mechem
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Lithium Al Hydride quick question
I was recently given a small BDH plastic bottle of reagent LAH. The bottle is quite old in appearance, prob. about 20 years old. In it is several
grams of a fine gray powder, about 1/3 full.
I put about 1/4 gram in a few ml of water, it fizzed a bit but nothing spectacular, a bit like putting anhy CaCl2 in water. I put a bit on a hotplate
and heated to about 400 deg-C it did not melt or change in appearance much, it just sat there.
Not sure how active this LAH is as it is supposed to melt at 150-C and decompose.
Does anyone have have any experience with LAH and weather it reacts more violently with water, as I feel this bottle might not be of much use.
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garage chemist
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I have some active and good LAH, and it only fizzes a bit when thrown on water as well.
You need to put it in some anhydrous ether, then it shows its real reactivity when you add a drop of wet ether or even water.
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Drunkguy
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Are u kidding? Fresh LiAlH4 fizzes like mad on contact with water!
It shouldn't stay good for 20 years, but if it's still gray then that indicates that it's still active.
I have saved some LiAlH4 for a few years and it changed from a gray powder which looks like zinc dust, into something white and brittle that indicated
decomposition.
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mechem
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Put a bit in ether, did not seem to dissolve even after 10-15 min/stir, added a bit of water as garage chemist mentions, only fizzed a bit giving off
H2.
Has the appearance of zinc dust, but does anyone know if it melts at 150C, as heated to 400c with no effect.
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Nicodem
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If you heated up to 400°C without it catching fire, then I would say that is indication enough that it is as good as dead. LiAlH4 is very pyrophoric,
that's why it is always handled under inert atmosphere. Exposing it to air is not a good idea (safety wise). That is, unless you heated it in inert
atmosphere but you did not mention.
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Gargi
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I too had a old stock of LAH ( probably 3 yrs old ) over the time the color degraded almost to off-white or like that of zinc dust.
putting a pinch of it in water showed no signs of reaction.
but beware active LAH can be nasty if in contact with water
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jizmaster
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Bought some LAH a couple of years ago, 100g, came in a god-damn plastic bag! Spilled it all over the place trying to get it into a few screw cap glass
bottles! Anyway it was white, i was pretty annoyed thinking it was useless since all the LAH i'd seen had been grey, and alumina is white. It fizzed
pretty vigorously when a little bit was put in water though, and got hot when i spilled some on my hand! Turned out to be the best LAH i've seen, much
better than the old grey stuff. Over the approximately 2 years it took to use it it turned light grey but still worked just as well.
You could see how much is still in there by adding a known mass to some ethanol and measuring the amount of hydrogen given off, like with borane.
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grind
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| Quote: | Originally posted by jizmaster
You could see how much is still in there by adding a known mass to some ethanol and measuring the amount of hydrogen given off, like with borane.
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I think ethanol is much too reactive for this purpose and a very vigorous reaction will occur.
May be better to suspend the LAH in THF and add slowly some ethanol or THF/water 10:1, this is not as dangerous.
LAH of exceptionally high purity is snowwhite. I guess the grey color is due to it contains a little bit of "suspended" aluminium metal.
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jizmaster
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Hehe, yeah good point!
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Drunkguy
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Really, well u learn something new every day!
I guess it is like the 5g of Cu(I)Cl I bought from Alfa-Aesar one time, I paid through the ass because it was stored under argon, and when it arrived
it was a light green color (it's supposed to be colorless when pure).
Or if you've ever bought photograde 1,4-benzoquinone, it is the most disguisting brown/black color you have ever seen in your life! Yuck! Pure
1,4-benzoquinone on the other hand is supposed to be much lighter in color, yellow/green IIRC.
Also, well done for pointing out that Al2O3 is the identity of the product after it has all degraded.
4LiAlH4 + 9O2 -> 2Al2O3 + 4LiO + 8H2O
or LiAlH4 + 4H2O -> Al(OH)3 + LiOH + 4H2
I guess some lithium oxide/hydroxide is in there also.
I remember when the police raided my house one time and they had cordened off the whole street to make the biggest scene you could ever wish to see.
I had some LiAlH4 still in the gold colored peal-top tin from Avocado that it arrived in about 3 years prior to this. I think I purchased 25g (not
from Avocado directly), but did not use it all up. I was at university alot at the time and the chemistry was just my bit on the side. In retrospect
thats incredibly dumb of me to be doing hobby chemistry while still an undergraduate. Anyway, I think i'd left about 10g in the tin and had not gotton
around to reusing it. I remember the police told my mum that I had enough of something to blow up the entire house! They later withdrew that statement
though which was obviously BS.
The ether is also a hazard when doing reductions. Bearing in mind that LiAlH4 can spark and ether is a demonstrably volatile and flammable solvent,
its all fuel for the fire if you have a little accident. The price of ether is almost as volatile as the actual solvent. When I bought a couple of
2.5L Winchesters I think I paid about £17 a bottle. Conversely, when I bought a 500ml bottle of anhydrous ether from VWR a few years later, I think
that sold for about £20.
[Edited on 28-3-2008 by Drunkguy]
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Foss_Jeane
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| Quote: | Originally posted by jizmaster
Bought some LAH a couple of years ago, 100g, came in a god-damn plastic bag! Spilled it all over the place trying to get it into a few screw cap glass
bottles! |
There's a reason for that: LAH + screw cap glass bottle= BOOM! You're lucky you spilled it otherwise you might have blown yourself
clear into next week.
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DJF90
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Apparently not as jismaster managed to keep it for 2 years, in what I assume to be these screw cap glass bottles...
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Fleaker
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I've never seen LAH come in anything but a thick vacuum-sealed bag in a can, in a box of vermiculite.
The material should be snow white and finely crystalline. Over time it will become gray and lose activity but it can be purified by making the
etherate and then evaporating off the ether in vacuo.
Neither flask nor beaker.
"Kid, you don't even know just what you don't know. "
--The Dark Lord Sauron
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vulture
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| Quote: |
There's a reason for that: LAH + screw cap glass bottle= BOOM! |
You have any references to back that up? I would hardly think LAH is a friction sensitive explosive.
[Edited on 17-5-2008 by vulture]
One shouldn't accept or resort to the mutilation of science to appease the mentally impaired.
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stoichiometric_steve
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| Quote: | Originally posted by vulture
I would hardly think LAH is a friction sensitive explosive.
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it's probably a very persistent lab myth, like the one with the Et2O peroxides...i know of an academic tutorial video which shows how LAH is correctly
transferred from the tin cans to a screw cap bottle. those guys gotta know where its @
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leu
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As far as screw cap bottles and LAH, there is nothing mentioned in Chapter 17 of Gaylord's Reductions With Complex Metal Hydrides but it does say
grinding it with a mortar and pestle is extremely hazardous so one might want to clean LAH off the threads before closing the cap just to be safe 
Chemistry is our Covalent Bond
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Nicodem
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LiAlH4 is very pyrophoric, but it certainly is not explosive, so all this about screw caps is just nonsense. Let's not make another chemical myth -
there are enough around already.
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panziandi
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My Lithium Aluminium Hydride came in pre-weighed plastic bags designed to dissolve in THF etc the bags came in sealed metal tins.
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Klute
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I had a bad experience with LAH in a screw bottle: It was new, and the first time I opened it (hopefully in a drybox) there must ahev been a certain
H2 build up as 1/4 screw off and bouf LAH dust all around the bottom of the box. At least 0.5g if not more (which is a certain volume considering the
very fine powder it is). I was expecting it it to catch fire at any moment, been worrie dof the 200mL of THF standing in a RBF in the box (made of
coated-wood). hopefully, nothing happened, I neutrlaized most of th epoxder with IPA, then cleaned up with a IPA-wet cloth, which wasn't the smartest
thing, but no fire or violent decomposition. Unfortunaly, the rim of the screw bottle is poxdered with LAH.. I got rid of most of it by gently
tapping the bottle, but it's still dusty. The bottle is kept in a can with a bed of CaCl2, so hopefully the LAH will just slowly degarde from any O2
present in the can. I am worried about the next time I'm going to open the bottle though, not having been able to flush it with argon last time
(theres in't much head space though).
So I think using wide neck screw bottles is more adviseable when repackaging LAH .
\"You can battle with a demon, you can embrace a demon; what the hell can you do with a fucking spiritual computer?\"
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Sandmeyer
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| Quote: | Originally posted by stoichiometric_steve
| Quote: | Originally posted by vulture
I would hardly think LAH is a friction sensitive explosive.
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it's probably a very persistent lab myth, like the one with the Et2O peroxides...i know of an academic tutorial video which shows how LAH is correctly
transferred from the tin cans to a screw cap bottle. those guys gotta know where its @ |
You can simply weigh LAH and transfer it to rxn flask like you do with any other powder, sure it will sometimes make you cough a bit, but it is not
much worse than handling NaOH cough-wise.
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stoichiometric_steve
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| Quote: | Originally posted by Nicodem
LiAlH4 is very pyrophoric, but it certainly is not explosive, so all this about screw caps is just nonsense. Let's not make another chemical myth -
there are enough around already. |
i'm not sure anymore if it is a myth, i read a guy somewhere on the net claiming that he powdered LAH pellets by smashing them with a hammer while the
pellets were wrapped in Al foil and at least every second pellet burst into flames.
i think that this methodology is a stupid idea.
another source says that you should just powder the pellets by picking at them with a spatula after putting them in the flask...
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ScienceSquirrel
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Lithium aluminium hydride can be ignited by friction so bashing it with a hammer would be a bad idea.
It could be powdered with a mortar and pestle inside a nitrogen filled glove box.
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stoichiometric_steve
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they say right here that they removed peroxides from TBME/MTBE with LAH. i thought TMBE/MTBE wasnt compatible with strongly alkaline substances, am i
wrong?
http://dx.doi.org/10.1016/S1570-0232(02)00371-9
also here
http://www.wipo.int/pctdb/en/wo.jsp?wo=2004106269&IA=WO2...
[Edited on 26-6-2008 by stoichiometric_steve]
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Maya
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Duh.................
I have not followed this thread but I suppose the purpose is to get the pure LAH out of the impure stuff.
in that case the only way to do this is to dissolve the LAH in pure dry ether
nothing else will work
I would stoichiometrictrly work out the mole conc of LAH from there and use it as is for its final purpose
nothing else would really make sense with such a labile compound
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