Purification of Ferrocene by Sublimation
Source: Tamara M. Powers, Department of Chemistry, Texas A&M University
Sublimation, the direct phase transition of a solid into a gas without first b…
1. Setup of the Schlenk Line
For a more detailed procedure, please review the "Schlenk Lines Transfer of Solvent" and "Degassing Liquids" videos in the Essentials of Organic Chemistry series. Schlenk line safety should be reviewed prior to conducting this experiment. Glassware should be inspected for star cracks before use. Care should be taken to ensure that O2 is not condensed in the Schlenk line trap if using liquid N2. At liquid N2 temperature, O2 condenses and is explosive in the presence of organic solvents. If it is suspected that O2 has been condensed, or a blue liquid is observed in the cold trap, leave the trap cold under dynamic vacuum. Do NOT remove the liquid N2 trap or turn off the vacuum pump. Over time the liquid O2 will sublime into the pump - it is only safe to remove the liquid N2 trap once all of the O2 has sublimed.
- Close the pressure release valve.
- Turn on the N2 gas and the vacuum pump.
- As the Schlenk line vacuum equilibrates, prepare the cold trap with either liquid N2 or dry ice/acetone.
- Assemble the cold trap.
2. Add 500 mg (2.7 mmol) of ferrocene to the base of the sublimation chamber.
3. Assemblyof the Sublimation Chamber
- Place the O-ring in the groove of the chamber base.
- Gently place the cold finger into the chamber base and make sure that the O-ring fits into the groove of the glassware.
- Secure the two pieces of the sublimation chamber using a clamp.
4. Connect the sublimation chamber to the Schlenk line and open the chamber to vacuum for 1 min. Close the vacuum valve on the sublimation chamber. The sublimation will be carried out under static vacuum.
5. Fill the cold finger with an ice bath.
6. Place the base of the sublimation chamber into a water bath heated to 80°C.
7. After the sublimation is complete, remove the sublimation chamber from the bath.
8. Close the stopcock on the Schlenk line.
9. Remove the Schlenk line tube from the sublimation chamber and repressurize the sublimation chamber by slowly opening the valve. Be careful! If the chamber is repressurized too quickly it will disturb the purified crystals on the cold finger.
10. Unclamp the sublimation chamber and remove the water from the cold finger with a pipette.
11. Carefully lift the cold finger out of the sublimation chamber.
12. Scrape the purified ferrocene from the cold finger and transfer to a vial. Record the weight of the purified product. If the compound being sublimed is air-sensitive, the entire apparatus should be brought into an inert-atmosphere glovebox prior to opening the sublimation chamber.
Sublimation is the phase transition of a substance from solid into gas without passing through its intermediate liquid phase. It is an important technique used for purification of organic and inorganic solids.
Usually the transition from solid to gaseous state requires passing through its liquid state.
However, reduced pressure and heating of a solid can lead to volatilization without melting, known as sublimation. The reverse process in which the substance passes from its gaseous to its solid state, is called deposition.
This video will illustrate the principles of sublimation, a typical procedure, and several applications.
At normal pressures, most chemical compounds and elements possess three different states of matter at different temperatures with a triple point at which all three states are present.
As seen in a phase diagram, vaporization and condensation - ?together known as distillation - may be performed at pressures above the compound's triple point.
On the contrary, sublimation and deposition occur only at pressures that lie below the triple point.
Sublimation can be performed using two types of apparatus, depending on the volatility of the solid: for highly volatile compounds, a makeshift sublimation chamber may be assembled from a beaker and a watch glass. This method is appropriate for compounds that sublime at or near atmospheric pressure and ambient temperature.
If vacuum and/or inert atmosphere are required, a specialized piece of glassware made specifically for sublimation is used. It is made of a glass cup, containing the crude solid, and a hollow cylinder, which contains a cryogen and fits over the top of the cup. An O-ring seals the base and cold finger, and a vacuum attachment makes up the rest of the apparatus.
After completing the sublimation procedure, the apparatus is disassembled in a fume hood or glovebox depending on whether the material is air-sensitive. Then the purified solid may be scraped off of the cylinder, while the non-volatile impurities remain in the cup.
Now that we have discussed the principles of sublimation, let's take a look at an actual procedure.
In a fume hood equipped with a Schlenk line, or dual manifold, weigh 500 mg of ferrocene in the base of a sublimation chamber.
Place an O-ring in the groove of the chamber base, and gently place the cold finger into the chamber base, making sure the O-ring fits. Then secure the two pieces of the chamber with a clamp.
Connect the assembled chamber to the Schlenk line, and open the chamber to vacuum for 1 min. Then close the vacuum valve on the chamber to continue the experiment under static vacuum.
Clamp the chamber to a ring stand and place the base portion of the chamber in an 80 ?C bath. Fill the cold finger with an ice-slurry, replenishing it as it warms.
After sublimation is complete, remove the chamber from the bath. Close the stopcock to the Schlenk line and detach the tube from the chamber.
Then, repressurize the chamber by slowly opening the valve to air in a fume hood or glovebox.
Use a pipette to remove water from the cold finger and unclamp the two pieces of the chamber. Then carefully lift the cold finger out of the sublimation chamber.
Scrape the purified ferrocene from the cold finger with a spatula, transfer to a pre-weighed vial, and record the weight.
500 mg of purchased ferrocene was purified via sublimation resulting in 493 mg isolated product with a yield of 99.6%. The proton NMR shows a singlet at 4.17 ppm, which integrates to 10 protons of the ferrocene. The absence of other peaks indicates that no impurities are present, and that the purification was successful.
Now that we have discussed a procedure for sublimation, let's take a look at a few applications.
Water can be sublimed using a process called lyophilization, also known as freeze drying. This is accomplished by freezing a water-filled flask in a dry ice acetone bath at -78 ?C and then applying high vacuum by attachment to a lyophilizer, where the water is recaptured in a cold finger.
Many mothballs contain a compound known as naphthalene, which is a simple polyaromatic hydrocarbon, consisting of two fused benzene rings.
Naphthalene sublimes at atmospheric pressure and 80 ?C and the gaseous form of this compound is toxic to moths.
You've just watched JoVE's introduction to Sublimation of Ferrocene. You should now understand the principles of sublimation, how to perform an experiment, and several of its applications. Thanks for watching!
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Q1: What is sublimation and how does it differ from distillation?
Sublimation is the direct phase transition of a solid into a gas without passing through the liquid phase. It occurs at pressures and temperatures below a compound's triple point. Distillation, by contrast, involves vaporization and condensation at pressures above the triple point, requiring the substance to pass through its liquid state.
Q2: Why is sublimation useful for purifying ferrocene and other inorganic solids?
During sublimation, the target compound vaporizes while non-volatile impurities remain behind in the solid state. The vaporized compound is then collected through deposition on a cold surface, yielding a purified solid. This separation is effective because impurities have different volatility properties than the desired compound.
Q3: What equipment is needed for vacuum sublimation of ferrocene?
Vacuum sublimation requires a specialized glass apparatus consisting of a cup for the crude solid, a hollow cylinder (cold finger) filled with cryogen, an O-ring seal, and a vacuum attachment. The apparatus connects to a Schlenk line for vacuum control and inert atmosphere during purification of air-sensitive materials.
Q4: What role does the cold finger play in the sublimation process?
The cold finger is a hollow cylinder filled with an ice-slurry or cryogen that sits inside the sublimation chamber. As the ferrocene vaporizes from the heated base, it contacts the cold finger and undergoes deposition, converting back to solid form on the cold surface where it can be collected and scraped off.
Q5: How does the triple point determine whether sublimation or distillation occurs?
The triple point is the unique temperature and pressure where all three states of matter coexist. Sublimation occurs only at pressures below the triple point, while distillation occurs above it. For ferrocene, with a triple point of 183 °C, operating below this pressure ensures direct solid-to-gas transition without melting.
Q6: What does the proton NMR result tell us about the purity of sublimed ferrocene?
The proton NMR showed a single peak at 4.17 ppm integrating to 10 protons, consistent with ferrocene's structure. The absence of additional peaks indicates no impurities were present, confirming that sublimation successfully purified the compound with a 99.6% yield from the original 500 mg sample.
Q7: How is sublimation applied in freeze-drying and mothball production?
Freeze-drying, or lyophilization, uses sublimation to remove water from frozen samples by applying high vacuum, recapturing water on a cold finger. Naphthalene in mothballs sublimes at atmospheric pressure and 80 °C; its gaseous form is toxic to moths, making sublimation an effective preservation method.