CHEM 211-2009
Experiment 6
Week 1
How Can We
Tell if Two
Solid
Compounds
Are the Same
or
Different?
A. Pre-lab Preparation Assignment:
1. Read: This handout
2. Read: Vacuum Filtration (Padías p. 26 & Figure 1-9, p. 28)
Recrystallization (Padías pp. 107-111)
3. Work through the material and link on the Experiment 6 Week 1 page of the course website.
4. Complete the Prelab Questions linked to the Experiment 6 page on the course website by
7:00 PM, Monday, October 19.
5. Consider possible responses for the QOW (See below)
6. In your lab notebook: (See Lab Manual pp. 16-19 for format.)
a. Enter this experiment into the Table of Contents with the initial page number.
b. Enter the title of this experiment and the QOW on the appropriate first left-hand page.
c. Prepare a Table of Compounds containing the bp's, densities, and water solubilities of the
potential solvents used for the recrystallization of crude unknowns. (See structures in D.
1. below)
d. Write the waste disposal instructions.
7. Bring your notebook and your ideas to the Tuesday AM Lab Discussion period on October
20 and be prepared to respond to the Question of The Week.
8. After the Tuesday AM pre-lab discussion, write the Procedure and set up a Results Section
corresponding to your Procedure.
B. Objective:
1. To gain experience with selecting a suitable solvent to recrystallize an unknown organic
solid.
2. To master recrystallization techniques for purifying solid organic samples by
recrystallization.
C. Question of the Week:
How can we choose an appropriate solvent to recrystallize an unknown solid
compound?
D. Key Terms/concepts/techniques:
 Solubility
 Recrystallization
CHEM 211 Experiment 6
2
Recrystallization & Solid Unknown
E. Introduction:
In the experiments over the last few weeks, we have developed an understanding of the
relationships between molecular structure, intermolecular forces and the effects of intermolecular
forces on physical properties of organic compounds. (BP, MP, solubility) In Experiment 6 we
apply this understanding to determine if different samples of organic compounds contain the
same or different compounds. In Experiment 6 each student will be given a sample of a solid
organic compound. However, there will be fewer different compounds than there are students in
each lab section. In fact, there will more than one student per compound and more than one
compound per lab section. The ultimate goal of Experiment 6 is for each student to identify the
group of labmates who have her/his compound.
As you have already seen, impurities in a solid sample can distort the properties of the major
component. So, our first goal in Experiment 6 is to purify our compounds so that we can be sure
that the observed properties are those of the compound and not of an impurity. In Experiment 5
we worked with column chromatography as a method for purifying solid compounds. It is a
useful technique for separating mixtures of solids, but often it does not yield sufficiently pure
samples for detailed studies. In Experiment 6 we will work with recrystallization, the most
common method for the final purification of solids and use the compound's melting point range
as a reasonable indication of its purity.
The first step in the normal procedure for the purification of a compound by recrystallization is to
choose a suitable solvent for the recrystallization. A good solvent for recrystallization is one that
dissolves only a small amount of the material to be crystallized at low temperature but a lot of the
material at high temperature. In this activity, you will gain experience in choosing a solvent and
carrying out a recrystallization. The structures of the solvents to be used for the solubility tests
are given below.
SOLVENTS
H
H
.. ..
C
.. ..
O
O
C
H
H
H
H
H
H
ethanol
water
The two solvent structures vary from weakly polar and limited hydrogen bonding ethanol to the
very polar and extensive hydrogen bonding water. All of the unknown compounds can be
recrystallized from one of these solvents or a mixture of them.
Once you have a pure compound, your goal in week 2 is to find the other student or students in
your lab section who also have your compound (assemble your "Solids Research Group").
F. Chemical Waste Disposal
1. Ethanol: waste disposal class 4.
2. Unknown solids and ethanol solutions of unknown compounds: waste disposal class 2.