Resolution through Conglomerate

From WikidChem

Slide 15: A conglomerate essentially is a mass made up of crystals, some R in orientation configuration, some S. Pasteur was able to use a microsope to actually see which pieces are R and which are S and then to pick them apart one by one in order to "resolve" the conglomerate into separate R and S components.

However, this method is laborious and definitely time consuming, and it's not always possible to see the difference. So a question is posed: is there an reliable, easier alternative?

Slide 16

Why, yes, in fact, there is! It involves stability of crystals in solution, which we will pause to examine here for just a minute.

Suppose there exists a crystal in solution which is stable, and thus will not grow or shrink in size. How is it stable? Well, in each crystal, there are molecules in the interior which are MORE stable than the outside solution, as well as molecules on the exterior (surface) which are LESS stable than the outside solution. The specific surface area to volume ratio of a stable crystal must be such that the AVERAGE stability of the crystal molecules is precisely EQUAL to the stability of the outside solution. And thus, this crystal is stable and will not grow or shrink.

So what happens for crystals that depart from this perfect ratio? Let's examine the two possibilities. The first: a smaller crystal. In a smaller crystal, there is less interior volume per surface area. Thus, there are fewer stable interior molecules to balance the unstable exterior molecules, and the average molecule now become less stable than solution molecules. Therefore, as the molecules will want to become more stable, they will go into solution to become more stable, and the crystal will shrink.

The second possibility is a large crystal. In this case, the exact opposite occurs: there are more stable interior molecules per unstable exterior molecules, and thus the average molecule in the crystal is stabler than the average molecule in solution. Thus, the crystal will grow, because the crystal state is more stable than the solution state.

• note that the critical nucleus is the smallest-sized nucleus that is conducive to the development of more crystalline structure- it is minimum size a seed crystal must be to cause a substance to crystallize out of solution; the critical nucleus must be larger than the metastable crystal size -AS

• I get the impression that the critical nucleus is the metastable crystal - anything larger, and the solute will crystallize out. I could be wrong on this, though. - NZG

(Very good so far. It would be good if you could finish up with the punch line of point 1 on the next slide to show how this is relevant to optaining a single enantiomer by simple filtration. Sorry I omitted including this in your assignment. - JMM)

• Pasteur conglomerates (RS) can be broken up and resolved through chiral-revolved seeding or chiral-revolved poison: in both cases, a certain seed crystal that has already been chiral-resolved (either R or S configuration) is used to jumpstart selective nucleation (crystallization)