Ferrocene – Structure & Chemistry
Ferrocene, Fe(C5H5)2, was first prepared in 1951. It was the first metal “sandwich” compound to be discovered, and its unusual structural and chemical properties were largely responsible for the growth of organometallic chemistry as an independent branch of study (organometallic chemistry is the study of molecules containing metal-carbon bonds).
Structure. The two five-carbon rings in ferrocene are parallel, and the ten carbon atoms are all equidistant from iron suggesting the presence of ten equivalent FeC bonds (below left). Experimental measurements show that the CC bonds in ferrocene are actually shorter and stronger than normal CC single bonds. The situation is similar to that found in benzene, a molecule that contains six equivalent, short, strong CC bonds (below right). Organometallic chemists draw a circle inside each ferrocene ring to reflect this bonding (below center). Note the similarity between this drawing and the analogous drawing of benzene.
The FeC bonds in ferrocene are not ionic, yet ferrocene is prepared by combining three ions, two C5H5– (cyclopentadienyl anion) and Fe+2 (see below). Cyclopentadienyl anion is prepared by a simple acid-base reaction in which a strong base is used to deprotonate 1,3-cyclopentadiene (pKa ~15). It may surprise you to learn that a neutral hydrocarbon can be as strong an acid as water. This behavior can be explained by noting that the conjugate base, cyclopentadienyl anion, contains 6 pi electrons and is predicted to be aromatic by Huckel’s rule (Sorrell p. 564-565).
The reaction that you will investigate in this experiment is one that is typical of aromatic systems: Friedel-Crafts acylation. Phosphoric acid and acetic anhydride are expected to form small amounts of a short-lived cation, CH3CO+. This cation attacks aromatic rings as an electrophile and forms a delocalized cation that subsequently loses a proton and forms a neutral product (Sorrell p. 572-3). In this case, the neutral product would be acetylferrocene. Since ferrocene contains 10 identical hydrogens, you might wonder whether polyacetylated ferrocenes can form. This possibility will be investigated as part of this experiment.
Chromatography – Overview
This experiment uses two chromatographic techniques, dry-column flash chromatography and thin layer chromatography (TLC), to separate compounds. Each one serves a different purpose. The dry-column flash method is a good preparative technique because it can process more material. We can actually purify all of our product using this technique. TLC, on the other hand, is a good analytical technique because it can be used with smaller amounts of material and provide information about the purity of these samples. The following sections, together with material in Padias, provide background information for these closely related methods.
Thin layer chromatography (TLC)
Read Padias, p. 164-174 to learn about chromatography in general and TLC in particular.
TLC can be applied in many ways and you will try out some additional applications of TLC in other experiments. This experiment, however, uses TLC only to “monitor a column-chromatographic separation” (Padias, p. 166). That is, you will use TLC to analyze fractions collected from the dry column. Your TLC results will tell you if a fraction contains only substance or multiple substances, and if only one substance, whether that substance is ferrocene or acetylferrocene.
Padias covers all of the important aspects of TLC:
- selecting a TLC “plate”
- spotting the plate
- developing the plate
- visualizing the plate
Because TLC is a flexible method, it can be performed in several ways and this is reflected in Padias’ description. To make things a little less ambiguous, here are some additional comments that relate to this particular experiment:
- plate selection
- We will provide you with plastic plates that have been coated with silica gel on one side.
- Silica gel readily absorbs solvent vapors in the lab, so do not attempt to store plates in your desk. Take only what you need.
- We will provide you with commercial capillary tubes. These tubes hold only a small amount of liquid, but if all of that liquid is deposited on a plate, the spot will be much too large. Small spots are required.
- Fractions collected from a chromatographic column are often too dilute for easy analysis. Therefore, it may be necessary to reapply liquid from one fraction multiple times to the same spot to build up enough sample for visualization. To keep the spot small, let it dry before you apply more liquid.
- As a rule, you will spot at least three samples on a plate and the middle spot will be a standard sample, such as authentic ferrocene. Padias Figure 3-28 shows several multispot plates. Avoid making plates like the upper drawings; the outer spots are too close to the plate edge. The three spots in the lower drawing are spaced just right.
- Take another look at Padias Figure 3-28. Notice that the spots are above the level of the developing solvent.
- Instead of using a jar for developing, use the tall beaker in your drawer that lacks a pour spout. Cover it with a small watch glass.
- Do not fiddle with your beaker while developing a plate. This will ruin your results.
- The compounds of interest, ferrocene and acetylferrocene, are both colored and can be detected simply by looking at the plate. Later on we will use UV and chemical detection methods for visualizing “invisible” substances.
- Instead of measuring and recording Rf values, you should make a drawing in your lab notebook that exactly duplicates your plate. That is, it should accurately reproduce the size of your plate, the locations of the initial spots, the locations and sizes of the final spots, the position of the solvent “front” at the end of the analysis. You should also include the developing solvent and label each lane. Padias Figure 3-29 shows what is expected (just leave off the distance measurements and calculated).
Dry-column Flash Chromatography
Read Padias, p. 174-181 to learn about column chromatography. You might also want to skim the sections that describe some other closely related techniques: flash chromatography, HPLC, and supercritical fluid chromatography.
Dry-column flash chromatography is related to column chromatography, but it differs in many important respects: the experimental apparatus, how we prepare the column, how we load the column, and how we run the column. (Is there anything left?) The dry-column flash method is much faster and more effective than the simple column method, and it is cheaper and safer than (but not as effective as) the widely practiced flash method.
The defining feature of the dry-column flash method is that suction is used to drain the “column” dry between fractions. The following description is adapted from two sources: L. M. Harwood, Aldrichimica Acta, 18, 25 (1985) and J. T. Sharp, I. Gosney, A. G. Rowley, “Practical Organic Chemistry”, Chapman and Hall, 1989, pg. 160-163. It has also been described in A.J. Shusterman, P.G. McDougal, A. Glasfeld, Journal of Chemical Education, 74, 1222 (1997) (DOI 10.1021/ed074p1222). Students and teachers who would like to use this technique to separate other mixtures (and use this technique on a different scale) should consult one of these other sources for more general instructions.
Briefly, dry-column flash chromatography requires you to set up a sintered glass funnel as if for a vacuum filtration (see below) and pack dry TLC-grade silica gel into the funnel.
The combination of funnel and silica gel is referred to as a “column”. You apply your sample mixture to the top of the column, and pass solvent mixtures of gradually increasing polarity, one at a time, through the column while you apply suction. The solvent is collected in a test tube (located inside the filter flask) and the size of the test tube dictates how much solvent you pass through the column.
The contents of each solvent “fraction” is then analyzed by TLC to determine its contents, and the fraction is worked up as deemed appropriate.