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May 11, 2006
A Day at the Rota-Vap
I've been spending a good part of the last couple of days rota-vapping down toluene. Why would I do such a thing, you ask? Because I ran a big column in the stuff, first time I've ever done that, I think. The chemists in the audience will guess that I examined many alternatives before settling on this one, and they are correct. Toluene's not especially toxic or smelly, but it is rather high-boiling compared to most of the solvents that we use for purifying things. You have to turn your water bath up and allow for plenty of time when you're taking a lot of it off. In this case, it turned out to be by far the best solvent for separating two closely running compounds.
Here's how I got there - it's a good illustration of a day-to-day chemical problem. I wandered through all sorts of common (and uncommon) solvent mixtures, checking each by thin-layer chromatography (a quick way to see if things separate). In ethyl acetate/hexane, the standard brew, you could just tell that there were two things in there after running a TLC plate three times. Switching to ether/hexane, which sometimes does the trick, was no help. Straight dichloromethane gave too fast-runnig a spot, and mixtures of dichloromethane-hexane didn't separate anything. Chloroform/hexane, on the other hand, wasn't too bad - not as good as toluene, but at least you could see some daylight in between the two spots. Isopropanol/hexane did no good at all.
I was trying here to run a bunch of different solvent types. Hexane is a common theme, since it's pretty much the plain vanilla of solvents. More polar stuff is added to it, and you see what happens. In this case, an ester co-solvent did a little bit of good, but ether and alcohol additions did nothing. Chlorinated solvents showed some promise - well, at least chloroform did. But it's an oddity, more polar than the others of its kind. Toluene was the only aromatic solvent I tried (it's really the only convenient one for chromatography), and something about its flat shape and electron clouds did the trick. This stuff is brutally empirical.
So toluene it was, with a little ethyl acetate at the end to speed the latter spot along. I got some mixed fractions, naturally, but quite a few clean ones, certainly enough for my purposes. (If I get bored or desperate, I can go back and re-run the mixed ones).
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