Sandra K. Koster, Ph.D. Lecturer
Department of Chemistry
4003 Cowley Hall
Office Hours: Summer hours are by appointment
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Ferrocene is a molecular complex of two cyclopentadienyl rings with
an iron atom. A "sandwich" structure has been established for ferrocene
in which all carbon atoms are equally bonded to iron. Ferrocene can be
viewed formally as a complex of two cyclopentadienyl anions and an Fe (II)
ion. The Fe (II) ion which has 6 valence electrons accepts 6 electrons
from each of the 2 cyclopentadienyl anions to achieve the 18 electron inert
gas configuration. Alternatively, ferrocene can be thought of as a complex
of 2 cyclopentadienyl radicals which donate 5 electrons each to an iron
(0) atom which has 8 valence electrons; this description also gives iron
an 18 electron inert gas configuration. (NOTE: Ferrocene is a non-polar
compound and the drawing below is not meant to imply that it is an ionic
Ferrocene is an orange crystalline solid that is stable to 400 0C and is resistant to acids and bases. Like benzene, ferrocene readily undergoes electrophilic substitution. In this experiment, we will acetylate ferrocene and separate the product from the starting material and diacetyl ferrocene using column chromatography.
Acetylation of Ferrocene. In a 50 mL round-bottom flask, place 0.40 g of ferrocene, 1.5 mL of acetic anhydride and 10 drops of 85% phosphoric acid. Equip the flask with a reflux condenser and a drying tube. Heat for 10 minutes on a steam bath. Pour the reaction mixture onto 5 g of ice in a 250 mL beaker. Use the liquid in the beaker for washing and a bent micro spatula for scraping to transfer the reaction mixture as completely as possible. Stir the mixture for a few minutes with a glass rod, then add 10.5 mL of 10% NaOH (mixture will still be acidic). Then add solid sodium bicarbonate slowly with stirring (watch for foaming) until the mixture is not acidic to litmus paper. Do not add a large excess of the NaHCO3. Stir and crush all lumps, allow the mixture to stand for 20 minutes, then collect the product by suction filtration on a Hirsch funnel and wash with water. Dry by pulling air through the solid on the Hirsch funnel for 10 to 15 minutes.
Column chromatography involves
the separation of compounds by the same mechanism as other chromatographic
techniques, i.e. differences in partitioning between mobile and
stationary phases. (See "Chromatography" in Pavia). It is like the other
methods in that a stationary phase is placed in a support through which
the mobile phase is passed. The stationary phase serves as an adsorbent.
Many compounds with varying functional groups may be used as the stationary
phase and several types of interactions can aid in developing the desired
separation (i.e. hydrogen bonding, electrostatic interactions, Van
der Waals forces, etc.) The major advantages of column chromatography are
its ability to handle large amounts of material and the ability to change
the eluting solvent throughout the course of the elution. This allows one
to remove impurities while the desired product remains essentially unmoved.
Then a solvent change moves the desired product through the column. Solvent
changes may include such things as changes in polarity, changes in pH,
or changes in ionic strength. The last two are used largely in biological
separations. Thus, by varying the stationary phase and by changing solvent
or solvent systems, an efficient separation may be achieved. In this experiment
the alumina separates components primarily on the basis of polarity; the
more polar components are held to the alumina more tightly and therefore
move through the column more slowly. Increasing the polarity of the solvent
moves all components faster.
Column Preparation. A clean, dry Chromatography column or 25 mL buret is aligned vertically (very important) and filled with 15 mL of petroleum ether (bp 30-60 0C, mostly pentanes). If there is no frit at the bottom of the column a very small plug of glass wool is pushed to the bottom of the column and a thin layer of about one cm of sand is added. Alumina (8 g) is slowly added and allowed to settle. Alumina is washed from the walls of the column with petroleum either. One cm of sand is added carefully to the top of the column. (If a glass column is stoppered tightly, it can be stored at this stage until the next lab period.) The stopcock is opened and the solvent is drained until the top layer of sand is just covered by the solvent.
Chromatography of the Reaction Mixture. Set aside a small sample (15 mg) of your crude product for TLC analysis. After the solvent is drained down to the sand, the rest of the crude product is dissolved in a minimum amount of warm methylene chloride, CH2Cl2, (1 mL) and gently added to the top of the column. Again, solvent is drained until the sand layer is just covered. To the column is then added 15 mL petroleum ether (30-60 0C) and the reaction mixture is eluted with petroleum ether until the yellow ferrocene band begins to come off the column. The solvent system is then changed by eluting with 20 mL of 10% ether-90% petroleum ether and then with a 50%-50% mixture of the solvents as needed. The solvent from the elution of the orange band of acetylferrocene is collected separately and rotary evaporated or evaporated under a dry stream of nitrogen in the hood. The purity of the compound is checked by tlc and mp, and the yield is recorded.
Turn in your sample in a appropriately labeled vial.
Thin Layer Chromatography.
thin layer chromatography (10 cm x 4 cm) plate is obtained and a horizontal
pencil line is gently drawn about 1 1/2 cm from the bottom of the plate.
On this line are spotted three solutions at equidistant intervals, each
of which is made by dissolving a few crystals of material in 1 mL of ether.
Crude product, final product and ferrocene respectively are used to make
up the three solutions. After the spots have dried, the plate is set in
a jar containing enough 10% ether-90% petroleum ether to wet the bottom
of the plate without submerging the spots. A piece of filter paper is also
placed against the inside of the jar to allow the atmosphere of the jar
to become saturated in solvent vapors. The sides of the tlc plate should
not touch the filter paper. The jar is closed and solvent is allowed to
move 80% up the tlc plate by capillary action. The plate is then removed,
the solvent front is marked in pencil and the plate allowed to dry. Visible
spots should be circled in pencil. The plate is then allowed to sit in
a jar saturated with I2 vapors
for a few minutes and any additional spots marked. The retention fraction
(Rf value) for each spot
should be calculated and the spots identified if possible.
A sketch of your plate should be included in your notebook.
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last modified 5/18/00