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Topic: LiAlH4 handelling  (Read 15029 times)

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Offline shrikantvarma

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LiAlH4 handelling
« on: June 19, 2010, 12:07:13 PM »
I am using LiAlH4 in my project work i use it by charging LiAlH4 first & then solvent tetrahydrofuran but flasck get hot while addition of tetrahydrofuran what is good process for LiAlH4 handelling?

Offline OC pro

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Re: LiAlH4 handelling
« Reply #1 on: June 19, 2010, 01:32:25 PM »
The THF should be pre-dried (e.g. refluxing with Sodium and Benzophenone). Then dissolve the starting material to be reduced in THF. Cool the reaction mixture with an ice-bath to minimum 5°C (measure internal temperature!). Then add the LiAlH4 in small portions. The first 5-10% of LiAlH4 will react with traces of water, therefore you observe rise of temperature and effervescence. At the end, you can add the LiAlH4 faster.
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Offline AC Prabakar

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Re: LiAlH4 handelling
« Reply #2 on: June 21, 2010, 03:22:40 AM »
Yes, solvents used for LiAlH4 should be dried.Toluene,ether also can be used as solvent.
Added to this,LiAlH4 should be handled in inert atmosphere(ex.N2/argon)

For better clarity check the MSDS of LiAlH4.

Offline discodermolide

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Re: LiAlH4 handelling
« Reply #3 on: July 10, 2010, 06:50:19 AM »
I am using LiAlH4 in my project work i use it by charging LiAlH4 first & then solvent tetrahydrofuran but flasck get hot while addition of tetrahydrofuran what is good process for LiAlH4 handelling?
OK you don't need to dry the THF, the LiAlH4 takes care of that. The heat you observe is the heat of solution of LiAlH4 in THF, also THF solutions of LiAllH4 are not stable due to polymerisation of the THF. This can lead to some nasty surprises. Use Toluene.
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Offline g-bones

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Re: LiAlH4 handelling
« Reply #4 on: July 11, 2010, 12:03:54 AM »
I am using LiAlH4 in my project work i use it by charging LiAlH4 first & then solvent tetrahydrofuran but flasck get hot while addition of tetrahydrofuran what is good process for LiAlH4 handelling?
OK you don't need to dry the THF, the LiAlH4 takes care of that. The heat you observe is the heat of solution of LiAlH4 in THF, also THF solutions of LiAllH4 are not stable due to polymerisation of the THF. This can lead to some nasty surprises. Use Toluene.



Thats quite scary advice.  It is always good practice to use dry solvents.  Any residual water will react with LiAlH4 to release hydrogen gas and heat; a recipe for fire.  Granted oxygen is needed for fire and inert gas should solve that but if the  solvent is too wet the reaction can be quite violent and result in a volcano-like event.  THF is a typical solvent for LAH reductions and can be okay.  granted toluene is okay too.     

Offline discodermolide

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Re: LiAlH4 handelling
« Reply #5 on: July 11, 2010, 01:18:02 AM »
I am using LiAlH4 in my project work i use it by charging LiAlH4 first & then solvent tetrahydrofuran but flasck get hot while addition of tetrahydrofuran what is good process for LiAlH4 handelling?
OK you don't need to dry the THF, the LiAlH4 takes care of that. The heat you observe is the heat of solution of LiAlH4 in THF, also THF solutions of LiAllH4 are not stable due to polymerisation of the THF. This can lead to some nasty surprises. Use Toluene.

I work in industry where we never dry our solvents. The THF has< 250 ppm water. I would be more worried about the peroxides.

I am using LiAlH4 in my project work i use it by charging LiAlH4 first & then solvent tetrahydrofuran but flasck get hot while addition of tetrahydrofuran what is good process for LiAlH4 handelling?
OK you don't need to dry the THF, the LiAlH4 takes care of that. The heat you observe is the heat of solution of LiAlH4 in THF, also THF solutions of LiAllH4 are not stable due to polymerisation of the THF. This can lead to some nasty surprises. Use Toluene.



Thats quite scary advice.  It is always good practice to use dry solvents.  Any residual water will react with LiAlH4 to release hydrogen gas and heat; a recipe for fire.  Granted oxygen is needed for fire and inert gas should solve that but if the  solvent is too wet the reaction can be quite violent and result in a volcano-like event.  THF is a typical solvent for LAH reductions and can be okay.  granted toluene is okay too.     
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Offline 408

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Re: LiAlH4 handelling
« Reply #6 on: July 11, 2010, 04:11:44 AM »
For microscale work, I use dry solvents.  For larger scale I admit I just use more LiAlH4.  Once the LiAlH4 is weighed, I add it to the cold solvent until gas evolution stops.  Then I weigh my remaining LiAlH4, and make up the difference to the stoichiometrically proper quantity. 

Offline pacifyer

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Re: LiAlH4 handelling
« Reply #7 on: July 13, 2010, 07:02:53 PM »
NEVER pour THF in a flask contaning LAH!!!!!

FIRE HAZARD!!!!!! :o :o :o :o

Add it cautiously and in small portions, to dry THF, instead...

Offline skbuncks

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Re: LiAlH4 handelling
« Reply #8 on: July 14, 2010, 07:34:19 AM »
I am using LiAlH4 in my project work i use it by charging LiAlH4 first & then solvent tetrahydrofuran but flasck get hot while addition of tetrahydrofuran what is good process for LiAlH4 handelling?

...you could just buy your lithal in ready made solutions...its far easy to handle and can be added dropwise to your reaction to control the exotherm.

skb

Offline discodermolide

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Re: LiAlH4 handelling
« Reply #9 on: July 14, 2010, 10:49:59 AM »
I am using LiAlH4 in my project work i use it by charging LiAlH4 first & then solvent tetrahydrofuran but flasck get hot while addition of tetrahydrofuran what is good process for LiAlH4 handelling?

...you could just buy your lithal in ready made solutions...its far easy to handle and can be added dropwise to your reaction to control the exotherm.

skb

I?ll say again solutions of LiAlH4 in THF or other ether solutions are not stable, within a short period of time even in the fridge the solvent starts to polymerise. I would recommend toluene or if it has to be an etheric solvent make it fresh each time.
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Offline LabRat01

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Re: LiAlH4 handelling
« Reply #10 on: July 14, 2010, 11:12:49 AM »
I am using LiAlH4 in my project work i use it by charging LiAlH4 first & then solvent tetrahydrofuran but flasck get hot while addition of tetrahydrofuran what is good process for LiAlH4 handelling?

...you could just buy your lithal in ready made solutions...its far easy to handle and can be added dropwise to your reaction to control the exotherm.

skb

Yeah, it's much easier to buy and use in a premade solution. Be carefull, I've seen a small fire caused by LiAlH4...

Offline pacifyer

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Re: LiAlH4 handelling
« Reply #11 on: July 14, 2010, 06:05:26 PM »
I would NOT prepare LAH solutions to be used sometime later. It is dangerous, and the actual concentration will be unknown after a while!!!

The best way is to use solid LAH, which is easy to handle, if you are careful.

Offline orgopete

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Re: LiAlH4 handelling
« Reply #12 on: July 17, 2010, 11:55:53 PM »
I see there have been a couple of comments about fire hazards. There was a similar post about fires with NaH to which I also posted a comment. A common factor that I have noted is that fires seem more prevalent in the absence of an inert atmosphere. If you had a fire and you were using an inert atmosphere, please describe how the fire occurred. Otherwise, I continue to believe fires can be averted by the use of an inert atmosphere, even with a pyrophoric reagent such as lithium aluminum hydride.

To that end, operationally, it is easier to charge a dry flask with LAH, evacuate and fill several times with nitrogen or argon, and then add your solvent dropwise. Even if the solvent contained water (or water itself is added), this reaction is easily controlled by the droprate. This is essentially how the quench of LAH is done. Adding solid LAH tends to have the effect of gas rushing out if given off during addition. Although I have done it that way many times, it also makes my pulse race a little higher.
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