From: Glen Brizius (brizius$##$psc.sc.edu)
Date: Fri Jul 02 1999 - 03:21:51 EDT
On the subject of drying THF, myself and other workers in my lab just =
use benzophenone / potassium. The melting point of the potassium is =
just below the boiling point of THF, so when the solvent begins to =
reflux / be collected in the still, the potassium is molten. This =
avoids the problem of crusty sodium with low surface area. We've used =
this method for many "dry solvent needed" reactions - organometallics, =
lithium species, grignards, etc - with no problem at all.
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.
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.
Glen Brizius
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