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Any help you could provide me would be greatly appreciated and I thank you for reading (even if you don't comment)

I am in an organic laboratory and as a 3rd chemistry major these concepts usually are pretty basic but recrystallization is kind of leaving me scratching my head. When you dissolve your compound with a small impurity in it, dissolve it in a solvent and heat, I am just confused on this part:

What I understand is that the solubility constant changes with temperature and in the case of organic compounds, solubility almost always increases with increasing temperature. So, if the compound is put in a solvent at room temperature and is heated until it is fully dissolved (solubility constant increased) then it is cooled and the solubility constant decreases and consequently solute starts precipating (crystal formation) My only issue is what is that makes the impurity go away? Thinking through it here, I know I'm so close to the Ah-hah moment but as of right now I'm mental blocked — I suppose.

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  • $\begingroup$ Wouldn't the impurities remain in the solution as the "pure" material precipitates? And you can separate via filtration. $\endgroup$ – 86BCP2432T Oct 6 '15 at 7:09
  • $\begingroup$ I don't understand that part. If they existed as solids initially, how are we supposed to know that the impurity doesn't precept. first? $\endgroup$ – joshuakatz Oct 6 '15 at 7:22
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    $\begingroup$ Sorry, that was only one type of instance of how the impurity "goes away". Since you're playing with the solubility properties, then the impurity gets separated anyhow. Does this link help at all? wiredchemist.com/chemistry/instructional/laboratory-tutorials/… $\endgroup$ – 86BCP2432T Oct 6 '15 at 7:28
  • $\begingroup$ Supposedly the impurity is present in minor amount, so it will not reach its own solubility limit and thus will not start precipitating. $\endgroup$ – Ivan Neretin Oct 6 '15 at 7:46
  • $\begingroup$ On an unrelated note: Welcome to chemistry.SE! Feel free to take a tour of the site. Consult the help center for any questions on how it works! $\endgroup$ – Jan Oct 6 '15 at 13:03
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You note that you are pretty close to the answer and you are. Let’s assume for a minute (I’m pretty sure the assumption is false, but it’ll get us a good way) that the solubility of the impurity is independent of the amount of desired product dissolved and vice-versa. Let’s also assume we have a mixture of $95~\%$ desired product and $5~\%$ side product which you want to purify by recrystallisation. The desired product has a solubility of $1\,\mathrm{\frac{mol}{l}}$ at $4\,\mathrm{^\circ C}$ and $10\,\mathrm{\frac{mol}{l}}$ at $80\,\mathrm{^\circ C}$; and the side product has the same solubility. And finally, let’s assume that you have $100\,\mathrm{mmol}$ total product (side and desired).

You then add $9.5\,\mathrm{ml}$ of your recrystallisation solvent and heat to $80\,\mathrm{^\circ C}$. That would be enough to dissolve both your $95\,\mathrm{mmol}$ desired product and your $5\,\mathrm{mmol}$ side product. You observe a clear (hopefully boiling because best solubility) solution.*

You then let your solution cool down and put it in the fridge overnight. Due to the decreased temperature, the solubility of both desired product and side product drop. The same volume of solution now dissolves only $9.5\,\mathrm{mmol}$ of both. Since the desired product is supersaturated ($95 > 9.5$), $85.5\,\mathrm{mmol}$ of the desired product will precipitate or crystallise. But the side product is still not supersaturated. We have $5\,\mathrm{mmol}$ of the side product dissolved, but the solvent could dissolve $9.5\,\mathrm{mmol}$. So all of the side product will remain in solution (ideally).

Next step is usually filtration and washing. The filtrate will now contain the saturated product solution which is undersaturated with respect to the side product. The side product didn’t disappear, it merely remained in solution. Wash your pure product, record an NMR spectra and rejoice upon its purity.


Of course, as I stated in the beginning, the assumption that solubilities be independent is wrong. It is also tendencially wrong to assume similar solubilities of desired product and impurity. One would attempt to select a solvent so that the product is not well soluble at low temperatures, but the impurity ideally is. However, the basic principle remains valid: The desired product will form a supersaturated solution and precipitate/crystallise while whichever impurity you have should still be soluble to not co-precipitate.


*: A note on how to do it practically: Usually, you would put your crude product into a flask into an oil bath with a refluxing condenser on top. You would then slowly, dropwise add solvent through the condenser until the solution is boiling and everything is dissolved. One wouldn’t go about saying ‘I should have $x\,\mathrm{mmol}$ so I should need $y\,\mathrm{ml}$ of solvent.’

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First of all, I am not sure what you mean by "solubility constant". Perhaps you mean just "solubility", which is the weight of a material that you can dissolve in a given volume of a specific solvent (at a given temperature).

You are also correct in stating that solubility usually (but not always) increases with temperature. This depends on the choice of solute and solvent.

There are two effects here serving to purify your impure compound. (1) As Ivan has explained above, a small amount of impurity (having a similar solubility) would remain in solution (being below its saturation concentration) while the main ingredient becomes saturated in solution as the solution cools and the solubility decreases, and therefore crystallizes out. (But--this would not work well if the impurity were relatively insoluble in the solvent.) (2) The formation of a regular crystal lattice, which is usually energetically more favorable than an irregular or amorphous solid, tends to favor the formation of crystals composed of a single chemical substance, rather than a mixture--especially if one substance is in large excess. So, if you repeatedly recrystallize a substance (each time, taking the crystals from the previous step and recrystallizing them using fresh solvent) you can sometimes achieve a very high state of purity.

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  • $\begingroup$ en.wikipedia.org/wiki/Solubility_equilibrium $\endgroup$ – joshuakatz Oct 10 '15 at 7:55
  • $\begingroup$ I'm not sure why you feel the need to school me, but you should check your soft skills bud. Your explanation added nothing and I'm not going to take solubility constant out of my vocabulary. Why wouldn't something be a constant just because it's temperature dependent? You seem like a real dope $\endgroup$ – joshuakatz Oct 10 '15 at 8:04
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    $\begingroup$ I am sorry, Joshua -- I can see how you would have been offended by the tone of my answer, so I have edited it accordingly. I have occasionally been known to act as a real dope, and my wife can corroborate on this. Please look at my edited answer and decide if it has any value to you on its merits. $\endgroup$ – iad22agp Oct 10 '15 at 10:52

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