the 9inch pipette and attached the balloon again (stopcock open). Repeat this 5-6 times so that the
ferrocene mixture has moved into the silica gel bed. The developing solvent (hexanes-ethyl
acetate 8:2) may then be added to halfway fill the column, the balloon attached with pinch clamp
open, and the column stopcock opened. Collect the eluate in a flasks or beakers switching to
tared round-bottomed flasks when the ferrocenes elute.
The separation does not take place unless the eluting solvent is passing through the column. The
least polar compound in the mixture, ferrocene, should elute almost with the solvent front. When
the acetylferrocene has been completely eluted, switch to hexanes/ethyl acetate (4:6) to move the
1,1'-diacetylferrocene more quickly. The separation should take no more than 15 minutes-hence the
name “flash chromatography”. Pour the hexanes/ethyl acetate eluate, collected between the colored
fractions, into the hexanes/ethyl acetate recovery bottle.
Returning to the elution operation now, you will notice, as each component elutes, some of the
solvent evaporates at the tip, leaving solid behind. Wash this solid off the tip into the flask with
ethyl acetate.
When all three fractions have been collected, evaporate the solvent on the rotary evaporator in a dry
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tared round bottom flask. It may be necessary to evaporate a solution portion-wise since a roundbottomed flask should not be filled more than 2/3. Residual solvent may be removed by attaching a
vacuum adapter and applying a vacuum from the water aspirator while heating on the steam bath.
Calculate the masses of ferrocene, acetylferrocene, and 1,1'-diacetylferrocene. Determine the percent
composition of the mixture.
Scrape out as much of the ferrocenes from the round-bottomed flasks as you can and place them into
the labeled beakers on the lab desk.
Flash Chromatography was introduced by W.C. Still and reported in the following article. The
method is widely used in organic laboratories. Still, W.C. J. Org. Chem. 1978, 43, 2923.
The Report should contain:
1.
2.
3.
4.
5.
6.
Title
Purpose
Structures of compounds
Procedure (reference and outline)
Observations
Results
ferrocene (mg)
acetylferrocene (mg)
diacetylferrocene (mg)
7. Conclusion
Possible Pre-Lab Quiz Questions:
1.
What is the purpose of this experiment?
2.
Compound A has an Rf of 0.75 and compound B has an Rf of 0.35. Which compound will
elute first from a silica gel column using the same solvent system as in TLC?
3.
Two of the substances used in this experiment, acetyl and diacetylferrocene, contain the
acetyl group. What is an acetyl group?
4.
Column chromatography is capable of yielding extremely pure substances. What accounts
for this? [Hint: it is common practice to collect many fractions is test tubes when performing
column chromatography on non-colored materials.]
5.
During the chromatography the solvent is changed from hexanes/ ethyl acetate (8:2) to
hexanes/ ethyl acetate (4:6).
(a) What is the reason for doing this?
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(b) Which is more polar?
(c) How would you prepare 200 mL of hexanes/ ethyl acetate (8:2)
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