East Tennessee State University Recrystallization Lab Report

TECHPurifying Acetanilide by
Recrystallization
Prepared by Carl Wigal, Lebanon Valley College
PURPOSE OF THE EXPERIMENT
Select an appropriate recrystallizing solvent. Separate and purify acetanilide from a mixture by recrystallization. Compare the melting points of
impure and recrystallized acetanilide.
BACKGROUND REQUIRED
You should know how to measure mass, in milligrams, and volume, in
milliliters. You should know how to measure melting points.
BACKGROUND INFORMATION
Impurities often contaminate organic compounds that have been synthesized in the laboratory or isolated from natural sources. Recrystallization
is a purification process used to remove impurities from organic compounds that are solid at room temperature. This process is based on the
premise that the solubility of a compound in a solvent increases with
temperature. Conversely, the solubility of the compound decreases as the
solution cools, and crystals form.
Very pure compounds can be produced by recrystallization. As a heated
solution of the desired compound cools, a small, pure seed crystal of the
compound forms in the solution. Layer by layer, additional molecules attach
to this crystal, forming a growing crystal lattice, as shown in Figure 1. The
molecules in the crystal have a greater affinity for other molecules of the
same kind than they do for any impurities present in the solution. In effect,
the process of crystal formation removes one kind of molecule from the
solution.
0)
·cC
co
Choosing a Recrystallizing
Solvent
Selecting an appropriate recrystallizing solvent to use is probably the most
difficult step of recrystallization. The primary consideration when choosing
a recrystallizing solvent is the extent to which the compound and impurities
a,
…J
a,
0)
ro
0)
C
a,
u
”
8
N
@
CENGAG E © 2004 Cengage Learning. ALL RIGHTS RESERVED. No part of this work covered by the copyright herein may be reproduced,
Learning·
transmitted, stored, or used in any form or by any means graphic, electronic,or mechanical, including but not limited to photocopying,
recording, scanning, digitizing, taping,Web distribution. 1nformatioo networks, or information storage and retrieva l systems, except as
permitled under Section 107 or 108 of the 1976 United States Copyright Act. without the prior written permission of the publisher.
27
28
Figure 1
BBB @
cryst allization
(a) ldentiml molecules attach
to one a11otlzer, formin g a
crystal lattice; (b) impurities
have different shapes or sizes
and do not layer
line B
temperature (°C)
Figure 2
Ideal solubility patterns of a
compound, line A, and
accompanying impurities,
lines B and C, at varying
temperatures
melting
individual molecules
crystal lattice
(a)
(b)
are soluble in the solvent at high and low temperatures. The graph in Figure 2
shows three possible scenarios for how the solubilities of the compound and
the impurities depend on temperature.
Ideally, the compound to be recrystallized should be very soluble in the
chosen solvent at elevated temperatures, but almost insoluble in the cold
solvent, as shown by line A. Impurities should be soluble in the chosen
solvent at all temperatures so that impurities stay in solution, as shown by
line B. Alternatively, impurities should be insoluble at all temperatures so
they can be filtered from the hot solution, as shown by line C.
Experimentation is needed to select an appropriate recrystallizing
solvent. Typically, several solvents are used to test the extent of solubility
of the compound. A small amount of the compound is mixed with a few
milliliters of each solvent. The compound’s solubility is observed at room
temperature and near the solvent’s boiling point. If the compound is
soluble in a solvent at room temperature, the solvent is not suitable. If the
compound is insoluble at room temperature and soluble near the solvent’s
boiling point, the solvent is a suitable candidate.
“Insoluble” is a relative term. All compounds are soluble to some
extent in every solvent. For example, benzoic acid in water has a solubility
of 6.80 grams per 100 milliliters at 100 °C. However, benzoic acid has a
solubility of only 0.34 gram per 100 milliliters in water at 25 °C. Benzoic
acid is typically listed as insoluble in 25 °C water.
When considering the solubility of an organic compound, a general
rule is like dissolves like. Polar organic molecules contain functional
groups that can hydrogen bond, such as -OH, -NH2, and -C02H. Polar
molecules are generally most soluble in polar solvents. Many organic
molecules are nonpolar. Nonpolar molecules are most soluble in
nonpolar solvents. A list of commonly used recrystallization solvents
is shown in Table 1.
The boiling point of the recrystallization solvent should be lower than
the melting point of the compound to be recrystallized. If the solvent’s
boiling point is higher than the compound’s melting point, the compound
will oil out. Oiling out occurs when a compound is insoluble in a solution
at a temperature above the compound’s melting point. As a result, the
compound is deposited as an oil, and not as crystals.
Another important criterion for selecting a recrystallizing solvent
relates to recovery of the compound. An abundant quantity of crystals
must be produced as the solution cools to room temperature or below.
The four major criteria for selecting a recrystallizing solvent are
summarized in Table 2.
29
Table 1 Commonly used recrystallization solvents, in order of decreasing polarity
Solvent
bp(°C)
Solvent
bp(°C)
Water
Methanol
Ethanol (95%)
Acetone
Ethyl acetate
100
65
78
56
77
Ethyl ether
Dichloromethane
Toluene
Petroleum ether
35
40
111
35-60
Table 2 Criteria for selecting a recrystallizing solvent
(1) Compound being purified must be insoluble in solvent at room
temperature
(2) Compound must be soluble in boiling solvent
(3) Solvent’s boiling point must be lower than the compound’s melting
point
(4) An abundant quantity of crystals must be recoverable from the cool
solvent
Often, the requirements necessary for successful recrystallization
are not met by a single solvent. In these cases, a mixture of two
solvents, called a solvent pair, is used. Two solvents are selected that
are miscible with each other, but have opposite abilities to dissolve the
compound. The compound to be recrystallized should be soluble in one
solvent (A) of the pair and should be relatively insoluble in the second
solvent (B).
To determine the proper combinations of the two solvents, the
compound is dissolved in a minimum volume of solvent A near the
boiling temperature of this solvent. Next, solvent Bis added to the boiling
mixture until the mixture becomes cloudy, indicating that the compound is
precipitating from solution. A few drops of solvent A are added to redissolve
the precipitate, producing a clear solution. Then the solvent pair is treated just
like a single recrystallization solvent. Common solvent pairs are ethanol and
water, acetone and ether, and acetic acid and water.
Dissolving the Compound
Once a suitable solvent is found, the recrystallization process is continued
by dissolving the compound in a minimum volume of boiling solvent.
Then a five percent excess of the solvent is added to the saturated solution
to prevent premature crystallization. For example, if 10 mL of a boiling
solvent is required to just dissolve a compound, five percent of 10 mL or
0.5 mL would be added to bring the total volume to 10.5 mL.
Decolorizing the Solution
Occasionally, a sample may contain a soluble impurity that produces a
colored solution, and that solution colors crystals that would otherwise be
colorless. In that case, activated carbon, or decolorizing carbon, is used to
remove these colored impurities from solution. Activated carbon has a
Cl
·cC
iii
.’.:]
a,
Cl
“‘
Cl
C
a,
(.)
‘