SEPARATING A MIXTURE OF FERROCENE AND ACETYL FERROCENE BY COLUMN
CHROMATOGRAPHY
We will be separating 40 mg of a mixture of ferrocene and acetyl ferrocene.
TARE the RECEIVING FLASKS
You will need to tare the three receiving flasks before you begin. Label them #1,#2 and
#3. If you skip this step you will not be able to calculate your percent recovery and you
will lose points.
PACK THE COLUMN
Rather than using a Pasteur pipet as your chromatography column, use the
Williamson micro-column with stopcock and filter tip contained in your microglassware kit. While standing at the hood, fill the chromatography column about 2/3 full
with Alumina and then pour the Alumina into a 10 mL Erlenmeyer flask. Add about 8
mL of hexanes to this flask (enough to cover the alumina well), cover and take to your
lab bench along with approximately 50 mL of hexanes in your TLC jar (capped). At
your lab bench, secure the column above a 125 mL Erlenmeyer flask, with the stopcock
closed. Add hexane to the column until it is nearly full, then transfer the slurry of alumina
in hexane using a plastic Pasteur Pipette whose tip has been clipped with sissors to create
a larger opening. To avoid overfilling the column, open the stopcock to allow the solvent
to drain as the slurry is being added to the column. Continue adding the slurry until the
column is one-half to two-thirds full of adsorbent and there is still at least 1 cm of hexane
above the level of adsorbent. The eluted hexane can be reused for this purpose.
(NOTE: Do not ever let the hexane level fall below the top of the silica gel!).
PREPARE THE SAMPLE
In the hood, add a small amount of dichloromethane to 40 mg of
the acetylferrocene:ferrocene mixture in order to dissolve the solid. Add approx. 120 mg
of alumina, stir, and evaporate the dichloromethane solvent completely by warming
with swirling over a hot water bath. Careful not to stop swirling lest the solution "bump",
spilling your contents all over the hood! Once the sample is very dry, transfer it to a
weighing paper so that it can be easily added to the column.
ADD THE SAMPLE TO THE COLUMN
Lower the solvent to just above the adsorbent level (about 1 cm) and add the
solid ferrocene/acetyl ferrocene/alumina mixture to the top of the column through the
funnel. Wash it down with a few drops of hexane, and then tap the column to remove air
bubbles. Open the valve, and carefully add new solvent dropwise as the eluent is
dripping out the bottom of the column. Try to keep no more than 1 cm of solvent above
the stationary phase. Add the drops of solvent to the side of the column so that the top
surface of the column is not disturbed. Once the 1 cm of solvent is no longer colored,
(meaning the solute has moved further down the adsorbent column), fill the column to the
top with the solvent and continue eluting the sample from the column. You can recycle
solvent as needed up until the actual elution of the mixture begins.
CHANGE THE SOLVENT POLARITY
Once you see a white band above the eluting layer (which suggests a separation of the
two components), change the polarity of the mobile phase to a 50:50 t-butylmethyl ether
:hexane. This will speed up the elution of the ferrocene and help to carry down the
acetylferrocene. Once it appears as though the yellow ferrocene is through the column,
but before the acetyl ferrocene begins to elute, change the receiving flask to flask
#2. Once you see the red acetylferrocene is about to elute from the column, you will
change flasks again to flask #3. Remember that the column must always be covered
with solvent; NEVER LET THE COLUMN DRY OUT—THIS CREATES
CHANNELS THAT RESULT IN UNEVEN SEPARATION OF THE MIXTURE.
TLC
While one partner continues with the elution of the column, prepare a TLC chamber with
a 25:75 mixture ether:hexane. During the course of the chromatographic separation, you
will need to spot two plates with the eluted solutions as well as with a standard
ferrocene solution. The TLC allows one to determine when a solute has emerged from
the column in cases where the solute is not colored (which is most of the time). It also
serves to confirm the identity and purity of fractions 1 and 3. If fraction two is pure
ferrocene, combine it with Fraction one. If it is pure acetyl ferrocene, combine it with
Fraction Three. Otherwise, discard it in the organic waste container. Be sure to sketch a
picture of your TLC plates in your notebook. Also measure the Rf of each spot carefully
before you throw the TLC plates into the inert waste container.
EVAPORATE SOLVENT
Once the separation is complete, Label fractions 1 and 3 with your name and place them
open in the fume hood so that the solvent may be evaporated overnight. You will need to
weigh the dry flasks and calculate the % recovery of each component in the next lab
period just prior to turning it in.
WASTE HANDLING
Dispose of any solvents, pure ferrocene and acetyl ferrocene in the organic waste
bottle.
Alumina can be ejected from the column into the marked waste beaker for
alumina with a blast of air through a tygon tubing held to the bottom of the column. TLC
plates may be discarded in the Inert Waste Plastic Bag.