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#1 2011-09-08 18:42:31


What solvent pair should you use for a recrystallization.

An organic compound is quite polar and is therefore very soluble in methanol, but insoluble in hexane. Why is this a poor choice for a solvent pair for recrystallization? What might be a solvent pair you could use to solve this problem?

Methanol and hexane don't mix. You need to have miscible solvents. … lity-table

If the compound is polar, you can use a solvent less polar than methanol such as acetone, dichloromethane, or diethyl ether. The other option would be to attempt to use a methanol water mixture. The key here is how polar the compound is. The more polar it is, the more likely it will dissolve in water. If it is less polar, then it is more likely to dissolve in a solvent less polar than methanol.

Practically speaking, water is cheap but is higher boiling and takes longer to evaporate. Organic solvents are more expensive, toxic, and lower boiling. Recrystallizations can often be performed faster with organic solvents for the final isolation of a pure (and dry) compound.

Also note, it is much faster to use a solvent mixture for a recrystallization than a single solvent. If a mixture of dichloromethane and hexane were used, it one can quickly add hexane to decrease the solubility. If the more soluble solvent is low boiling, one can boil it off more quickly. If you are trying to dissolve your compound in 95% ethanol and then concentrate the solution so it will be supersaturated when it cools, then it may take too much ethanol to dissolve your compound initially and this must be evaporated off. Because ethanol is relatively high boiling, this will take some time. If your compound is quite soluble in ethanol, then the concentration of impurities in the filtrate will be much higher and more likely to leave a residue. This also creates an operational problem of trying to filter a solid with a very small volume of solvent.

I often liked to recrystallize from a 1-10% solution. If I had 0.1 to 1 gram of a compound, I liked to work with 10-15 mL of solvent. This resulted in a filtrate that was sufficiently dilute that I did not have to wash my solid with more solvent. I did not like to risk dissolving my compound nor precipitating out impurities from residual solvent. The success of this strategy also depends on the mp of the solid, the nature of the impurities and their amounts. Low mp solids are difficult to crystallize if there are polar impurities, especially from organic solvents. They can also be difficult to crystallize from alcohol-water mixtures as they may tend to oil out. 

If you have polar impurities, an alcohol-water mixture is often a good choice as the polar impurities are more likely to remain in solution. A TLC of your compound can identify whether impurities are more or less polar than your compound. If the impurities are less polar, then an organic solvent mixture is generally preferred as your compound is likely to be the least soluble and first to crystallize.


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