Benzophenone is a colorless solid, tetrahydrofuran is a colorless liquid, and sodium metal is unremarkable stuff. When pure, it's silvery and bright, but it tarnishes easily in the presence of oxygen or water, leaving it looking like any other metal - kinda greyish silver. If you mix the three together and boil them for a few hours (sometimes you don't even have to do that if everything's very pure and dry), you end up with an impossibly dark royal blue-purple solution. This is benzophenone ketyl radical (and the related benzophenone dianion - reduced one step further by one more atom of Na0).
Oxygen or water will react with ketyl radical, meaning you can produce dry, oxygen-free THF. This is desirable because many reactions won't work with even traces of water. The benzophenone actually decomposes a little bit, producing benzene and some other contaminants, but these are present at fairly low levels (and they usually don't interfere with your reaction).
If the still pot has to remove too much water, you end up with a clear solution again (rarely), or a rust-colored slime (happens more often). Often boiling it for a few hours again will dry it out, but usually, you end up having to break down the still, safely disposing of the sodium chunks. This can take a whole afternoon.
Stills are pretty dangerous, so you tend to see kegs of dry solvents used in industry. The solvents are passed over columns of drying agents right before use to ensure they're completely dry. I've even seen this setup in a few academic labs (usually younger guys). Ketyl drying might not make it another generation - it's already a sort of oral tradition, and economies of scale will make other methods of getting dry solvents cheaper and cheaper.
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See http://membership.acs.org/c/ccs/resources/CHS_10_4_distillation.pdf for some of the dangers of stills, and a discussion of the trade offs betweens stills and column systems.
Incidentally, I've definitely distilled solvents this way. I wouldn't expect to see this disappear from academic laboratories (particularly the less well funded ones) until insurance mandates it. That being said, I'm a physical chemist, and we don't even have any solvent stills or stills of any kind.
Uncle Al begs to differ. There is benzophenone ketyl (blue-green) and benzophenone dianion (deep red). The combo is the extraordinarily huge epsilon deep purple wonder. Aside from being dry (~10 ppm water) and free of oxygen, Na/benzopehone-distilled THF is also freed of peroxides. Any rotovap can be a peroxide sensor - once.
The only purples that come close are Crystal Violet and (CO)5[W,Mo,Cr]S=CPh2. S=CPh2 itself is an extraordinary cerulean blue (better than vanadyl sulfate). M(CO)6 + S=CPh2 + h(nu) in solvent. No doubt an inorganiker can pontificate as to why only one goes on.
Fixed, thanks.