Good examples of solvents with high boiling points commonly used by organic chemists include dimethylformamide (DMF), dimethylsulfoxide (DMSO), and, at least in the ‘old days’, hexamethylphosphoramide (HMPA). If you are fortunate to want to remove the solvent from something that is non-volatile and not too heat sensitive, the common approach is to use a rotovap to strip off the solvent. The procedure is done at reduced pressure (either water aspirator, ~25 mm Hg, or vacuum pump (< 0.1 mm Hg) and the application of heat (water bath kinds of temperatures).
There’s a number of tricks you can use, depending on the solubility of your compounds.
- As mentioned, chromatography can be helpful if you can find a solvent that will elute your compound separately from the solvent.
- sometimes, drying by centrifugal evaporation is your only method, and you just have to be patient. My preference was for freeze drying, which could be done in trays and run for long periods with little babysitting needed.
- Here’s a trick I discovered when doing natural products isolation using butanol: in some cases, you can find a nonpolar solvent that will partition preferentially into a separate phase. In my case, tea polyphenols are highly soluble in water but prefer to be in butanol or ethyl acetate. However, I could force the compounds of interest back into the aqueous phase with a tiny amount of methanol and lots of hexane or pet ether: the hexane lowered the polarity of the butanol phase to the point where they would prefer to be in aqueous solution.
- Another trick is by forming salts- carboxylic acids in some cases will ionize and prefer to be in a aqueous solution of ammonium bicarbonate. Once back in aqueous solution, ammonium bicarbonate can be either neutralized or under gentle heating will break down into carbon dioxide and ammonia and can be removed evaporatively. Bicarbonate salts can also be neutralized and the compound transferred back to a lower boiling point solvent.
