Please make sure that you read the chapter about how to set up glassware for reaction under inert gas (page 81-89 in reader, Winter 2011) with emphasis on the distillation setup and the use of the Schlenk line. Those are important topics that should be familiar to you after Thursday's introduction.You should feel confident to set the distillation setup up when needed without spending four hours on it.

Step 1: (This step should be done as a group!)

It might be a good idea to use either a 500 mL Schlenk flask or a 500 mL three-necked flask to hold the dimer in the beginning. This would allow you to pour (or inject the dimer) after you are done flame drying the setup.

The Vigreux column (= the long glass tube with the spikes on the inside) is necessary to ensure separation of the monomer (b.p.: 40 ^oC) from the dimer (b.p.: 170 ^oC). The temperature required for the cracking of the dimer is about 180 ^oC.

Although cyclopentadiene is not extremely air-sensitive (but it is very flammable!), it does not hurt to flame dry the distillation setup since the presence of oxygen causes problems in the formation of KCp. The anionic species is extremely air sensitive and even trace amounts can cause problems. Pure potassium cyclopentadienide (KCp) is white. The more oxygen is present during the formation, the darker the color will be. Traces lead to a pink product, which gradually goes over purple to dark brown due to increased polymerization and oxidation.

Even though the monomer does not polymerize directly, it should be kept at low temperatures (preferably -78 ^oC) until used.

The distillation should be slow (1-2 drops per second) and be monitored by a properly placed thermometer at the head of the distillation. The heating mantle has to be plugged into the Variac (under the fume hood) and not straight into the wall plug! The setup has to be open to the Schlenk line in order to release the pressure upon heating (=vent!). Some cooling could not hurt either (in at the lower end, out at the upper end).

Do not distill the flask to dryness! Alkenes (like ethers) tend to form peroxides which usually have higher boiling points than the alkenes (why?) and that are extremely explosive!


Step 2: (to be carried out under inert gas)

The DMSO used in the experiment should be deaerated to minimize the amount of oxygen in the reaction.

It is not really necessary to grind the KOH to a fine powder. KOH is very hygroscopic and will absorb water very fast, especially when it is powdery (why?). You should try to dissolve as much as possible in the DMSO.

After the CpH monomer is added, the mixture has to be stirred for 15-30 minutes (usually turns purple or brown during this time). Then the solution (suspension) of ~6 g FeCl₂ * 4 H₂O in ~10 mL DMSO is added.

The mixture has to be stirred for a minimum of 30 minutes. During this time an orange precipitate should form, which is sometimes difficult to see if the mixture is very dark brown in color. The mixture is poured over ice-cold deoxygenated water (obtained by several pumping-purging cycles). The product and other organometallic compounds precipitate.

The crude product is recrystallized from petroleum ether (what is that?). The container has to be capped because ice crystals will form at -78 ^oC if left open. Further purification can be accomplished by sublimation (will be demonstrated in lab).


Step 3: Synthesis of Acetyl ferrocene

Ferrocene is first dissolved in acetic acid anhydride affording a red solution. Then 85% phosphoric acid is added slowly. The color usually becomes a little darker at this point. After heating the mixture to ~70-80 ^oC for 20 minutes, it is cooled to room temperature and then poured in a 1M NaOAc solution, and the entire mixture brought to a pH= 5-7 using solid sodium bicarbonate.

Next, the entire mixture (including the precipitate) is extracted with ethyl acetate (How much solvent is appropriate here?). The organic layer is washed with water, saturated sodium bicarbonate solution and then dried over anhydrous MgSO₄ (How do you know that you added enough drying agent?).

The solvent is removed and the remaining crude product purified by flash chromatography on SiO₂ using petroleum ether first (to elute the unreacted ferrocene) and then a mixture of ethyl acetate and petroleum ether (5:95) in order to obtain the final product.(Note: The column required has to be bigger than the one shown in the picture since a larger amount of material is processed!)

Step 4: Characterization for ferrocene and acetyl ferrocene

IR spectrum (ATR), melting point, ¹H and ¹³C-NMR, UV-Vis spectrum, MS (see reader)

Step 5: Problems to be considered

1. A student distills the monomer using a vacuum distillation. He observes a boiling point of 30^oC at p=2 mmHg.

a. What did the student isolate?

b. What is going to observe in the subsequent reaction of the distillate with potassium hydroxide?

c. Which problem is he going to face?

2. Why does the procedure ask to minimize the exposure of potassium hydroxide to the air, particularly when it is ground?

3. Which solvents could be used instead of petroleum ether in the recrystallization step?

4. Which precautions have to be taken if the sodium bicarbonate is added in step 3?

5. How much stationary phase should be used in the column chromatography step in step 3?

6. Which solvent should be used for the characterization using NMR spectroscopy?