Sciencemadness Discussion Board

List of items needed to get started in air-free chemistry?

MichaelBijanAfghani - 15-11-2021 at 18:18


On the glassware and connections side of air-free chemistry, here's what you need:

* Schlenk line
* Vacuum pump
* Cold trap
* Dry ice
* Vinyl tubes that fit 1/4" barb
* adapters, 1/4" barb to male 24/40 ground glass
* adapters, 1/4" barb to luer lock
* luer lock needles
* cannula
* 24/40 septa
* blowtorch
* oil vial
* drierite drying tower

I already have most of these and can't wait to be able to do grignard reactions and maybe even play with organolithiums. however i'm less familiar with the gas supply side. I'm not familiar with nitrogen cylinders and how exactly you connect them to the schlenk line. I think you can buy nitrogen cylinders at welding shops like Praxair, then I think you need a regulator or something? Do welding shops carry those too? Whatever it is, the output of the nitrogen cylinder should be 1/4" barb.

Edit: Seems like regulators are really expensive. Can you make do without one? Just go directly from cylinder, to 1/4" barb adapter, to schlenk line?

Michael Bijan Afghani

[Edited on 16-11-2021 by MichaelBijanAfghani]

timescale - 15-11-2021 at 18:36

The regulator is absolutely essential if your cylinder is the industrial/medical kind. If you have "demonstration bottles" containing only a small amount of gas, you might be able to get away without it.

Gas in cylinders is supplied at hundreds of psi. The regulator drops this to a low pressure suitable for charging your Schlenk line. Try to put hundreds of psi into your Schlenk line and you lose a lot of money, very quickly. I wouldn't trust the main valves on cylinders to have the precision to add small, controlled quantities of gas.

In the setups I've seen, low-pressure nitrogen (or whatever) from the regulator is fed to the apparatus through a low-pressure gas valve and a gas bubbler, while the pressure is closely watched on an attached gauge.

Honestly, if you have the capacity for an air-free apparatus, the regulator shouldn't make much of a dent ;) and I think you can find inert gas at welding stores, where it's sold for TIG and similar welding techniques, along with the regulators. If what you're doing is sensitive enough to need ultra-high-purity gases, these might be harder to obtain.

MichaelBijanAfghani - 15-11-2021 at 18:42

Quote: Originally posted by timescale  
In the setups I've seen, low-pressure nitrogen (or whatever) from the regulator is fed to the apparatus through a low-pressure gas valve and a gas bubbler, while the pressure is closely watched on an attached gauge.


From what I've seen in my university research experience, the gas bubbler comes after the schlenk line, not before. The nitrogen flow rate is maintained at around one bubble per second. I never got to learn nitrogen delivery because the nitrogen in academic fume hoods comes from a nozzle on the side and the setup is all hidden behind the walls.

Michael Bijan Afghani

[Edited on 16-11-2021 by MichaelBijanAfghani]

macckone - 16-11-2021 at 07:14

You really do not need to be 'air free' to do grignard reactions.
Excluding moisture is the key thing.
The second thing to exclude is CO2 unless you are planning on reacting it with CO2.
Air has very little CO2 but it will reduce the desired product ever so slightly.

If you are doing lithium chemistry, you want to use argon instead of nitrogen.
Lithium will react with nitrogen.

karlos³ - 16-11-2021 at 07:50

Gignard reactions are not hard to run, you certainly don't need most of that.
At the hive Bwiti ran one in a mason jar in his kitchen.
Successful.

If you have good Mg(thin shavings), a tiny piece of iodine and pre-dried your solvent with molsieves, its really pretty easy and will easily start.

Texium - 16-11-2021 at 12:07

I would need to find the source again, but it's been proven that properly dried molecular sieves are actually more effective than benzophenone ketyl at drying solvents. There's two reasons that sodium benzophenone ketyl is still used: one is because there are a lot of old professors who insist on using the old fashioned methods, the other is simply because it looks so cool. ;)

From a more practical standpoint, distilling ethereal solvents also removes stabilizers and oxidation products, so there is a tangible benefit to it, though sieves are still superior for dryness, so putting freshly distilled ether over dry sieves is fantastic.

macckone - 16-11-2021 at 13:16

Recent review of sodium/benzophenone:
https://pubs.acs.org/doi/10.1021/acsomega.8b01707

And the seminal 2010 paper on why molecular sieves are better:
http://ccc.chem.pitt.edu/wipf/Web/Solvent_Drying.pdf

Vogel was originally written in 1948.
It has not been radically changed.
The 5th edition was released in 2003.

The long and short is that having dry solvent is helpful but unless you are doing chemistry in the rain having an air free environment is not necessary.
Keeping stuff as dry as possible improves yields but it isn't the end of the world if there is a little moisture present.
If you use excess magnesium (which you should) and add iodine and let it sit before adding your halogen compound, the magnesium will dry your solvent.

timescale - 16-11-2021 at 16:42

Quote: Originally posted by MichaelBijanAfghani  
From what I've seen in my university research experience, the gas bubbler comes after the schlenk line, not before.


Different purposes, probably: Bubbler into the line for gas scrubbing and backflow prevention if you're using the gas to backfill vacuum; bubbler out of the line for flow monitoring and backflow prevention if you need a steady flow of inert gas.
Quote:

You really do not need to be 'air free' to do grignard reactions.
Excluding moisture is the key thing.


Yup, as I understand it the reason diethyl ether is so popular as the Grignard solvent is its high volatility - the ether vapor will push out all the air. When I did it in school we didn't even use a drying tube.

Water should react with and destroy the organomagnesium reagent, but though this will lower yields, it's not necessarily unsafe. Butyllithium on the other hand...

karlos³ - 16-11-2021 at 16:55

Quote: Originally posted by MichaelBijanAfghani  

Why does vogels say I have to dry diethyl ether with benzophenone sodium ketyl? Shouldn't molecular sieves be enough like you said? I guess benzophenone ketyl is more suitable for solvent stills in academic labs which constantly need dry ether on tap. But it's a hassle for drying small amounts of ether for one reaction

Michael Bijan Afghani

[Edited on 16-11-2021 by MichaelBijanAfghani]

I have no idea why Vogel's says this, but I can tell you that leaving it to stand over NaOH for a month is more than sufficient to dry ether.
Tell that Vogel's, they need to update their entry ;)
Because this does work.

Molsieves are sufficient, if you have good ether right from the bottle, leave it a day over sieves and you will be astonishend.
No worries, grignard reactions are not that hard as some people make them sounds.
Everything is in the proper preparation.

E: I ran a few grignards just lately, I got THF from a friend and dried it simply over molsieves, 4A of course.
And I can tell you, when I started the reaction I quickly had to put the ice-bath under it! :o
But as said, in THF.

In ether, they do run even easier (and more violent! prepare not to let them run dry, that possible..).
Grignard reactions are not the highly advanced syntheses some people make them out to be.
And the same is true for the friedel-crafts.
Somehow people think those only work for the most advanced chemiists.
But that is not true, absolutely not.

[Edited on 17-11-2021 by karlos³]

BromicAcid - 16-11-2021 at 20:42

You can go without a regulator just main-lining off the cylinder but:

1) You will waste a lot of nitrogen, even if you baby it. To ensure you have a constant supply you have to keep bumping it up as the cylinder pressure drops. You never really have the consistency.
2) If something plugs there's nothing to stop the pressure from building toward the cylinder pressure and rupturing along the way.
3) You won't know you're out until you're pretty much out of nitrogen.

I have run a few reactions that I have run inerted with argon that I've done without a regulator. In those cases though I have run the reaction in a flask in a glove bag and used the argon cylinder to do a set of three purges of the bag. Then occasionally topping off the bag to keep pressure on the inside. None of these situations are optimal and they can require babysitting.

[Edited on 11/17/2021 by BromicAcid]

Texium - 17-11-2021 at 08:21

You can do without the Schlenk line and the regulator entirely by using argon or nitrogen balloons for your reactions. Simply fill a balloon directly from your cylinder, attach it to a cut off plastic syringe with needle, and stick it through a rubber septum on your flask. Stick a second open needle on there and the balloon pressure will force all the air out of the flask through the open needle. Then you just remove the open needle, maybe refill your balloon at this point, and presto, you've got a flask under inert atmosphere. Sure it doesn't give you quite as inert of an atmosphere as evacuating and backfilling through a Schlenk line, but most of the time it's sufficient.