From: Manuel Toennis (Manuel.Toennis$##$ch.tum.de)
Date: Fri Jul 02 1999 - 09:09:00 EDT
> Glen Brizius wrote:
> Diethyl ether stills are a little trickier, as the boiling point is so =
> low. We've found that the drying species that seems to work the best =
> here is a potassium / sodium alloy, which has the advantage of being a =
> liquid at room temperature. This alloy can be prepared (by the =
> courageous) by melting potassium / sodium chunks in an evacuated flask =
> by means of a heat gun, then transferring the alloy (which looks =
> remarkably like mercury) to the still via pipet. This again overcomes =
> the problem of these large metal chunks developing a crust and becoming =
> inactive.
To my experience you only have to use pieces of sodium and potassium
(1:2), add them together to the ether still and boil for several
hours. The alloy forms after some time (ok.: once it took a long
time...) As the potassium reacts with the water, etc in your ether,
you will have to add potassium from time to time, but no further
sodium.
>
> Of course, with these liquid alkali metals you have to use extreme =
> caution, we've already had an incident involving refilling an ether =
> still that resulted in a fire extinguisher being discharged. But =
> overall, these methods seem to work well.
The alloy can ignite spontanously if exposed to the air, if you have
it stick on a spatula, dont touch anything with it ( it is impressing
to see what happens, if you 'clean' the spatula by rubbing on a paper
towel !)
The stuff can be 'disarmed' with decomposition in pentanol.
Ether dried several hours (over night) over Na/K alloy (without
Benzoephenone!) gives only very little evolution of gas with LiAlH4
and gives very good results with the corresponding reductions (no
alcohols are formed by reduction of an amide).
But if such ether still isn't dry enough, a final drying step with
LiAlH4 is recommended.
By the way: the blue colour of the benzoephenoneketyl shows only that
theres no oxygen (radicals) present, it is absolutely no sign for
absence of water !
Bye, Manuel
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