chemrox - 20-11-2014 at 00:53
Have any of you used chunk LAH in place of LAH powder? I acquired a can sometime back thinking it would be easier to store and less likely to ignite
in humid air. I had the powder and just ran out. I want to try the hydride for hydrogenolysis of an heterocyclic arylhalide.
Dr.Bob - 20-11-2014 at 08:26
The chunk should work, may be hard to disolve it quickly, but it will work. The main issue is stirring, if you use a mechanical paddle, it might
clog or break it. For a magnetic stirbar, I don't think it will be too bad. You could try to break it up a little in a mortar and pestle or in thick
platic bag with a hammer (might want to purge bag with argon or N2 then squeeze most out.)
chemrox - 21-11-2014 at 12:14
I don't know the hardness of the reagent. The plastic bag method you propose has definite appeal because I wouldn't have to use a solvent to macerate
it. So whether I can break it up in a bag without ripping the bag is the issue. I would pre-flush the bag with Ar. I have all the stirring options
available so there's no worry there. I need to get it fine enough to react more or less at the same time. I'd probably run the reduction in ether or
THF under Ar. Have you any idea how hard the stuff is?
aga - 21-11-2014 at 12:27
Use several layers of re-sealable bags.
Bash with a rubber hammer, or if a steel one is necessary, use a piece of wood over the bagged material before bashing the hell out of it.
Dr.Bob - 21-11-2014 at 12:31
The piece of wood works, or even better put a piece of flat iron or aluminum on top. I have used an old aluminum heating block to break ice, dry
ice, and many other hard reagents like that before, the flat surface keeps the bag from breaking as easily. I would suggest always doing that
operation somewhere safe. Outdoors in the winter is a cold and dry place to do hydride preparation. And if the bag should break or catch fire, LAH
is great for melting snow, for those in NY.
zed - 24-11-2014 at 19:33
Always preferred the paste; LAlH4 in mineral oil. Formerly a product offered by the Ventron Corporation. A division of Morton-Thiokol.
Pretty safe to handle.
Now, you got yourself a wolverine in a plastic bag, and your gettin' ready to hit it with a rubber hammer.
I like the idea better, of shaking the LiAlH4 inside the can, with a couple of ball bearings. Maybe put a plastic bag around the outside of the can.
In other words, Ball Milling.
DJF90 - 25-11-2014 at 00:06
You can purchase pelletised LAH reagent which is safer than powder due to the limited surface area during dissolution. If you're intending on using an
ethereal solvent then i wouldn't worry about chunks as they'll all dissolve eventually (assuming they're not large rocks... these would benefit from a
reduction in particle size). If you're looking to charge solid LAH to a reaction mixture the consider the use of a solution. If tou're looking to use
a slurry in inert solvent (toluene?) then you're going to have to reduce the particle size. As zed suggests, ball milling (possibly with toluene
present?) could provide a good option.
[Edited on 25-11-2014 by DJF90]
adk - 7-12-2014 at 19:00
I have had success by gradually adding the pellets to mag stirred solvent (I guess you will be using some kind of ether; Et2O, THF etc).
I find that the pellets are a lot easier to handle than the powder. LAH powder sticks to everything (static!) and then ends up spilling everywhere. I
do not recommend the bag/mallet idea unless you want a fire or some caustic burns.
Some kind of gentle inert gas flow (balloon & septa, gas inlet etc) is always a good idea when working with LAH.
Let us know how you go.
A