[ginkgobilbo]

Our prof usually posts some questions for us to think about, a few of them I'm not sure how to answer. I also had some questions of my own. I think they are relatively rudimentary lab questions, but I feel a bit behind because theory and techniques were never extensively discussed in previous semesters. I want to make sure I have all the bases covered.

Our resolving agent was D-tartaric acid, methanol our solvent. We had to cover our beaker with foil while the crystals formed. Why was this necessary? Methanol is volatile, so leaving the beaker uncovered over a few days would have drastically reduced the volume of methanol. Our desired product, the (-)-amine salt is not as soluble as the (+)-amine salt, thus it crystallizes out slowly. The amount of methanol actually needed to dissolve the tartaric acid was less than 60mL, yet we used 450mL of methanol for the experiment. Was this to prevent reduction of product quality? If methanol was allowed to evaporate freely the less soluble diastereomer would have come out of solution. Is this the only reason we cover the solution?

The lab procedures suggested after collecting the crystals to reduce the mother liquor to half its volume then setting the solution aside for more crystals to form. We did not have enough time in the lab to do this, but I was just wondering why we had to reduce the mother liquor. We were given no data regarding the solubilities of the amine salts, but if there was too much solvent would that have increased the possibility of losing the desired product as more would have stayed in solution?

When I was performing the vacuum filtration there was a lot of white residue left during transfers of the methanol solution, it literally coated my beakers. I noticed this happened to everybody in the lab. Was that mostly the (-)-amine salt? My prof mentioned that the racemic amine given to us at the beginning left a white residue as it evaporates. I'm curious, what is that white residue? The grade of the amine was never mentioned, so I'm guessing the residue is just impurities.

After collecting the crystals we had to break up the salt using 50% NaOH. One of the questions we were asked to think about was whether it would have been possible to use KOH. Would it have made that much of a difference at 50%?
 

Thank you for reading through that, if you did, or even some of it. Any help and hints would be much appreciated.

[discodermolide]

I would guess that the first solid is one of the diastereoisomeric salts. During vacuum filtration the other one crystallised out, hence the white residue.
This has happened to me many times. You filter off the solid and get a clear mother liquor and rapidly crystals of the other salt precipitate out of solution.