1. Cracking the Cyclopentadiene Dimer (Figure 3)

Cyclopentadiene undergoes a Diels-Alder reaction with itself to give dicyclopentadiene. This reaction is reversible, so cracking is accomplished using La Châtelier's principle to drive the reverse reaction by distilling the cyclopentadiene monomer (b.p. 42 °C) away from the dicyclopentadiene dimer (b.p. 170 °C). The dimerization reaction is slow when the cyclopentadiene is kept cold, but it must be freshly prepared to successfully synthesize ferrocene.

1. To a 25-mL round-bottom flask, add a stir bar and 10 mL of dicyclopentadiene dimer.
   CAUTION: Be careful to only use dicyclopentadiene and cyclopentadiene in a fume hood because they are both stench chemicals.
2. Attach the round-bottom flask to a fractional distillation apparatus (see "Fractional Distillation" module in the Essentials of Organic Chemistry series), and place in an oil bath on a stirring hot plate. Be sure to have the collecting flasks in an ice bath. Clamp the apparatus in place.
3. Set the hot plate to 160 °C and gently stir the solution.
4. Fractional distil ~ 5 mL of the Cp monomer from the dimer (39-42 °C).

Figure 3. Cracking of dicyclopentadiene.

2. Synthesis of Ferrocene (Figure 4)

1. To a 100-mL Schlenk flask, add a stir bar and 15 g of finely ground KOH.
   CAUTION: KOH is very corrosive and hygroscopic! The KOH should be ground in a well-ventilated hood and used immediately after preparation.
2. Add 30 mL of 1,2-dimethoxyethane to the flask while stirring.
3. Connect the flask to nitrogen and place a rubber septum over the neck of the flask.
4. While stirring under nitrogen, add 2.75 mL of cyclopentadiene via syringe. Allow this to stir for at least 10 min prior to addition of the iron.
5. While the reaction is stirring, add a stir bar, 3.25 g of ground FeCl₂·4H₂O, and 12.5 mL DMSO to a separate 50-mL Schlenk flask. Place a septum on the neck and stir under nitrogen until all of the iron has dissolved.
6. Cannula transfer the iron solution to the cyclopentadienyl solution under nitrogen dropwise over the course of 30 min. For a more detailed procedure, please review the "Synthesis of a Ti(III) Metallocene Using Schlenk Line Technique" video in this Inorganic Chemistry series.
7. Once the addition is complete, stir for an additional 30 min.
8. Meanwhile, in a beaker cool 45 mL of 6 M HCl by adding crushed ice (50 g) directly to the solution.
9. Once the reaction is complete, pour the mixture onto the slurry, and stir for a few minutes. Orange crystals should form.
10. Collect the crystals on a Büchner funnel (fitted with filter paper), and wash the precipitate with water.
11. Allow the solids to dry in the air.

Figure 4. Synthesis of ferrocene.

3. Purification of Ferrocene. Purify the product by sublimation (for a more detailed procedure, please see the "Purification of Ferrocene by Sublimation" video).

4. Characterization of Ferrocene

1. Prepare and collect a ¹H NMR spectrum of ferrocene.
2. Collect an IR spectrum of ferrocene.
3. Collect a cyclic voltammogram of ferrocene (see the "Cyclic Voltammetry" video in the Essentials of Analytical Chemistry series).