Organic Chemistry: Techniques and Transformations
1
LABORATORY 3
Crystallization
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, use of charcoal if needed, filtering a hot solution, washing and
drying crystals.
Introduction
Organic compounds synthesized in the laboratory or isolated from natural sources are
often contaminated with impurities. Recrystallization is a method 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, the solubility of a compound will decrease as a solution cools
until the solution becomes saturated, and crystals form.
Recrystallization can produce very pure compounds. When a warm solution of a
compound cools and reaches the saturation point, small seed crystals of the compound
form in the solution. Slowly, additional molecules of the compound attach to the seed
crystal and the crystals grow. Since molecules in the crystals 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.
In this laboratory you will have the opportunity to carry out the recrystallization of two
organic compounds: benzoic acid and naphthalene. The first experiment will give you
practice in the technique as you recrystallize benzoic acid from water. In the second
experiment, your goal will be to purify an impure sample of naphthalene by
recrystallization. You will then assess its purity by taking a melting point.
Reading and Pre-Lab Assignments
Read the following sections in
The Organic Chem Lab Survival Manual: A Student’s
Guide to Techniques
by James W. Zubrick, John Wiley & Sons, Inc.
• Interpreting a Handbook. You will need to use these resources to look up data
for benzoic acid, naphthalene, and methanol.
• Recrystallization. See especially “General guidelines for Recrystallization”,
“Activated Charcoal,” and “Working with a Mixed-Solvent System,”.
C
O
OH
Benzoic Acid Naphthalene
