Fun with CaCl2
In my earlier post on "Harry Potter Chemistry", I ranted on the popular portrayal of chemistry as leaving out workup/purification of the reaction mixture in order to obtain a pure product. In this installment, I'm gonna just rant about wash liquids.
So you've got your reaction mixture, in some sort of organic solvent, and you want to clean it up. There's all sorts of aqueous - water based - solutions you can wash your mixture with to clean it up. Suppose there's base in your reaction mixture, you can remove it by shaking it up with aqueous acid. Or you can remove acidic impurities by washing with aqueous base. Easy and fun. Maybe you want to remove something more exotic. Like iodine, for example? Wash with aqueous sodium sulfite, it'll reduce it to iodide which is soluble in water and you can just get rid of it that way.
Here's a crazy wash liquid I've been using - 10% aqueous CaCl2. I've been using it to remove fluoride from various reactions that I have been doing. Sometimes it's important to get rid of every last trace of fluoride from a reaction. Suppose fluoride removes a protecting group, and then you take that deprotected compound on to a coupling with another compound containing the same protecting group - a little bit of leftover fluoride can really make a huge mess, especially if you're working microscale and have nothing to waste. And it's not readily visible on 1H NMR, or TLC, you could totally be fooled if you don't bother to take a careful 19F NMR and even that isn't necessarily a guarantee of freedom from fluoride. So if I want to get rid of F-, I always do, as my final wash, a calcium chloride wash these days just to be extra, extra paranoid. Sure, water will extract the vast majority of it, but I'm disinclined to take chances with fluoride these days.
Aqueous CaCl2 is rather a weird wash liquid, because unlike most wash liquids it doesn't solubilize F-. In fact, fluoride is completely insoluble in CaCl2 solution! It falls right out as CaF2.
That's fine with me, though, it's totally fine. CaF2 is plenty inert, so long as there's no acid around, so it won't screw up the next step so bad even if a little stays in. And I can just filter it out later on, when I'm filtering off my drying agent anyway. One thing I worry about though... if I get CaF2 in my glassware it's not going to be removed by water, or soap, or any solvents..... hopefully I can remove most by scrubbing .... but the rest will just lurk and lurk until one day I add some acid and poof! It's nasty, reactive, glass-etching, poisonous as heck hydrofluoric acid! Well, that's why this is pretty much a microscale technique for me.
Sure I could just put the reaction through a calcium-salt packed column, but like I said, valuable milligrams. This is a goofy workup, but it works for me, and in the end what matters is what makes me happy, right? And what makes you happy. So think of calcium chloride next time you have to clean up fluoride from your reaction.