Recrystallization Notes
Recrystallization and Melting Point Determination
W. H. Bunnelle, L. A. Meyer, R. E. Glaser (Version 3)
Introduction
Consider what happens when a solid material is placed in a solvent in which it has a low solubility. Not much! A small fraction of the solid will dissolve, but the rest will just sit there. (Actually, it doesn't just 'sit there' since an equilibrium is in effect, with solid molecules going into solution, and an equivalent number of dissolved molecules reforming solid, but we see the same amount of undissolved material). How can we get the entire solid to dissolve? One way is to heat the solution -- most materials are more soluble in hot solvent than in cold. Suppose we heat to the boiling temperature of the solvent (the highest temperature which is practical), and the entire solid goes into solution. If we now let the solution cool, what will happen?
Obviously, at some temperature, the concentration of solute will exceed its solubility, and the solid will start to come back out of solution - it will crystallize. By the time we return to the initial temperature, nearly all of the original material has formed crystals, which can be removed from the solvent by filtration.
Now, suppose that the original solid contained small amounts of impurities. These would go into solution as well, but since the impurities are present only in small amounts, they remain soluble when the temperature is lowered again. Thus, the crystals, which come out of solution, are more pure than the original sample. This is the essence of purification by recrystallization (Fig. 1).
[pic]
Figure 1: An impure solid in: (A) cold solvent in which it has low solubility, (B) solvent that has been warmed up to dissolve all of the solid, and (C) solvent that has cooled down to the original temperature. The crystallized solid is now pure, while the impurities remain in solution.
Recrystallization
Recrystallization is the preferred method for purification of organic
W. H. Bunnelle, L. A. Meyer, R. E. Glaser (Version 3)
Introduction
Consider what happens when a solid material is placed in a solvent in which it has a low solubility. Not much! A small fraction of the solid will dissolve, but the rest will just sit there. (Actually, it doesn't just 'sit there' since an equilibrium is in effect, with solid molecules going into solution, and an equivalent number of dissolved molecules reforming solid, but we see the same amount of undissolved material). How can we get the entire solid to dissolve? One way is to heat the solution -- most materials are more soluble in hot solvent than in cold. Suppose we heat to the boiling temperature of the solvent (the highest temperature which is practical), and the entire solid goes into solution. If we now let the solution cool, what will happen?
Obviously, at some temperature, the concentration of solute will exceed its solubility, and the solid will start to come back out of solution - it will crystallize. By the time we return to the initial temperature, nearly all of the original material has formed crystals, which can be removed from the solvent by filtration.
Now, suppose that the original solid contained small amounts of impurities. These would go into solution as well, but since the impurities are present only in small amounts, they remain soluble when the temperature is lowered again. Thus, the crystals, which come out of solution, are more pure than the original sample. This is the essence of purification by recrystallization (Fig. 1).
[pic]
Figure 1: An impure solid in: (A) cold solvent in which it has low solubility, (B) solvent that has been warmed up to dissolve all of the solid, and (C) solvent that has cooled down to the original temperature. The crystallized solid is now pure, while the impurities remain in solution.
Recrystallization
Recrystallization is the preferred method for purification of organic