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advice sought on LAH

advice sought on LAH - Organic Chemistry Forum

advice sought on LAH - Organic Chemistry Forum


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Old 02-09-2007, 10:04 AM
FJC2007
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Default advice sought on LAH



Hi

When working with LAH dryness of THF (or solvent you are using) is
very important. The common procedure to dry THF (and ethers) is using
metallic Na. You must know Na is very reactive with H2O, and a
dangerous exothermic reaction occurs, so in general you must avoid
contact with water. I use Na in a paraffin dispersion (from Aldrich),
although sodium comes in many ways (kerosene, lingots, etc). I think
sodium in paraffin is very safe, but when you need sodium for
reactions, paraffin dispersion could be a problem. Besides paraffin
could dissolves well in warm or hot THF or diethylether. You must use
a good grade quality THF (HPLC quality or similar) to ensure that it
has very low concentration of water. If you don't have a good THF, you
must dry it previously with anhydrous salts (for hours or days
depending of the quality).Remember that ethers could produce organic
explosive peroxydes when they are stored for long times, so, if you
have a THF with unknow origin better you test it for peroxydes (look
for procedures in a organic-analytical books). If you need to
eliminate peroxydes from ether there is a lot of methods you find in
books. The usual method is to use a FeCl3 solution to transform
peroxydes in oxydes that precipitate and could be filtered off. Then,
having a good THF you proceed to distill it with sodium. You need a
closed distillation system with a stream of a inert gas (nitrogen,
helium, rgon, etc) Oxygen must be avoided. So you put very carefully
some sodium (in this case sodium dispersion in paraffin) in a round
bottom flask, with stirring or shaking. Remember that the amount of
sodium depends of the water percentage in THF. When you finished it,
you begin to distillate THF (be care with water in the refrigerant/
condenser). To make sure if THF is dry, before distillation you can
put a little amount of benzophenone; sodium reacts with benzophenone
in absence of water to make a radical species with a intense blue
color. So when you see that colour you must be sure that THF is
completly anhydrous and then start the distillation. Fresh, distillate
THF should be use immediatly. So you must have your reduction system
ready. I usually pour the THF in the flask and close it (sometimes you
can use something filled with a dissicant salt like CaCl2 or passing
through a stream of an inert gas). Next, I add the LAH in the flask
with anhydrous THF and close the system again: Handle LAH is like to
handle Na, you must be very careful and always avoid contact with
water, protect your eyes and avoid to aspire LAH (I recommend you use
a lab gas mask or something similar). Try to weigh it once. To put
your reactant into the suspension of LAH in THF I recommend you use an
addition funnel and put your reactant previously dissolved in
anhydrous THF into the funnel, so you add the dissolved reactant into
the LAH/THF mixture very carefuly (drop by drop) because many
reactions with LAH are exotermic (it depends on the reaction features,
some reductions with THF take place at very low temperatures). That's
all. If you have any question about it just mail me. Note: rememeber
to be very careful, try always to perform all in a fume-hood or a very
ventilated place and with all protection needed. To wash materials
with traces of sodium or LAH don't use water, try to use first
alcohols or acetone.

Bye

FJC

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