Purifying CDCl3

Every chemist who dabbles in organic synthesis from time to time uses deutero-chloroform. In fact, it is probably the most commonly used NMR solvent in synthetic chemistry. Do you ever stop to think about what is really in the bottle? Most chemicals come with a standard set of impurities, and we usually ignore the ones that don't affect us. But occasionally when you run into a problem you can't solve, looking at the assumptions you make can make all the difference.

Last week I had a problem purifying a borane that I was synthesizing: Ph2BCl. Doesn't really matter why, but after trying countless recrystallizations, sublimations, washings, etc. I was frustrated and had no idea what the problem was. My colleague suggested that I try purifying my CDCl3 to make sure I wasn't destroying it in the NMR tube. Turns out that wasn't the problem, but it was a good suggestion and I am posting my method for quick and dirty purification for those of you who might have the same problems.

The two most common impurities in CDCl3 are HCl and EtOH. The ethanol is a stabilizer, the HCl a decomp product. You can quickly remove the HCl by filtering through a column of basic alumina, but the procedure below is a more thorough method.

1. Wash the chloroform 3x with water (and once with brine)
2. Dry and basify over K2CO3 (I waited a few hours stirring the CDCl3 over an excessive amount of base)
3. Filter away the excess base
4. Distill into a clean/dry flask--you can rinse the flask with CDCl3 first if you want to be sure there is no residue clinging to the flask walls 
5. Dry over 3A molecular sieves and degas if necessary by sparging with inert gas (Ar, N2) or 3 freeze/pump/thaw cycles

That CDCl3 should be pure enough for most applications. Good luck!