In this exercise we will be separating a mixture of Ferrocene and Acetyl Ferrocene by Column Chromatography.
Ferrocene is an organometallic compound in which Iron(II) is coordinated to two cyclopentadienyl anions:
Once you have read the text, you will need to prepare your notebook with the following:
The purpose of this lab is to gain familiarity with the column chromatography technique.
Table of Reactants/ Products/ Solvents
Need ferrocene, acetylferrocene and solvents
Make this an outline of what you will do, not how you will do it.
Leave room to the right of the procedure for notes/ observations/ changes on the day of the experiment.
The procedure is not in your textbook, but rather is given below (pdf version):
We will be separating 40 mg of a mixture of ferrocene and acetyl ferrocene.
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 micro-glassware 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, 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. 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 minimum amount of dichloromethane to 60 mg of the
acetylferrocene:ferrocene mixture in order to dissolve the solid. Add approx. 200 mg of
alumina, stir, and evaporate the dichloromethane solvent completely by warming with swirling over a hot water bath. Now the sample can be 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 in such a manner that the top surface of the column is not disturbed. Run the solvent down near to the surface of the adsorbent several times to ensure that the sample moves down the column in as narrow a band as possible. Once the solvent above the adsorbent is clear (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.
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 by adding an equal portion of t-butylmethyl ether to the hexane. This new eluent
will carry down the acetylferrocene. Change flasks at this point and label it #2.
Once you see the red acetylferrocene eluting from the column, you will change flasks again and label this last fraction #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!
You can recycle solvent as needed up until the actual elution of the mixture begins.
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 is emerging 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.
Once the separation is complete, place the contents of fractions 1-3 in the FERROCENE/ACETYL FERROCENE recovery bottle so that it may be recycled. (NOT the organic waste bottle)
Dispose of pure ferrocene and acetyl ferrocene in bottles labelled as "recycled ferrocene" and "recycled acetyl ferrocene" containers so they can be used again.
Place TLC plates and alumina from the column in the plastic container for solid waste.
Alumina can be ejected from the column into a waste beaker with a blast of air through a
tygon hose held to the bottom of the column. Discard TLC solvents in the container for Organic waste.
During the lab:
The preparation of the column is tricky, but the separation itself is foolproof.
Don't allow any dry alumina at your lab bench. Alumina is toxic when inhaled. Try to minimize the solvent vapor in the room by keeping all solvents capped whenever possible.
Ferrocene sublimes, so be careful when removing the solvent from fraction 1 not to overheat the solution. You may use a hot water bath and a stream of nitrogen to evaporate the solvent. Weigh your ferrocene product.
Be sure to sketch your TLC results in your notebook. The plates themselves can be saved temporarily in a plastic bag until you are finished measuring the Rf's.
Complete the following summary sheet and turn it in along with a copy of your data/observations: word
This lab is due one week from the day that it is scheduled.
|©Kathleen Armstrong||May, 2009|