Author Topic: Silica gel in LAH reductions  (Read 2194 times)

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IudexK2

  • Guest
Silica gel in LAH reductions
« on: October 26, 2001, 06:43:00 PM »
If not using an inert atmosphere, would it bee beneficial to add silica gel or another dehydrating agent to suck up any water that may get in there?

Osmium

  • Guest
Re: Silica gel in LAH reductions
« Reply #1 on: October 26, 2001, 07:55:00 PM »
No, the LAH will be the stronger dehydrating agent.

foxy2

  • Guest
Re: Silica gel in LAH reductions
« Reply #2 on: October 27, 2001, 04:01:00 AM »
What does an inert atmosphere have to do with water?

Use the silica gel to dry the system then remove it and proceed. A small tube of fresh silica gel can keep out moisture.

I have several ideas for easy inert amosphere's in a reduction.

Helium is the obvious one.
Disposable helium tanks for ballons or better yet just get a tank of helium.  A 210 cu ft tank is only about $50+deposit, then buy a regulator and you are set.

Here is my novel idea, Hydrogen
For reductions this should bee excellent, No?  I am not sure if there would bee risks besides the obvious flammability of Hydrogen.  Just set up a nice H2 generator by dripping H2SO4 on magnesium or iron or aluminum.  Dry this gas and pipe it into the reaction.  The flow rate is easily controlable by very slow dripping of H2SO4 from an addition funnel.  Added Hydrogen can't hurt a reduction.
Here is Drone's H2 write uo.

https://www.thevespiary.org/rhodium/Rhodium/chemistry/hydrogen.html



Anyone see any problems with this procedure?

Do Your Part To Win The War

IudexK2

  • Guest
Re: Silica gel in LAH reductions
« Reply #3 on: October 27, 2001, 04:53:00 PM »
Getting an inert gas is not the problem - I have lots of argon. Its just I dont understand how to keep the gas in there [confused]

Rhodium

  • Guest
Re: Silica gel in LAH reductions
« Reply #4 on: October 27, 2001, 04:58:00 PM »
You run a slow continous stream of argon through the apparatus the whole time.

lugh

  • Guest
Re: Silica gel in LAH reductions
« Reply #5 on: October 27, 2001, 04:59:00 PM »
Generally some sort of trap is used, such as bubbling the exiting gas through a U-tube filled with mercury (as illustrated in Vogel's)  :)

terbium

  • Guest
Re: Silica gel in LAH reductions
« Reply #6 on: October 28, 2001, 12:29:00 AM »
Getting an inert gas is not the problem - I have lots of argon. Its just I dont understand how to keep the gas in there
This is one of the reasons that argon is superior to nitrogen, and especially, hydrogen and helium. Argon is heavier than air, it will displace the air from your flask and push it out the top of your reflux condenser; even so people still will use a continuous stream of argon into the flask and a (mineral oil) bubbler as the gas exits the condenser.

That said, there is no need for inert gas when performing LAH reductions. At most use a calcium chloride drying tube though you really don't even need that.

yellium

  • Guest
Re: Silica gel in LAH reductions
« Reply #7 on: October 28, 2001, 12:38:00 AM »
Note that the keyword is a slow continous stream of argon. Don't overdo it, because otherwise you'll blow away a lot of your solvent.

terbium

  • Guest
Re: Silica gel in LAH reductions
« Reply #8 on: October 28, 2001, 12:43:00 AM »
Note that the keyword is a slow continous stream of argon.
Yes, good point. That is why the device on the outlet is called a bubbler, the key word being bubble.

Osmium

  • Guest
Re: Silica gel in LAH reductions
« Reply #9 on: October 28, 2001, 02:10:00 AM »
You guys must be damn wealthy, wasting all that argon!
Attach a balloon to the top of the condenser.

terbium

  • Guest
Re: Silica gel in LAH reductions
« Reply #10 on: October 28, 2001, 05:57:00 AM »
You guys must be damn wealthy, wasting all that argon!

Geez, you must be pretty damn impoverished if you have to worry about the cost of a few liters of argon.

IudexK2

  • Guest
Re: Silica gel in LAH reductions
« Reply #11 on: October 28, 2001, 01:17:00 PM »
Damn right I'm impoverished.

Lino

  • Guest
Re: Silica gel in LAH reductions
« Reply #12 on: October 29, 2001, 12:21:00 AM »
'udexk ma man. Don't go wastin' yo money on balloons! Go down the seedy side a town an' pick up a few used condoms. They wash out good as new. In fact, I know a man… :)

Linoleum: the 13th element!  Now available at all good DIY stores… & Walmart.

IudexK2

  • Guest
Re: Silica gel in LAH reductions
« Reply #13 on: October 29, 2001, 01:32:00 AM »
will do  ;)

Osmium

  • Guest
Re: Silica gel in LAH reductions
« Reply #14 on: October 29, 2001, 01:37:00 AM »
> Geez, you must be pretty damn impoverished if you have to
> worry about the cost of a few liters of argon.

Well, last time I checked prices for chem grade Ar suitable for anhydrous reaction conditions it wasn't THAT cheap, actually not cheap at all, a fucking reusable balloon certainly costs less, especially when doing a 12 hour oxidation-sensitive reflux.

lugh

  • Guest
Argon Purification
« Reply #15 on: October 29, 2001, 02:55:00 AM »
From Ind Eng Chem 42 (10) 2095-6 (1950): Argon may be dried by bubbling through sulfuric acid, followed by passing through a tower packed with anhydrous magnesium perchlorate, the hydrogen, nitrogen and oxygen can be then be removed by passing through -20 to +60 titanium powder heated to 850 degrees C, commonly known as a gettering furnace  :)

Phosphole

  • Guest
Re: Argon Purification
« Reply #16 on: October 29, 2001, 08:07:00 AM »
You forgot about the oxygen!  :o

To remove the oxygen, bubble through an alkaline solution of pyrogallol (trihydroxybenzene).
   - Purification of Laboratory Chemicals, 4th ed (Armarego & Perrin)

terbium

  • Guest
Re: Argon Purification
« Reply #17 on: October 30, 2001, 08:10:00 AM »
You forgot about the oxygen!

To remove the oxygen, bubble through an alkaline solution of pyrogallol


Uhm, no, he didn't.

the hydrogen, nitrogen and oxygen can be then be removed by passing through -20 to +60 titanium powder heated to 850 degrees C, commonly known as a gettering furnace 

But the commercial argon that I have used had already been highly purified to remove reactive gasses.