There are myriad techniques to purify compounds. In the organic labs I teach, we go through the basic ones: crystallization, distillation, column chromatography etc...
Of all of the available techniques, my favorite is sublimation. Sublimation rocks AND rolls. Sublimation describes the change of a solid to a gas. If the gas is cooled, the solid can reform. [Bonus points if you know the name of this process. It's not condensation].
Purification by sublimation involves heating a solid in a flask that has a cold finger above the solid. This can be done under vacuum or at room pressure.
Recently I had my organic students synthesize ferrocene. This reaction involves the deprotonation of cyclopentadiene with potassium hydroxide. Most hydrocarbons would never be deprotonated by potassium hyroxide, but the cyclopentadiene is easily deprotonated because the product is aromatic. Upon reaction with iron II, it forms a nice orange sandwich compound.
This product is easily purified by crystallization from hexane, but sublimation is a lot more fun. If a product can be purified by sublimation, I will do it. Without question.
Here are some pictures I snapped of the purification by sublimation of student synthesized ferrocene.
Here's the crap before purification:
Here is the setup. The cold-finger is cooled with water. The hotplate is heated to "4." Whatever that means.
After about an hour, the ferrocene deposits on the side of the flask. I usually put a Al foil tent around it to contain the heat and keep the whole flask warm. That way more of the product ends up on the cold finger.
Here's the cold finger with some purified ferrocene upon it.
This is the crap left behind.
And this is the final product.
I love sublimation!!!!!!
UPDATE: Here are some products I have purified by sublimation.
16 comments:
Deposition.
Do you freshly crack your cyclopentadiene for the kids? When doing the Diels-Alder reaction in grad school, we always had to crack the stuff right before the lab and then keep it in -80 baths.
One of my students once said "Why not just let the stuff warm up? It's the same principle, that way we don't need to worry about taking melting points." I couldn't fault his logic, but I chastised him for taking the easy way out. I reminded him "you want to go to med school...you don't want to end up like me."
Hmmm. I didn't know you could set up a sublimation that way (perhaps because my organic I lab was microscale and therefore useless). Will that work for aromatics without the icky metal? (Actually, most of what I make doesn't do so well with sublimation. Just curious.)
Matt wins, though I'm sure he isn't the only one who knows that. That's just one of my pet questions I ask my students. Most Gen Chem students don't know the answer.
I crack the stuff before hand and keep it in the freezer. I just also tried a Diels Alder with Maleic Anhydride (classic). Turns out the anhydride was crap and the lab was a waste. I hate that.
Ψ*Ψ I'm going to add an update of things I've purified my sublimation. One of them is aromatic.
Are you doing the classic D-A with Maleic anhydride and then treating it with acid to get it to cyclize on itself? That's what we did for the D-A lab at ND. We always referred to the product as "Chemical X", which then turned out to be what gave the Power Puff girls their amazing abilities.
Hooray! I'm a winner! See, it's important to come to work early, kids. I'm also available for "Phone-a-Friend".
I remember in my microscale orgo lab (circa 1995), we extracted caffeine from tea leaves using methylene chloride (I think), and purified it with a cold finger.
Later, I tasted it... boy, pure caffeine sure is bitter!
Sublimation is great . . . when it works. Typically works best with aromatics and highly symmetrical compounds.
Did it every year in one of my O-Chem labs. Isolate caffeine from tea, with sublimation to purify. Lab works great, but I never looked forward to it. Imagine 15-20 students, each with 20 tea bags in 300mL of boiling H2O. Makes you nauseous.
The synthesis of bis-(diphenylphosphino) ferrocene used to be a staple of the Cambridge UK undergraduate labs, sublimation and cyclopentadiene cracking included (maybe it still is?). The students must have made buckets of the stuff. We used to speculate that the TAs sold the stuff on to Strem etc, thus keeping us old timers happy with cheaper catalysts.
My sophomore organic lab did acetylferrocene. I don't seem to remember it working well for most students. My favourite lab from a class was a Sonogashira coupling. It worked beautifully and the Pd plated out.
As a side note...I'm considering a post on undergrad-proof reactions.
Ψ*Ψ, this sounds like a good idea. I would include the ferrocene one, along with aspirin synthesis, and hydrogenation of olive oil. I've had good success with these.
I forgot about that one! Aspirin synthesis is always good. Hexaphenylbenzene was cool also.
When I synthesized aspirin in my sophomore organic lab, I had this "lab partner" who demanded she did everything (we liked to refer to her as "Beaker").
Long story short, my aspirin turned out green. I quickly found a different lab partner.
When I did it in an undergrad lab we used a heatgun :D
i hv a question plz help:
can acetylferrocene be collected by sublimate? and why?
tnx mill...
I did sublimations like this in my organic chem III lab to purify coumarin and camphor. We used ice in the cold finger and used a filter flask with an aspirator to lower the pressure though. They are a fun, but recrystalizations are cool too.
And i synthesized ferrocene in my inorganic chem lab, not organic. You do organometallic stuff in organic?
i must thank you for the efforts you've put in penning this blog. excellent blog post .
www.n8fan.net
Post a Comment