Over at Org Prep Daily, Milkshake did an excellent post earlier this month on extraction techniques. It’s well worth looking over, even for experienced lab crew. Solvent extractions are a way of life for organic chemists, a fact that has not changed since the beginning days of the science, because (for one) we still do the bulk of our reactions in one sort of solvent or another, and (two) because the bulk of our reactions need to have garbage removed from them, and this is the first line of cleanup. (Here it is in real life - scroll down).
For those readers who aren’t chemists, three paragraphs of explanation: extraction works on the “like dissolves like” principle. A look at a bottle of oil-and-vinegar based salad dressing that’s been sitting for a while will show the familiar layers, with the oil on top and the aqueous layer below. If you were to take samples out of each and analyze them, you’d find that they contain rather different parts of the dressing mixture. The oil layer will have the compounds that can’t dissolve in water very well – organic pigments like carotenes or lycopenes, for example. They’re better off in with the oil molecules; they don’t have any polar molecular features strong enough to horn in on the hydrogen bonds that water uses to stick to itself.
Down in the water layer, on the other hand, is all the stuff that has such polarity. All the amino acids and most proteins will be there, as will the sugars and other soluble carbohydrates. These compounds have a lot of groups (hydroxyls, amines, carboxylic acids) that can interact strongly with the small, polar water molecules. Since there’s a lot of vinegar in there, too, acid-base chemistry will be a factor. The compounds with strongly basic groups (fishy amines and the like) will be protonated by that acetic acid, and the resulting salt is a natural for the water layer. A few base-containing things that might otherwise be just as happy in the oil layer will be pulled in by this effect.
So if you have a messy mixture of stuff, you can separate the greasy components from the polar ones by shaking up the lot in a mixture of water and some solvent that’ll form a separate layer. Sometimes you’ll want one layer, sometimes the other, depending on what your product is like, but most of the time organic chemists are throwing out the water layer and keeping the other one. There are other tricks – for example, if your compound’s acidic or basic, you can adjust the aqueous layer the other way to hold it as a salt, wash out all the other goo with solvent, and then change the acidity so that your compound will now go into a fresh solvent wash. In all these cases, you drain off the appropriate layers with one of these.
I would not like to hazard a guess at how many extractions I’ve done. Shaking a sep funnel is such a basic act of organic chemistry – every time you mix something up and wait for the layers to separate, you’re participating in a rite that goes back to the days when labs were only illuminated by sunlight or fire. It’s one of the few things that a scientist from the 1850s would immediately recognize if teleported in front of my fume hood, that’s for sure (the fume hood itself would be a revelation, for starters).
I have to say, though, that Milkshake’s nom de blog is an unfortunate one for the topic, since one of the worst things that can happen to you during an extraction is a thick emulsion. That’s when the layers don’t want to separate – millions of tiny droplets of each component decide, for various irritating reasons, that they’re happy where they are, thanks, and don’t pair off with their former comrades. The result is a thick, opaque mess, so the name for the most intractable emulsions is, naturally, “milkshake”.
One of the things he mentions as seldom seen these days is actually one of my favorite pieces of lab apparatus: the liquid-liquid continuous extractor. Sad to say, I don’t have one these days, but I could find or buy one if the need arose. There’s something appealing about setting up a continuous, automatic purification. Being able to see the results over time (the solvent pot in these things gets uglier) makes you feel as if you’re getting something done even if all you’re doing is standing there watching the extractor. A fine apparatus it is, and worth a post of its own some day. . .