I've seen, in a few places, that it isn't recommended to heat LAH (in THF) using heating mantles. The reasoning being that LAH can ignite at around
125C and the mantles heat unevenly.
What about using a voltage regulator that would keep the temperature in the flask at around 50C?
Also, I was wondering if the regulator could, at some point, fail and send the full 110 volts?
entropy51 - 28-1-2012 at 18:56
It's been a long time since I ran an LAH reduction, but my recollection is that they didn't require heating. It was necessary to add the compound to
be be reduced at a slow rate in order to maintain a gentle reflux from the heat of reaction. I vaguely recall that one particular reduction may have
required some heating, and if this is so I'm sure we used a mantle since that was the only heat source we ever used, except of course oil baths for
distillations.
I don't know about the solid state voltage regulators, but the only way I've ever known a Variac autotransformer to fail was a blown fuse or bad
brushes, in which case they failed open circuit.
With LAH I think the danger is the presence of unsuspected moisture, not the uniformity of heating. Nicodem - 29-1-2012 at 02:10
Reductions with LiAlH4 are indeed commonly performed in refluxing THF. In my experience, some reductions are really slow even at reflux, particularly
reductions of bulky or secondary amides can be annoyingly slow (I could never reach full conversion in some occasions even after 2 days of reflux).
Some other types of substrates, such as esters, ketones and aldehydes are said to generally proceed rapidly at room temperature (though I never used
LiAlH4 for these as there are better options available). I suspect the freshness of the reagent also has a great influence on the reaction rate and
efficiency.
Sometimes reversing the reaction selectivity by forming the "acidic" reducing reagent alane (AlH3) in situ from the "basic" LiAlH4 allows for
reducing the less electrophilic functional groups (such as amides) much faster than LiAlH4 itself does. I have no experience with this alternative and
can't say if it really solves the problem, but if it does, it can perhaps allow you to avoid reflux conditions. In any case, the solvation of LiAlH4
is exothermic enough to heat THF to reflux and so is the initial reaction with the substrate which has to be added slowly and carefully (so a reflux
condenser and some kind of bath as a heat sink is needed anyway, even if you want to do the reaction at ambient temperature).
Anyway, I would avoid heating mantles for refluxing such reactions. After all, you are heating a pyrophoric substance in a highly flammable solvent
and heating mantles have a certain annoying statistical tendency to cause heat shock related breaking of the larger flasks. LiAlH4 also thermally
decomposes via disproportionation at higher temperatures. In an oil bath you can at least avoid the overheating by keeping the reaction mixture level
above the oil level and application of good stirring (mind that LiAlH4 attacks teflon, so your stirrbar will look nicely carbonized after the
reduction). It just seems safer to use an oil bath, but you are free to use a heat mantle if you take all the precautions and possibly avoid leaving
the reaction unattended (and keep flammable solvents out of the vicinity!).soma - 30-1-2012 at 17:57
Would you have any (online) literature references? I used to have a paper that discussed this but haven't been able to find it.
I was wondering what acid to use to acidify LAH. I think in the article they used P2O5. Was wondering how to determine how much P2O5 to add?
Thanks.
[Edited on 31-1-2012 by soma]entropy51 - 30-1-2012 at 19:10
Would you have any (online) literature references? I used to have a paper that discussed this but haven't been able to find it.
I was wondering what acid to use to acidify LAH. I think in the article they used P2O5. Was wondering how to determine how much P2O5 to add?
Thanks.
[Edited on 31-1-2012 by soma]
The problem with workup of LAH reductions is to obtain something that is
filterable. A workup that I learned a long time ago is that for x grams of LAH first add dropwise x mL of H2O, then dropwise x mL of 15% NaOH, then
finally dropwise 3x mL of H2O. This generally produces a precipitate that can be filtered and washed. Nicodem undoubtedly knows of a better way.soma - 30-1-2012 at 20:22
Sorry, I was referring to this:
Quote:
Sometimes reversing the reaction selectivity by forming the "acidic" reducing reagent alane (AlH3) in situ from the "basic" LiAlH4 allows for reducing
the less electrophilic functional groups (such as amides) much faster than LiAlH4 itself does.
Looking for online references and for how much P2O5 to add to acidify LAH.Nicodem - 31-1-2012 at 13:09
A workup that I learned a long time ago is that for x grams of LAH first add dropwise x mL of H2O, then dropwise x mL of 15% NaOH, then finally
dropwise 3x mL of H2O. This generally produces a precipitate that can be filtered and washed.
That sounds OK, but the violent quench with water should not be underestimated. A couple of drops can cause strong heating and violent foaming of the
evolved hydrogen. What I generally do (but everyone has his own preferred technique), is to first let the oil bath cool to ambient temperature, then
dilute the reaction mixture with some methyl t-butyl ether or diisopropyl ether and then quench with 5M NaOH(aq) in a syringe (1 mL per g LiAlH4) by
adding very, very dropwisely while intensively stirring still in the bath which is now a heat sink (don't forget to remove the septum with the balloon
from the top of the condenser to let the rapidly formed hydrogen out). After some 30 min stirring and making sure there is no more hydrogen bubbling,
I filter off the gray solids, wash them with some more ether and then work up the filtrate, usually via some washing or extraction protocol, depending
on the type of the product (that's why I dilute with a water non-miscible ether). The filtering always go easily.
Looking for online references and for how much P2O5 to add to acidify LAH.
I don't know exactly what you mean by "online references", but examples of alane based reductions can be found in many scientific articles. I'm sure
you can take a few minutes to do a literature search. P2O5 is not an acid and I never saw it mentioned for in situ alane preparation. Concentrated
H2SO4 is generally used as a cheap acid (be careful and strictly follow the protocol or else you risk serious accidents!). Other nonreducible
anhydrous acids can be used as well. Otherwise, this topic has already been discussed at the forum.soma - 31-1-2012 at 14:16
Quote:
P2O5 is not an acid and I never saw it mentioned for in situ alane preparation.
The article I saw used it. Since it's the anhydride of phosphoric acid it would be similar to fuming sulfuric but without the unpleasant "fumes".soma - 9-11-2013 at 19:42
P2O5 is not an acid and I never saw it mentioned for in situ alane preparation.
It's the anhydride of phosphoric acid - just as fuming sulfuric is the anhydride of sulfuric acid?
[Edited on 10-11-2013 by soma]soma - 10-11-2013 at 23:23
Oops - sorry.
Didn't notice I had already posted the same thing about a year ago.
[Edited on 11-11-2013 by soma]chemrox - 16-11-2013 at 02:14
I had a contract making Grignard reagents for an engineer. I used a 22L flask with a heating mantle and was refluxing 10 liters of ether at a time.
The contract was about three years. I never had a problem. At the start of the reactions I had to heat up the ether to push the air and any water
vapor out as well as start the reax. Then I had to get the mantle out of the way and cool the pot with ice water using turkey basters. Once the reax
could be controlled by the addition of the halide solution the mantle went back. Borosillicate glass should obviate heat shock worries. Any time you
work with pyrophoric agents fire precautions should be in place. For LAH reductions I have a sand bucket. It is often necessary to reflux THF or
ether to add the material to be reduced with a Sohxlet modified for continuous throughput. I think there's a lot of hype among bees about the
'dangers' of LAH. @Soma: no progress in the year huh? (just poking fun).
I forgot about the voltage regulator part. I now use a controller that measures temperature in the refluxing solution. I use a thermometer adapter
with Viton 0-rings to hold the probe. The controller cuts out when the solution T rises above the T I set for the process. I run this through
another cutout that measures water flow. This allows me to leave the reax running overnight when I need to. As Nicodem pointed out, some LAH
reductions can go on for days. Reductive aminations can take a week...automation => peace of mind.
[Edited on 16-11-2013 by chemrox]zed - 21-11-2013 at 17:01
My Buddy, Dr. Death, once destroyed a small building via LiAlH4. But, he did it during the quenching process. Large reaction vessel, Diethyl Ether,
and lots of LiAlH4. Heat built up, the process ran out of control, his reaction vessel vented lots of Ether and H2, and Kablooie!
Wisely, he had run out of the building, a garage, just before it blew.
[Edited on 22-11-2013 by zed]
[Edited on 22-11-2013 by zed]vulture - 22-11-2013 at 01:50
Excess LAH can be destroyed safely (no H2 formation) by adding ethylacetate.
[Edited on 22-11-2013 by vulture]soma - 31-12-2013 at 14:08
Reductive aminations can take a week...automation => peace of mind.
I don't think I'd be able to do those kind of reactions. -- Too nervous. I don't trust automation. Maybe if the lab was in a disposable house with
nothing that could catch fire around it. Or maybe if there were multiple fail-safe devices that would go off if the others failed.
There was some news story recently about something that had about 5 fail-safe devices and 4 of them failed? Can't remember at the moment.
[Edited on 31-12-2013 by soma]madscientist - 14-3-2015 at 01:56
I have extensive experience with LAH reductions - AlH<sub>3</sub> solves the issues with sterically hindered amides. The difference is a 2
day reflux with 4 eq. of LAH in THF affording 50% conversion, and full conversion with 1 eq. AlH<sub>3</sub> in less than 30 minutes in an
ice bath. I generate the reagent by dissolving LAH in Et<sub>2</sub>O, and in another flask, AlCl<sub>3</sub>, in the same
solvent - and both chilled (addition of Et<sub>2</sub>O to AlCl<sub>3</sub> is exothermic and you want to bang in the solvent
fast to swamp the exotherm.) The AlCl<sub>3</sub> is added to the LAH, followed by the substrate, ideally in Et<sub>2</sub>O
or in a minimal volume of THF. The Fieser workup suffices.
I've heard of AlH<sub>3</sub> reductions taking down some amides and esters at -78C in minutes and I believe it. The thing to remember is
AlH<sub>3</sub> reduces amides in the order tertiary > secondary > primary, while LAH displays reversed reactivity.
Also, AlH<sub>3</sub> is great for conjugated aldehydes/ketones/esters/amides when strict 1,2 reduction is desired. Simply remember that
when your substrate is an electrophile, you need an electrophilic reducing reagent. AlH<sub>3</sub> is electrophilic (and oxophilic.) LAH
is nucleophilic, hence 1,4 reduction competes.
