chemrox
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cautions regarding use of NaH in synthesis
Are there particular cautions regarding use of sodium hydride that are different from using LAH? How would the two compare? I read a process where
one worker used a 52.9% NaH in mineral oil but it wasn't clear from the writeup if that was because it was what he had. Anyone with experience using
NaH is asked to reply. Thanks, CRX
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grind
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LAH is more dangerous. NaH wetted with oil doesn´t ignite in the air as easy as LAH, but reaction with water is very vigorous. Don´t heat it with
DMF to elevated temperatures or explosion can occur.
Oilfree NaH can be considered as dangerous as LAH. It´s ability to ignite in (moist) air is even greater compared with LAH.
Always use carefully dried solvents when working with NaH. Use of dry argon or nitrogen is necessary for transfering oilfree (dry) hydride, you can
omit this in the case of NaH wetted with oil.
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chemrox
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My NaH is dry in the can so should I start by getting it into oil? Does mineral oil off the shelf need to be dried? How?
"When you let the dumbasses vote you end up with populism followed by autocracy and getting back is a bitch." Plato (sort of)
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Klute
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You could throw a small chunk of sodium in it. So it will also get rid of any dissolved oxygen. Overwise, sacrifice a little NaH perhaps.
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chemrox
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I like the Na idea! Have you done it?
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Gargi
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NaH Handling
Its always better to use NaH wetted in oil, Usually an approximate qty of wet NaH is taken in reaction flask fitted with silicon septum ( Tare of
Flask noted beforehand ), Hexane is introduced by the way of syringe and rinsed the NaH a few times, a positive pressure of N2 or Argon will then
drive out the hexane, weigh back the RBF and charge the remaining reactants working backward with mole equivalents.
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Klute
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| Quote: | Originally posted by chemrox
I like the Na idea! Have you done it? |
Yes, but for Na metal only as we have NaH in oil at work.. But to transfert/cut needed amouts of Na metal, we keep some old Na chunks (the small
slightly oxidized/hydrolysed whitish peices left in empited cans of sodium) to dry fresh oil used to cover the sodium during cutting and transfert,
leaving the chunks for 15-30min before removing them and decanting the oil from any particules. When we repack sodium portions (20-75g) that really
needs to stay fresh, we do that under argon with schlenk filtering. The fresh sodium never bubbles once introduced in the treated oil.
We usually use pentane for rinsing Na/NaH as it evaporates even faster than hexanes, and is notably less toxic. Again, adding oldish sodium to
bottled pentane insures it is completly anhydrous.
What are you going to use the NaH for? Enolates? Just curious.
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Nicodem
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| Quote: | Originally posted by chemrox
My NaH is dry in the can so should I start by getting it into oil? Does mineral oil off the shelf need to be dried? How? |
Are you sure? It's been a long time since I heard of naked, unparafinized NaH. I don't even know if it is commercially available anymore. It seems
unusual that you would have unparafinized NaH available. Better check the label again. It should say something like 60% NaH in paraffin or whatever
percentage.
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unworthy of the scientist. Some of the greatest men of science have publicly repudiated a theory which earlier they hotly defended. In this lies their
scientific temper, not in the scientific defense of the theory. - Weston La Barre (Ghost Dance, 1972)
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Nick F
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I've never had to use the stuff myself, but I noticed that the stuff in the labs at my university (~50% NaH in hexanes) did look quite dry, more like
a clumpy powder rather than a paste, which I would have expected. So maybe it just looks like you have dry NaH, but don't?
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grind
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| Quote: | Originally posted by Gargi
Its always better to use NaH wetted in oil |
Some reactions need the dry NaH, the oil-wetted hydride doesn´t work there.
NaH wetted with oil, i.e. the commercially available 60% dispersion, is in most cases a free-flowing powder that looks dry, but is not.
As far as I know, only Aldrich sells really dry NaH (assay 95%).
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Ozone
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My stuff is in mineral oil. I transfer the material to a tared flask through a Buna septum, rinse the material several times with pentane (with argon
purge to blow-down the solvent) to remove the oil and re-weigh to determine mass of the NaH.
This gives the mass as dry NaH, under argon.
Seems to work well,
O3
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grind
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| Quote: | Originally posted by Ozone
Seems to work well, |
That´s the usual method to remove the mineral oil. Instead of pentane you can use toluene, benzene, THF, hexane, ether and other solvents. But it´s
nearly impossible to completely remove the oil and the NaH treated in this way is not comparable in reactivity with "a priori" dry hydride.
[Edited on 23-3-2008 by grind]
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jizmaster
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| Quote: | Originally posted by grind
LAH is more dangerous. NaH wetted with oil doesn´t ignite in the air as easy as LAH, but reaction with water is very vigorous. Don´t heat it with
DMF to elevated temperatures or explosion can occur.
Oilfree NaH can be considered as dangerous as LAH. It´s ability to ignite in (moist) air is even greater compared with LAH.
Always use carefully dried solvents when working with NaH. Use of dry argon or nitrogen is necessary for transfering oilfree (dry) hydride, you can
omit this in the case of NaH wetted with oil. |
I've never had LAH ignite in air while weighing/transferring it? The NaH in oil is pretty safe to handle, i wouldn't particularly want to work with
the 100% stuff though! Someone in uni told me how they ordered some and it came in a plastic bag, caught fire while transferring it!
The 60% stuff still goes nuts with water though, i started a fire with it by throwing the spatula i'd used to weigh it in the sink, which was wet and
full of acetone 
Thanks for the heads up about DMF though, i didn't realise that, how does that happen do you think?
How would it sometimes help to use dry NaH? I can't see that the oil would make a difference to its reactivity, it'll just dissolve in the solvent
surely? I used to wash it with petrol but it was horrible, trying to remove the solvent without getting the hydride in with it. I soon gave up and
just used it as is which worked fine, though i never used it for anything tricky, usually deprotonating DMSO or alcohols. The oil is easily removed
during chromatography since it's usually much less polar than the product.
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grind
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| Quote: | Originally posted by jizmaster
Someone in uni told me how they ordered some and it came in a plastic bag, caught fire while transferring it! |
This is not unusual. Transfers are better done in a glove box filled with dry argon.
| Quote: | Originally posted by jizmaster
Thanks for the heads up about DMF though, i didn't realise that, how does that happen do you think? |
There is a warning in the ALFA chemicals catalogue:
"CAUTION! Thermal runaway reactions can occur with NaH".
But I don´t know, what exactly happens.
| Quote: | Originally posted by jizmaster
I can't see that the oil would make a difference to its reactivity, it'll just dissolve in the solvent surely? |
The different reactivity concerns only a few special cases. In most reactions there is no difference.
In most solvents (hexane, ether, THF, toluene....) the oil will dissolve surely. Possible exceptions are DMF or DMSO.
[Edited on 26-3-2008 by grind]
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indazole
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Hi,
everybody knows that the use of NaH in DMF can lead to violent exothermic decomposition on scale-ups, but I red that the replacement of DMF with a
DMF/Toluene mixture could be a safer alternative; so, I lost such informations, and I tried to find the articles but I didn't found.
Could anyone help me, could anyone indicate any articles in which the mixture is used whith the target of a safe process?
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zed
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My buddies used to utilize large quantities of LiAlH4 powder, that was pacified with a coating of mineral oil. Made a nice thick paste. The material
was fairly easy to work with. Never had a mishap during initial transfer. Used no special precautions.
I've been quite surprised to hear, that contemporary chemists seem unfamiliar with the material. It was a product offered by Ventron. Which I
believe, was a division of Morton-Thiokol.
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indazole
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I suppose that you didn't want to answer me...
Quote: Originally posted by zed  | My buddies used to utilize large quantities of LiAlH4 powder, that was pacified with a coating of mineral oil. Made a nice thick paste. The material
was fairly easy to work with. Never had a mishap during initial transfer. Used no special precautions.
I've been quite surprised to hear, that contemporary chemists seem unfamiliar with the material. It was a product offered by Ventron. Which I
believe, was a division of Morton-Thiokol.
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zed
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DMF is an Amide. I would never have considered DMF as a hydride solvent, for the simple reason that Amides are not inert to hydride reduction.
It may be, that Amide reduction does not proceed at low temperatures. But, when the reaction mixture reaches a specific elevated temperature, it
starts to "go". Thereby elevating the temperature even higher.
Bingo! Runaway reaction.
Also note; hydrides themselves are subject to spontaneous decomposition upon heating. No solvent or other reactants required. The mis-matched
elements are happy to return to their native states if given the opportunity.
If the decomposition itself, fails to be extremely exothermic, the gaseous expansion presents its own peril. Less than a hundred CCs of NaH, upon
decomposition.... yields ~22 L of flammable H2 gas.
Me? I'm no expert, but I would look for another solvent.
[Edited on 30-10-2010 by zed]
[Edited on 30-10-2010 by zed]
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