Evaporation to obtain a solid

[BlackJack21]

Say you had an organic compound with a relatively low melting point (say 50 degrees celcius) that was currently in a solution with a boiling point of say 100 degrees celcius. If you wanted to evaporate the liquid to obtain the solid, how would you do this?

 

I'm confused since the BP of the solution is higher than the temperature needed for the solid to crystallize. So if you make the solution hot enough to evaporate the liquid the solid will never crystallize out. If you heat it high enough to boil off the solvent at what point do you know to stop heating and let the compound crystallize out. Could you just boil off the liquid and the residue your left with is your compound?

[Suxamethonium]

You have 2 options here:

 

1) Recrystalise from the current solution.

 

2) Distill the water away from the product (which will melt) then recrystalise from a more suitable solvent to obtain the required purity.

[John Cuthber]

The "official" answer is to distil off the solvent under reduced pressure (so it boils well below 50C).

The practical answer might be just leave the solution in an open dish with the lid off so the solvent evaporates.

[Suxamethonium]

The practical answer might be just leave the solution in an open dish with the lid off so the solvent evaporates.

 

With a solvent boiling at 100^oC I would hazard a guess that that method is not exactly practical (unless you have the time to wait). If the compound melts (and doesn't decompose) I don't really see that it really matters that much what temperature/pressure you do the distillation at, but it would be quicker at least.

[BlackJack21]

I have tried to distill off the solvent at 50 degrees and high pressure but it just won't evaporate. I can't increase the pressure anymore (I have a very good vac pump) so I increased the temperature. The solution eventually decomposed before I could get the solvent to go. Any suggestions?

[Fuzzwood]

You DEcrease the pressure with a vacuum pump. And honestly don't see what a melting point has to do anything with this, just distill the solvent off at w/e temp it boils at under reduced pressure. The molten compound won't come over as it won't boil.

[hypervalent_iodine]

The obvious question I'm surprised no one has asked yet is what solvent are you using? If it's water, then you can remove it azeotropically. It takes ages, but it means you don't have to use temperatures that are too high.

[Suxamethonium]

Yeah true, at first I think I subconsciously assumed that it was water. In further posts I considered it irrelevent if the targeted compound only melts and doesn't decompose- so didn't really think to ask.

 

(To the OP): About the 'solution' decomposing, what exactly decomposed (i.e. the solvent or solute?). It would be great to have an idea of the class of solvent and compound you are trying to extract- might be an easy way to seperate it. For example, a possible way to seperate an amine might be by precipitation as the HCl salt (might still need to distill solvent, but the MP of the compound should be higher).

[BlackJack21]

I was given an "unknown" light yellow oily liquid with the task to evaporate it an get a crystalline solid. I'm not really sure what the solvent is or what the compound is however I believe it is an organic compound. I only believed it to be decomposed based on the rapid color change and change in texture. Yellow-orange oily clear liquid to black globular oil. I have been able to reduce the liquid from ~100mL to ~20mL but I can't find a practical way to reduce the last portion without burning it.

[hypervalent_iodine]

Can you get it characterised or is this for coursework?

[mississippichem]

The obvious question I'm surprised no one has asked yet is what solvent are you using? If it's water, then you can remove it azeotropically. It takes ages, but it means you don't have to use temperatures that are too high.

 

If the OP can ever figure out which solvent it is he can consult wiki's (only somewhat trusty) table of azeotropes:

 

table of azeotropes

 

I actually used the little known ethanol/toluene azeotrope last week and was saved at least two more hours or rotovaping.

Edited March 1, 2012 by mississippichem

[CaptainPanic]

I was given an "unknown" light yellow oily liquid with the task to evaporate it an get a crystalline solid. I'm not really sure what the solvent is or what the compound is however I believe it is an organic compound. I only believed it to be decomposed based on the rapid color change and change in texture. Yellow-orange oily clear liquid to black globular oil. I have been able to reduce the liquid from ~100mL to ~20mL but I can't find a practical way to reduce the last portion without burning it.

You can also attempt steam distillation to remove a little more of your solvent. Using steam to remove the solvent will allow you to use a lower temperature, and still remove the solvent. You won't easily recover the solvent (it's mixed in steam in a low concentration), so only use this if you don't care about your solvent.

 

If a vacuum distillation does not remove any more liquid, and steam distillation also fails, then I would try crystalization next (ok, it's not evaporating the liquid, as in your assignment). If nothing happens at room temperature yet, cool it down, and see if any solids appear. Remove the solids, and cool further. With a bit of luck, your solid material is a pure component.

 

Question: do you need to figure out a method to do the separation? Or do you also need to perform the separation with a high recovery of the solid material (i.e. do you have a target of, say, 90% that you need to recover in pure form)?

[hypervalent_iodine]

The other way is to try and do a liquid-liquid extraction. If the compound is organic and the solvent isn't (I'm assuming it's water), you should be able to pull it out into an organic solvent like ether or DCM. Worth a shot, anyway.

[ewmon]

I was given an "unknown" light yellow oily liquid with the task to evaporate it an get a crystalline solid.

Notwithstanding the stated method of evaporative distillation, perhaps freeze distillation (fractional freezing) could work as well, at least to a reasonably high concentration where a vacuum pump could more easily draw off the remaining solvent.

[CaptainPanic]

The other way is to try and do a liquid-liquid extraction. If the compound is organic and the solvent isn't (I'm assuming it's water), you should be able to pull it out into an organic solvent like ether or DCM. Worth a shot, anyway.

Yeah, I thought of that too. But I was assuming that the solid is present in a relatively high concentration in the solvent (the solid affects the vapor pressure of the solvent, which no longer wants to boil at lower temperatures). That tells me that the solid has an affinity for the solvent, (like salts or sugar in water, or naphthalene in non-polar solvents).

 

So, when the solid has more affinity for the current solvent than any extraction liquid, you will need loads and loads of this extraction liquid. Eventually you will get the solid out, but I wonder if that doesn't just increase your problem.

 

That said, it might actually work, and it is worth a try. I can't tell from where I'm sitting.

[hypervalent_iodine]

Realistically if the solvent boils at 100 degrees C, a rot evap would get rid of most of it and a high vacuum pump would do away with the rest.

 

So, when the solid has more affinity for the current solvent than any extraction liquid, you will need loads and loads of this extraction liquid. Eventually you will get the solid out, but I wonder if that doesn't just increase your problem.

 

 

Well, the idea would be to extract it into a more volatile solvent such as ether, which would make drying the compound considerably easier. Also, you might not need as much extraction solvent much if you wash it with saturated brine. This is, however, based on possibly faulty assumptions as to the nature of the compound.

 

Also, just noticed this:

 

I'm confused since the BP of the solution is higher than the temperature needed for the solid to crystallize. So if you make the solution hot enough to evaporate the liquid the solid will never crystallize out. If you heat it high enough to boil off the solvent at what point do you know to stop heating and let the compound crystallize out. Could you just boil off the liquid and the residue your left with is your compound?

 

 

You would heat it slowly until you distilling solvent across and leave it until it stops evolving. If temperature is a problem, used reduced pressure. This has been said before in this thread but I don't think we've gotten an answer as to whether or not it's been tried. If the compound melts, it's really not an issue. It won't boil across unless you massively overheat it and it will solidify again once you cool it down.

Edited March 1, 2012 by hypervalent_iodine

[John Cuthber]

The obvious question I'm surprised no one has asked yet is what solvent are you using? If it's water, then you can remove it azeotropically. It takes ages, but it means you don't have to use temperatures that are too high.

 

Whereas if it isn't water you might still be able to distil off the liquid azeotropically.

In fact, if it's not water then there's a poor to fair chance it forms an azeotrope with water.

Incidentally, if the substance is air stable then he could have left the stuff to dry by now. It wouldn't have lost all the solvent but it might have been concentrated enough to start to crystallise.

One advantage to working significantly below the melting point is that, in the latter stages, you have a solid suspended in a small amount of solution. If you filter that solid off it's likely to be rather purer than the stuff left behind.

Also the saturated solution of the stuff , in eqm with the solid has a definite vapour pressure of solvent above it. In a vacuumt will lose that solvent until all the solvent has gone. However if the stuff is warm enough to be molten you have a mixture of two liquids. The partial vapour pressure of solvent will fall as its mole fraction falls. For any given pressure there will always be some solvent left in the mixture.

So, by working under vacuum you can get a purer product.