Question

Under what circumstances would we want to use a solvent pair for recrystallization instead of a single solvent? What can cause 'oiling out'?

Answer 1

  Sometimes there is no single solvent that will give you a good recrystallization, hence, purification of your organic compound. In that case you must choose a solvent pair to do the job. The first solvent (at its boiling point) will dissolve the compound allowing you to filter and otherwise purify the compound. The minimum amount of the 1st solvent should be used. This is determined by observation of the solid as you add the solvent. The second solvent (also added at the boiling point) will provide a change in the polarity of the solvent system. As you add the 2nd solvent you will notice a point at which the mixture becomes cloudy. It is at this point you place the system into a ice-water bath and allow it to cool. The solid should crystallize at this point allowing you to recover it through filtration.

Two causes of oiling out :

1] Larege impurity : It prevents crystallization to occur .

2] When the melting point of solute is relatively very low melting point when compared to that of boiling point of solvent or solvent system .Oiling out can happen.It occurs less frequently