Organic Chemistry: Techniques and Transformations
1
LABORATORY 3
Recrystallization
Concept goals:
Relationship between structure, polarity and solubility, role of temperature
on solubility, characteristics of a good recrystallization solvent.
Operational goal:
Develop the skill to perform a recrystallization effectively, including the
use of minimal solvent, washing and drying crystals.
Introduction
Organic compounds synthesized in the laboratory or isolated from natural sources are
often contaminated with impurities. Recrystallization is a widely used purification
technique for removing impurities from organic compounds that are solid at room
temperature. This method relies on the observation that the solubility of a compound in a
solvent generally increases with temperature. Conversely, as a solution cools the solubility
of a compound will decrease until the solution becomes saturated and crystals form.
Recrystallization can produce very pure compounds. The choice of recrystallization solvent
is critical. A good recrystallization solvent will not dissolve the solid compound at low
temperatures, but will dissolve it at high temperatures. The cold solvent also needs to
dissolve the impurities. A typical recrystallization procedure involves dissolving the impure
solid in a minimal amount of boiling solvent, followed by a cooling period. If insoluble
impurities are found in the sample during heating, additional steps (hot-gravity filtration)
are taken to remove them before cooling. After the hot solution cools and reaches the
saturation point, small seed crystals of the compound will form in the solution. Slowly,
additional molecules of the compound attach to the seed crystal and the crystals grow.
Since molecules in the crystals typically have a greater affinity for other molecules of the
same type than they do for any impurities, the process of crystal formation gives rise to
relatively pure crystals. The impurities originally present in the compound are left in
solution. The solution containing impurities is then removed from the pure crystals via
vacuum filtration.
Typically, a single recrystallization solvent is chosen for purification; however, some solids
are not easily purified from a single solvent and require a ‘mixed’ solvent. A mixed
recrystallization solvent contains two miscible solvents. The solid compound must be very
soluble in one (solvent A) and nearly insoluble in the other (solvent B). Typically, the
impure solid is dissolved in a hot solvent A, followed by addition of solvent B to reach
saturation. At this point, seed crystals will form and grow.
In this laboratory you will carry out the recrystallization of two organic compounds:
benzoic acid and naphthalene. The first experiment gives you the opportunity to practice
the technique by recrystallizing benzoic acid from a single solvent (water). In the second
experiment, your goal will be to purify an impure sample of naphthalene by
C
O
OH
Benzoic Acid Naphthalene
