explain crystallization for orgo

[Visible Ghost]

Please explain crystallization. How is a compound purified by dissolving it in a solvent it is soluble in?
thx

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[Hailstorm]

Without getting into salt effects and such, in a very quick and dirty conceptual manner:

Let's say you have a 98% desired product / 2% contaminant product.

Lets say that you have 1g of total product (.98g desired product, .02g contaminant), and that they're both equally soluble to .02g/mL of solvent at a given temperature 0.

Heat up the solvent and dissolve all the solute. Gradually cool it down (to 0) so that they become less soluble and your desired product precipitates out, while your contaminant stays in solution.

If you put them in 1 mL of solvent, .02g of each will dissolve, and the remaining .96g of your desired product will precipitate out.

Of course there's Ksp and such involved here, but in broad strokes that's what's going on.

 

[Visible Ghost]

Thanks!

Do we assume that the contaminant is more soluble in the solvent than our desired product? A higher Ksp.

Is this theoretically always the case? This was what I was having problems understanding

 

[Hailstorm]

Thanks!

Do we assume that the contaminant is more soluble in the solvent than our desired product? A higher Ksp.

Is this theoretically always the case? This was what I was having problems understanding

It's not really an issue that the contaminant be more soluble in an absolute sense (the solubility could be less in terms of moles per litre), but because you're normally starting with substantially more moles of your desired product.

ie: if the solubility at 0 degrees C is 1mol/litre for your product, and .1 mol/litre for your contaminant:

If you have .1 mol of contaminant in 9 moles of product and you do a recrystallization you'd end up with (9-1->) 8 moles of product and (.1-.1 -> ) 0 moles of contaminant. I think you're focusing a bit too much on the specifics than the concept that they're looking for.

Keep in mind that you need to dissolve both the contaminant and the product at one temperature (usually boiling).

Don't show a physical chemist the above math and expect them to be pleased though.