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  •  » Ether or ethanol for a recrystallization?

#1 2008-02-19 13:42:23

orgopete
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Ether or ethanol for a recrystallization?

Yahoo Answers:
Solubility in diethyl ether vs ethanol?
Why is diethyl ether used for dissolving organic compounds but not for recrystallization? Why is the opposite true for ethanol?

I expect you will get a lot of answers for this question, but probably not many like mine. Most books describing recrystallization use difficult solvents for recrystallizations. I generally used the opposite solvents.

For example, ether was a favorite solvent for recrystallizations because I could complete a recrystallization very quickly. This is how I always did them. I wanted to start with a good solvent that would dissolve my sample. If some material didn't dissolve, I filtered it to remove that impurity. I knew it was an impurity if most of the sample dissolved quickly and a small amount did not. Most of my samples were not soluble in hexane so I just started adding hexane while heating on a steam bath. I kept my concentration in the 0.1-10% of solvent range. The ideal was to use a small amount of ether if it would dissolve readily and make up the rest with hexane. Because ether was low boiling, it was easy to evaporate, and the less I had to add, the less I had to evaporate. As the temperature of the flask began to rise, I knew the ether was largely evaporated. At the cloud point, I removed it from the heat, seeded it, and let it crystallize. I could do most recrystallizations in about 10 minutes, start to finish. I did it that way because it was really fast.

If I had something that was too polar, I liked acetone-water. Same situation. Acetone dissolves most things and relatively low boiling. Add the least amount possible. Because the boiling points are much higher, it takes longer.

From my experience, if you do a lot of recrystallizations and want to get pure samples, most chemists gravitate to mixed solvents. The best pairs are a low boiling solvent in which a sample is easy to dissolve and a higher boiling solvent in which the sample has low solubility.

What I described was a dynamic process. What is in the books is a formulaic process. Add 10 mL of 70% ethanol to recrystallize. I tried to show students how I could recrystallize a sample, but I couldn't write a formula or tell exactly how much to use.

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