Ferrocene, C10H10Fe

Ferrocene is a molecule with a sandwich-type structure. It's an iron atom sandwiched between two five-membered aromatic rings.

Orange 5
Smelly 5
Stable in air, unlike most other metallocenes 2

Molecular weight: 186.03
Boiling point: 246ºC *
Melting point: 173-174ºC
Heat of vaporization: 74 kJ/mol 8

I'd tried to draw a text art picture, but it hasn't turned out very well. It's probably better to look at a real one from one of my references. Reference 5 has it drawn the way I can best picture it.

Ferrocene was the first sandwich-type molecule to be discovered.3 Its discovery has revolutionized the study of organometallic chemistry. 4

Ferrocene was portrayed in Nature magazine in 1952 as a planar structure of an iron atom attached to the non-conjugated carbon of two cyclopentadienes. 3

That same year, Ernst Otto Fischer proposed a cone structure (which looked like two cones stacked vertically, point to point) as the structure of ferrocene. At the same time but independently, Geoffrey Wilkinson and Robert Woodward discovered the sandwich structure of ferrocene as the correct one. Because ferrocene was air-stabilized, it had to have a stable electronic configuration, which was not the case with the previously proposed structure. 2 Their work was verified when NMR data indicated all the carbons as equivalent and IR data indicated all the C-H stretches as equivalent. 3

Like benzene, the cyclopentadienal rings of ferrocene often reacted in electrophilic substitutions, so the molecule was named ferrocene, the "ene" part analogous to benzene. 2

Wilkinson and Fischer received the Nobel Prize in Chemistry in 1973 for their discovery of ferrocene. 1

In sophomore organic chemistry lab last spring, I synthesized ferrocene and used some of the product to synthesize a derivative, benzoylferrocene.

2 Cyclopentadienal anions + Fe2+ ----> ferrocene

Ferrocene + AlCl3 + benzoyl chloride (C6H5COCl) ----> benzoylferrocene

It was important that the reactions not be exposed to air before their completion. The cyclopentadienal anions decompose in air, and the ferrous ion, Fe2+, is oxidizer in air. Because there's no way a sophomore organic chemistry laboratory could provide a glove box for each student to perform the oxygen-free experiment, we covered our test tubes with rubber tops and bubbled nitrogen into the liquid with syringes. 5

Procedure 3
Ground potassium hydroxide (KOH) was combined with liquid dimethoxyethane (DME, C4H10O2) in a test tube. Nitrogen was bubbled through to ensure that no air was in the test tube.

Crushed iron (II) chloride (FeCl2) was combined with dimethylsulfoxide (DMSO, (CH3)2SO) in a separate test tube. Nitrogen was also bubbled through this test tube.

Cyclopentadiene was added to the KOH/DME test tube using a syringe. The hydroxide part of the KOH reacted with the cyclopentadiene by removing a hydrogen, creating a cyclopentadienal anion. The DMSO mixture was then added to this test tube by syringe. The ferrous ion from the FeCl2 reacted with the cyclopentadienal anion to produce ferrocene.

After the reaction was complete, the opaque solution was added to a beaker containing ice and hydrochloric acid (HCl). This neutralized excess KOH. The ferrocene crystals were separated by filtration. The crystals could have been purified by sublimation at this point.

Ferrocene can undergo the reactions that aromatic compounds can normally undergo, including electrophilic substitution, nucleophilic substitution, addition, etc. In the conversion of ferrocene to benzoylferrocene in my lab, the Friedel-Crafts acylation, a type of electrophilic substitution, took place.

The ferrocene produced was dissolved in benzoyl chloride. Aluminum chloride (AlCl3) was added. The mixture was shook and allowed to react for a few minutes.

Distilled water, followed by hexane as a solvent, were then added. The mixture was separated by column chromatography. The ferrocene created a rust-colored band, and the benzoylferrocene created a yellow-colored band of liquid. 3

Relieved to have finished the arduous year of organic chemistry class and laboratory, I thought I'd never think about ferrocene again. That was not the case.

This summer, I worked in a combustion laboratory in the Environmental Engineering department at Washington University. My research necessitated inserting iron particles into a flame by passing them through a piping system with some other gases and into a porous sphere. The mixture passed through the pores in this sphere; thus, the iron had to be in gaseous form, along with everything else.

Obviously, solid iron could not practically be sublimed and released into the flame. Ferrocene, however, was eligible for use in the project, since it takes very little heat to sublime ferrocene. I heated ferrocene to only 80-140ºC and produced more than the desired amount of ferrocene vapor. When the ferrocene entered the flame, the hydrocarbon parts were consumed in the flame, and the iron particles clumped together. 5

Ferrocene is often used as a fuel additive. It preserves diesel fuel, produces leadless gasoline, eliminates smoke in fuel oils, improves power, and cuts down on oil consumption. 6

Its derivatives are also useful. For example, Fe(C5H5)2+ is used in a pen meter to measure blood glucose levels for those with diabetes. 7

1. Chemistry in Action. http://www.ul.ie/~childsp/CinA/Issue49/fam_chem.html
2. http://stanley.chem.lsu.edu/webpub/4571-Notes/chap9-Cp.doc
3. Organic Chemistry Laboratory Manual. Washington University. Spring 2003.
4. Wooley, Karen, Ph.D. Washington University Chemistry Professor. Chem 257 Lecture 03-17-03.
5. Personal Experience. See my final research project report on http://www.env.wustl.edu/REU/2003/Students/Lankester/Joanna_Lankester.htm
6. AAA International Corporation. http://www.aaainternational.com/T/ferrocene.html
7. www.chem.gla.ac.uk/~louis/chem2x/lect5.doc
8. National Institute of Standards and Technology Chemistry Webbook. http://webbook.nist.gov/chemistry/

* - I've read 249ºC elsewhere, though.

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