May 30, 2016

Homework Help: To Sheryl: polarity

Posted by DrBob222 on Wednesday, September 27, 2006 at 3:56pm.

The exact question is: Benzyl alcohol (bp 205 degree C) was selected by a student to crystallize fluorenol (mp 153-154 degrees C) because the solubility characteristics of this solvent are appropriate. However, this solvent is not a good choice. Explain.

So the melting point looks okay and both would seem to be nonpolar, according to your researches. What else could it be?

I talked to my friend at lunch. He has a Ph. D. in organic chemistry. The OH on the fluorene (to become fluorenol) is somewhat acidic making it somewhat polar. But the REAL problem is that benzyl alcohol (which isn't very polar either)is just too good a solvent; i.e. the same reasoning we went through on the biphenyl/benzene crystallization. The fluorenol is so soluble in benzyl alcohol that very little can be removed after solution so it is a poor solvent for purposes of recrystallization. I hope this information is useful. BTW, I thought some more about the Buchner funnel/saturated solution question we were discussing. We know, of course, that cooling will take place and that some of the solute probably will ppt which makes solute appear as a solid on th funnel as well as in the liquid that goes through the funnel. Therefore, a good separation is not obtained BETWEEN THE SOLUTE AND THE SOLVENT. But that may not be the real question. Why did we filter the saturated solution in the first place. If because we want to seaparate some solid impurities from the solute in the saturated solution, we will have the solid impurities along with some of the crystallized solute in the Buchner funnel. Again, if the solute is what we are interesting in collecting, then the yield suffers because some of the product has been removed from the solvent and is mixed in with the solid impurities.

Thanks for the extra effort. Just got back from lab.

Hope you had a nice lunch with your friend.


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