The LAH will remove any water still left in the THF, if you use a slight excess of LAH, it should destroy any water just fine. Unlike some
reactions, trace water will not hurt an LAH reaction, if you have a slight excess. However, they are easy reactions to set fire to, so please do it
in this order, unless you have a good reason not to.
Purge the rbf with Nitrogen or other dry gas, you can always use some dryrite to dry it, but most compressed N2 is fine. Mostly, you want to remove
the oxygen from the flask, as that is bad when combined with LAH and an ether. The LAH and ether or THF will often release some heat when combined,
so best not to be in oxygen or air.
Once the flask is dry and inert, put the LAH needed in the flask and purge it again, carefully, as LAH is easily blown about.
Once you are confident that there is no oxygen present, you can transfer the THF or ether into the flask via a syringe or cannulae safely. It may
bubble a bit at first, due to both the LAH reacting with any traces of water, and also the LAH complexing the ether as it dissolves. But if there is
no oxygen this cannot start a fire. Many people chill the reaction flask now in ice water, making sure not to crack the rbf while stirring, as that
will be exciting.
Once the LAH is in solution or suspension (depending on the concentration) the next step is to dissolve the substrate into the THF and SLOWLY drip it
into the LAH, at least at first until you see how reactive it is. In some cases you can add it quickly, but if added too fast, the whole thing can
erupt like a volcano and shoot fire out the top. I have seen it done by someone else, so I know it can happen.
If you try to add LAH powder to the THF or worse yet, ether, it can create a dust cloud that will ignite readily in air and cause a fire, this is the
WRONG way to do this. I have seen this also, shortly after telling someone not to do that, and they did it anyway and burned their entire hood.
DON'T do this, unless you know exactly what you are doing, and are well trained in fire control and away from people.
Hope I am not being too bossy, but I have seen the results of bad LAH reactions, they must be done carefully, and are best done by adding substrate to
a solution of LAH. There are likely some exceptions to that rule, but I don't know any off hand. The temp needed to keep the reaction under control
depends on the substrate and scale/concentration. For acids, you might have to heat it to get a reaction, for ketones, you likely need to cool it to
keep if from overheating. I am setting up a ketone reduction currently, which is iced to keep it slow, but the substrate must be added slowly so as
to not blow it up from overheating the solvent.
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