Re: advice sought on LAH

[istoutgrill]

On Feb 9, 6:04 am, "FJC2007" <francisco.ja...@gmail.com> wrote:
> 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

I don't know where you learned to do synthesis, but feeding HPLC-grade
THF into your still is an egregious waste of money. Not necessary.