LAB 8: COLUMN CHROMATOGRAPHY
PRE-LAB:
1. Complete the Safety section for Lab 8.
2. Ferrocene and acetylferrocene are considered “organometallic” compounds. What are the
molecular formulas for these compounds?
3. Looking at the structures of ferrocene and acetylferrocene below predict how you could
differentiate the two compounds using IR.
4. Draw hexane and diethyl ether – how are they different.
PURPOSE: You will learn how to separate complex mixtures of solids or liquids and collect
the various components using column chromatography. In this experiment, the separation of
ferrocene and acetylferrocene will be done.
BACKGROUND INFORMATION:
Column chromatography is a technique in which an absorbing material (usually alumina
or silica gel) is placed in a glass column and a mixture is separated by passing it down the
column under the influence of an eluting solvent. The solution is collected as it drains from the
column and the pure component is collected by evaporating the eluting solvent. It is important to
never let the column of absorbent become dry during the process. Alumina comes in four grades
of activity depending on its moisture content.
To prepare the column, one generally fills the column (which looks like a burette without
graduation markings) to about 2/3 full with a suitable solvent. The alumina is then added slowly
with the stopcock partially opened so that the adsorbent settles evenly in the chromatography
column. It is important that the alumina be nicely distributed in the column for best separations.
Next, the mixture to be separated must be carefully added to the top of the column. This
is usually done by making a very concentrated solution of the mixture and adding it drop wise
from a dropper to the top of the column. It is important to have the minimum amount of solvent
above the column at this point so that a concentrated band of mixture can be applied to the top of
the column. Again, be careful that no part of the alumina column becomes dry.
Once the sample is placed on the column, an eluting solvent is added and the solvent
dripping from the bottom of the column is collected. If the components are colored (as in our
experiment) it is easy to see how they move down the column. Fractions are collected in a preweighed flask or beaker, the solvent is evaporated and the resulting material is weighed and its
melting point is determined.
Components of a mixture are separated based on differences in polarity and/or molecular
size. Ferrocene is less polar than acetylferrocene due to the added polarity of the acetyl group.
The acetylferrocene ‘sticks’ to the alumina more than the ferrocene when one elutes with a non
polar solvent such as hexane and this is what causes the separation.
O
C
Fe
Fe
Ferrocene
M.Mass 186, mp 172-174oC
Acetylferrocene
M.Mass 228, mp 85-86oC
CH3
EXPERIMENTAL PROCEDURE:
1) Prepare a chromatography column in the hood by placing enough hexane in the column to fill
it about 2/3 full.
2) With the stopcock partially opened, add enough powdered alumina to this column to fill the
column 2/3 full. Never let any of the alumina become dry. At this point, drain the hexane from
the column until it is about 1/2 inch above the top of the alumina. SAVE this hexane – you can
re-use it.
3) Mix 75 mg of the ferrocene - acetylferrocene mixture with about 100 mg of alumina.
4) Carefully open the stopcock to a very slow drip rate. As this is happening, add the
alumina/sample mixture to the top of the column. Add one ml of hexane at a time to keep the
column from running dry and to concentrate the sample on the top of the column.
5) Once the sample is fully adsorbed on the column add about 5 - 10 ml of hexane and continue
the elution process at a reasonable rate. Add more hexane to the top of the column as needed.
Watch the colored material as it moves down the column and when it reaches near the bottom of
the column place an empty pre-weighed beaker below the stopcock to collect the eluting
solution. Continue to collect the solution until all the colored material has been collected. Set
this container (called beaker A) aside for now.
Question: What is in beaker A?
6) Place a new container under the stopcock to collect eluting solvent and now start to add a
50/50 mixture of hexane - diethyl ether as the eluting solvent. At this point, the other colored
substance will begin the move down the column. Collect it as you collected the other sample in
a separate pre-weighed beaker (called beaker B).
Question: What is in beaker B?
7) Using a slow stream of air in the hood, evaporate the hexane from beaker A and the hexane ether from beaker B. Weigh each of these containers, determine the mass of each product and
calculate the percent recovery for each.
8) Take the melting point of each recovered sample. Dispose of the solids in the appropriate
waste container.
IMPORTANT INFORMATION ABOUT THE REPORT:
Be sure to calculate and report the percent recovery of each substance. Show your
calculations and use correct significant figures. Give your observed melting points and compare
them to the literature values. Discuss your results.
You will be given the IR spectra of ferrocene and acetylferrocene. You should include in your
discussion a paragraph that describes the two spectra and how you could use them to determine if
your column was successful.
Questions to consider:
Why is chromatography important in organic synthesis?
What is more polar hexane or ethyl acetate? What is more polar ferrocene or acetylferrocene?
How does this relate to column chromatography?
END OF EXPERIMENT.
© 2010 Carberry and Carreon