what is the theoretical background of the mixed solvents recrystallization ?

[jarmenkill]

might one of you guys please tell me what is the theoretical background of the mixed solvents recrystallization ?

[mississippichem]

might one of you guys please tell me what is the theoretical background of the mixed solvents recrystallization ?

 

What about them? There are many factors to consider.

[Horza2002]

Do you mean how it works? Heres goes then:

 

Recrystallisation: typically, you need to use a solvent in which the product is soluble in hot solvent but not in the cold. So you dissolve your crude product up in a hot solvent and then let it cool down. As it cools, the product crystallises out while the impurites stay in solution allowing you to simply filter off the product.

 

With a mixed solvent system it could be for two reasons:

 

1. Your crude product only dissolves in the mixture of solvents (e.g. a DCM methanol mixture) so you need to use them before you can attempt the recrytalisation
   
2. You use one solvent to dissolve all the crude product and then add the second, which your product is insoluble in, to force the recrystalitsation. An example of this is sodium chloride dissolves in water, then add a few drops of acetone and the sodium chloride crystalises out
   

[jarmenkill]

thanks

You use one solvent to dissolve all the crude product and then add the second, which your product is insoluble in, to force the recrystalitsation. An example of this is sodium chloride dissolves in water, then add a few drops of acetone and the sodium chloride crystalises out

 

so what is the effect of the second solvent? i mean why it decrease the solubility of the crude ? could you please give me a molecular approach of why this happens?

[Horza2002]

I assume you are aware of how solubility arises? So bascially, when you add the second solvent (the one that does not dissolve the product), the proudct can no longer stay in solution.

 

For the spdium chloride example I gave above, in water, the sodium and chloride ions are seperate and surronded by water molecules. This system is more stable than the crystal form of sodium chloride so it dissolves. However, when you add acetone, the situation changes. While the oxygen atoms of the acetone are still able to coordinate to the sdoium ion and stabilise it, there are no hydrogen atoms avaliable for donation to stabilise the chloride ion. As a result, the chloride is far less stable that it would be in the solid form. At a critical concentration of acetone, the negative stabilisation of the chloride outweighs that which the sodium is stabilised and so the solid form of sodium chloride is once again favoured and so crystalisation occurs (see the attached file for a diagram).

 

In fact, I did a crystalisation today in the lab. I had a phsophonium salt that dissolved in toluene (probably because of the pi-interactions resulting from the three benzene rings my product had and that of the toluene). However, once I added a few drops of diethyl ether (which is incapable of forming pi-interactions), my product started to crystalise out. Again, this is is a toss up between how stable the solution form is and how long it takes for your second solvent to disrupt this system.

Doc1.doc

[mississippichem]

so what is the effect of the second solvent? i mean why it decrease the solubility of the crude ? could you please give me a molecular approach of why this happens?

 

I like the sodium chloride example because it is simple.

 

Water, being such a polar molecule is allowed coordinate the charged sodium or chloride ions. The water molecules form what is called a solvation shell. Remember that water also has two lone pairs of electrons mostly residing on the oxygen atom. If some less polar, yet still lone pair containing solvent is added, water can donate hydrogen bond interactions to that solvent. One can see how this disrupts the structure of the solvation shell. Water molecules now have two things on their agenda (to use a grossly cheesy personification); coordinate to charged ions, and interact via-hydrogen bonds with the co-solvent. This reduces the number of available water molecules that can contribute to solvation shells.

 

One important thing to consider is that the co-solvent must be at least somewhat miscible with the other solvent. Recrystallization techniques are often employed on solutions that are already near saturation (maximum amount of solute is dissolved). So if only a small amount of the other solvent is added, that can "distract" enough water molecules to force percipitation. If this happens rapidly enough, the ions will form into a low entropy, highly ordered lattice known as a crystal.

 

To get the crystal, the particle size (and therefore particle surface coulombic potential) must grow rapidly enough to force other ions out of their solvation shells. Some impurity molecules will not be allowed into the crystal on steric or symmetry grounds, which is what makes this a powerful purification technique.

 

Hope this helps.

[jarmenkill]

thanks a lot

really helpful