[asokhi]

While I was preparing for my lab I learned that I will not be given the amount of solvent to be used to carry out a recrystallization and will have to figure it out on my own. I am confused as to whether I should actually figure this value out using math or pay close attention in the lab and use the volume of solvent at which the solid just dissolves. Any thoughts?

[psychoNOT]

Can you find the value for the solubility of the compound in whatever solvent is being used, ie 1g/10ml?  If so then I believe you could calculate it mathematically and then perhaps add a bit in excess to ensure all the solute is dissolved.

[cpncoop]

I'd start with 5 volumes, heat it up to just under boiling, and slowly add in additional solvent until everything dissolves.  Then cool to r.t. and then on ice to collect xtals.

[Med90]

you can just dissolve the compound in polar solvent like ethyl acetate or methylene chloride (use as little as possible) with gentle heating. Slowly add non-polar solvents like hexane until the solution appears cloudy. After that add 1 or 2 drops of polar solvent and keep it to form crystals...

[orgopete]

you can just dissolve the compound in polar solvent like ethyl acetate or methylene chloride (use as little as possible) with gentle heating. Slowly add non-polar solvents like hexane until the solution appears cloudy. After that add 1 or 2 drops of polar solvent and keep it to form crystals...

This sounds like the advice of someone with experience in recrystallizations. I very much concur with this advice. This advice is even more valuable if impurities are present. Put a little sand in a sparingly soluble mixture or an non-polar impurity in methanol or water. If you can start with your sample easily dissolving, the insoluble materials can be recognized as impurities and removed. Then by using a miscible co-solvent that your compound is not soluble in, one can very quickly bring a solution to the saturation point.

[cpncoop]

From the question, it sounded like the type of solvent would be provided, and amounts were the only thing that needed to be performed. 

But as a side note on crystallizations, the best way, in my opinion, to approach a crystallization problem is to start by screening neat solvents as opposed to mixed systems.  Mixed systems can be very tricky at scale, and often when you reach the supersaturation point (cloud point), you've gone too far, and end up oiling out your compound.  When I've done screening experiments in the past, you dissolve the compound of interest in about 10 volumes of a solvent, and let it slowly evaporate, then look at the residue via microscopy.  This will at least tell you if the substrate is capable of crystallizing from the solvent, and is a good starting point

[Med90]

exactly, addition of excess non polar solvent will cause the sticky (oily) residue. So care should be taken at that point. When it just starts to give cloudy appearance while adding drop of non polar solvent (hexane), one should stop adding more.

[orgopete]

But as a side note on crystallizations, the best way, in my opinion, to approach a crystallization problem is to start by screening neat solvents as opposed to mixed systems.  Mixed systems can be very tricky at scale, and often when you reach the supersaturation point (cloud point), you've gone too far, and end up oiling out your compound.  When I've done screening experiments in the past, you dissolve the compound of interest in about 10 volumes of a solvent, and let it slowly evaporate, then look at the residue via microscopy.  This will at least tell you if the substrate is capable of crystallizing from the solvent, and is a good starting point

Agreed, do not screen mixed solvents. If your sample is soluble in methanol and insoluble in water, methanol-water can be used as a mixed solvent. Hexane and others can be done similarly. If you reach the cloud point with hexane, it is easy to add a drop or two of methylene chloride to keep the sample from oiling out. Mixed solvents are an easy way to reach the saturated concentration without distilling off much solvent.

Recrystallizations are really experiments. If you are trying to purify a kg of a sample to avoid chromatography, run a tlc to determine whether the impurities are more or less polar (approximately) than your product. If you have polar impurities and it becomes cloudy upon addition of a non-polar solvent, that may be the impurity. If it behaves as an impurity as further addition only slowly increases the cloudiness. A small amount of silica gel can remove the impurity. You could continue or perhaps you should try to isolate the least polar compound by using a polar solvent mixture. Non-polar impurities are removed from polar solvents with charcoal.