EXPERIMENT #1
SOLUBILITY TESTS AND RECRYSTALLIZATION
INTRODUCTION:
The preparation of compounds in as pure a state as possible is a crucial aspect of organic chemistr y.
Techniques used for this purpose include the recrystallization of solids, the distillation of liquids,
and chromatography. In this laboratory session, you will be presented the purification of salicylic
acid by recrystallization from an appropriate solvent after examining the solubility of some typical
organic compounds in various common laboratory solvents.
OVERVIEW OF THE EXPERIMENT:
This experiment has two parts:
Part I: Solubility tests of various compounds in various solvents to help reinforce the concept of
"like dissolves like".
Part II: the purification of an organic compound by recrystallization and the use of decolorizing
charcoal.
EXPERIMENTAL PROCEDURE:
Read through and familiarize yourself with the experimental procedures described below. Read
the General information on recrystallization below
Check the chemical hazards associated with the following chemicals (as this is part of your
prelab worksheet write up, see format under “learning activities” of the module ):
anthracene, benzoic acid, phthalic acid, resorcinol, sodium naphthionate, ethanol, hexane,
toluene, salicylic acid, charcoal.
Part I. Solubility tests
The five compounds to be tested are anthracene, benzoic acid, phthalic acid, resorcinol and the
sodium salt of 1-naphthylamine-4-sulfonic acid (sodium naphthionate). The solvents to be used
are ligroin (an alkane mixture which behaves like hexane), toluene, ethanol and water. Structur es
of these compounds and solvents are shown below.
O
O
SO Na
3
HO
OH
OH
NH
2
Benzoic Acid
Phthalic Acid
Toluene
OH
O
Anthracene
OH
Ethanol
OH
Resorcinol
n-Hexane
H O
2
Water
Sodium
Naphthionate
Each compound is to be tested in each solvent using the procedure indicated below. For each test
determine whether the compound is soluble in the cold solvent, and, if not, whether it is soluble
when the solvent is heated.
Add a spatula tip of each compound to separate dry test tubes. Add 3 mL (~3 pipettes) of solvent,
using the Pasteur pipettes provided. Take care not to contaminate the solvent bottle.
If the solute does not dissolve, heat the test tube carefully, using the steam bath. If the solid
dissolves completely, it may be considered soluble in the hot solvent; if some but not all dissolves,
it may be recorded as moderately soluble, and further small amounts of solvent should be added
with heating until solution is complete. Cool the hot solutions by means of an ice bath and, if
necessary, induce crystallization by scratching the walls of the test tube with a stirring rod.
Crystallization could also be induced by evaporating some of the solvent, on the steam bath, if too
much was added to dissolve the sample. Record the following observations in a 5 x 4 matrix chart
for each possible pair of compound and solvent: solubility in hot and cold solvent and crystalline
form - needles, plates, prisms, amorphous.
To conclude this section, make suggestions as to the solvents (or solvent pairs) you would
recommend for the recrystallization of each compound.
Part II. Purification and crystallization of salicylic acid
In this experiment 1 g of crude salicylic acid containing 0.2% Alizarin Yellow R will be purified
by recrystallization.
O
COOH
OH
O N
OH
Salicylic Acid
2
N N
OH
Alizarin Yellow R
Combine 1 g of the crude salicylic acid and 25 mL of hot distilled water in a 150 mL beaker and
swirl on a hot plate until the boiling point is reached. Then add more hot water, 2 mL at a time,
and determine the amount of water required to just dissolve the sample at the boiling point.
Next continue heating the mixture on the hot plate and add an excess of hot water (50-100%) in
order to avoid later difficulty during the filtration process.
Remove the solution from the hot plate, swirl it, and cautiously introduce the required amount of
decolorizing charcoal (a spatula tip). Use approximately double the minimum amount calculated
in your pre-lab preparation. Reheat to boiling and filter the hot solution by gravity through fluted
filter paper. Wetting the paper with a few drops of boiling solvent will prevent the solution from
crystallizing in the cold filter paper and plugging it. If the first few drops of filtrate contain carbon,
watch for the point when the filtrate is clear, change to a clean receiving flask, and reheat and
refilter the first filtrate. To assure a quantitative transfer of product, rinse the original flask with a
few milliliters of fresh water, heat to boiling, and add slowly to the upper rim of the filter paper
while rotating the funnel. Set the clear filtrate aside to cool.
After crystallization at room temperature has ceased, place the flask into a beaker of ice water to
promote further crystallization. Filter the product by suction. (Wet the filter paper with water first
before applying suction so that a tight seal is formed). If crystals remain in the Erlenmeyer flask
after it has been emptied, the filtrate, which is saturated with solute, can be poured back into the
crystallization flask as often as necessary to help rinse out the product. Fresh solvent should be
used only in the final wash to free the crystals from this "mother liquor". If the crystals are fairly
large and the crystalline form is to be preserved, gravity filtration is sometimes more advantageous.
The product should be colorless. Air-dry the crystals in an appropriately labeled (Names, Lab day,
Compound) plastic beaker at least overnight. Weigh sample before acquiring infrared (IR)
spectrum. More on what an IR spectrum is and how the product sample is prepared to obtain this
spectrum will be presented in experiment 2 of the term.
Calculate % recovery.
The IR spectrum of the product will be posted in the “content” section of the module.
Suction Filtration:
Be sure your filter flask is clamped properly, as shown above.
Insert the Buchner funnel with rubber fitting and ensure a tight fit. Cover the bottom of the funne l
with a piece of filter paper. Wet the paper with an appropriate solvent (something that your product
is insoluble in) and turn on the vacuum gently to form a seal. This is important because it prevents
your crystals from sliding under the filter paper when you add them to the funnel.
Scrape the sides of your beaker to suspend your crystals in the mother liquor. Carefully swirl your
crystals in the beaker to generate momentum and quickly pour them into the Buchner funne l
(Suction is gently on and filter paper is still moist. You may need to re-wet your filter paper right
before you are ready to pour your product in).
The goal is to transfer your entire product to the funnel in as few attempts as possible. If some
crystals remain in your beaker, turn off the vacuum and pour the mother liquor from the filter flask
back into your beaker. Repeat the “swirl and dump” method until you have achieved complete
transfer. Make sure the filter flask is not filled up with your filtrate. Keep the level of filtrate low
in the flask. At the end, you may increase the vacuum strength to dry the crystals you have
collected. Always keep the filter paper with your product, which you will store in a plastic beaker.
Once the paper is dry, it is much easier to scrape off.
GENERAL INFORMATION ON RECRYSTALLIZATION
Typical procedure
A typical recrystallization procedure is as follows. The compound of interest, contaminated with
the impurities, is suspended in the minimum amount of hot solvent. Enough hot solvent is added
until the compound dissolves. In certain cases, a specific percentage of excess hot solvent is added.
At this point the solution is filtered to remove any insoluble impurities. The solution is then
allowed to cool. The desired material crystallizes out of solution leaving the more soluble
impurities behind in solution. The crystalline material is obtained by filtration. In this way the
desired material is purified from both less soluble and more soluble impurities. After allowing
solvent to evaporate from the crystals, a highly purified sample of the desired compound is
obtained. Recovery of pure material is maximal if it has a very low solubility in cold solvent.
If a melting point determination or other test after the first recrystallization indicates that the
material is still impure, it must be recrystallized a second or even third time to obtain the required
degree of purity.
Choosing the solvent
The dissolving capability of solvents is largely determined by the functional groups that they
possess, which in turn determines the polarity of the solvent. If the functionality of the compound
you wish to purify matches that of the solvent, it is likely that the compound will dissolve in that
solvent readily. For example, hexane will dissolve most alkanes and methanol will dissolve many
low to medium weight alcohols. Hence the phrase: "Like dissolves like".
While this principle is a useful starting point for solvent selection, it will often be the case that
such a solvent will be too powerful and it will therefore be difficult to get the dissolved substance
to crystallize out of solution, even when it is cold. Using the Table of Solvents (page 30) as a guide,
one should then switch to a slightly less powerful solvent - one that is either slightly more polar or
one that is slightly less polar. This new solvent may have the desired property of dissolving the
compound only upon heating. This is why, for example, water is chosen for the recrystalliza tio n
of salicylic acid in this experiment. Water is a very "poor" solvent for salicylic acid at room
temperature; yet it is an excellent solvent for recrystallization because it can dissolve appreciable
quantities of salicylic acid at 100 °C. Another guide is that the solvent power of a solvent
belonging to a specific functional group usually increases with increasing boiling point of the
solvent. For example, ethanol dissolves about twice as much of a given solute at its higher boiling
point than does methanol.
A few more points for solvent selection are the following.
-
The solvent must not react with the compound. For example, a material that undergoes acid-
catalyzed decomposition should not be recrystallized from acetic acid. Another form of reaction
of the solvent with the solute is incorporation of solvent as solvent of crystallization. This will lead
to a pure substance, but an unexpected melting point. Use of a second solvent can detect this.
-
The solvent must not be heated past the melting point of the compound. This is to prevent
melting the solid rather than dissolving it.
-
If the purity of the solvent is unknown, it should be distilled before use. You can assume that
all of the solvents in this laboratory are of suitable purity and distillation will in general not be
necessary.
Amount of solvent
The most common error in recrystallization is to use too much solvent. The proper amount of
solvent required for a recrystallization is determined by dissolving the solute in just enough hot
solvent to effect solution and then to add a small excess of the solvent, depending upon the type
and number of manipulations to be carried out. A larger excess of solvent is usually required if the
solvent is very volatile and the solution is to be filtered while hot. In addition, and although no
crystal formation may be observed on the filter paper, quantitative transfer of solutions always
requires that paper and funnel be rinsed with some additional hot solvent. The total filtrate may
then be evaporated to the desired level and set aside to allow crystallization to proceed. When the
solution has acquired room temperature and crystal formation has ceased, a common practice is to
chill the product in an ice bath before filtration. This process will often promote additiona l
crystallization.
Solvent Pairs
Occasionally a single suitable solvent for recrystallization cannot be found. If this is the case, a
mixture of two solvents (a so-called “solvent pair”) may be useful. A useful solvent pair consists
of two miscible solvents having different solvent powers. There are two common ways of using
a solvent pair:
The first technique applies if the boiling point of the "good" (i.e. the more powerful) solvent (e.g.
ethanol, b.p. 78°C) is lower than that of the "poor" solvent (e.g. water, b.p. 100°C). The compound
is dissolved in a reasonable amount of the "good" solvent which has been heated until near the
boiling point; the "poor" solvent is then added drop wise until the solubility limit is reached and
cloudiness is observed. A few drops of the good solvent are then added until the solution is again
clear and the solution is then allowed to cool slowly.
A second method can be used, if the "good" solvent is more volatile than the "poor" solvent. In
this technique, the compound is dissolved in a small amount of "good" solvent (e.g. methyle ne
chloride, b.p. 39 °C). The "poor" solvent (e.g. hexane, b.p. 69 °C) is added drop wise down the
side of the flask. Cloudiness will develop where the drops of "poor” solvent contact the "good"
solvent. The cloudiness disappears when contents of the flask are mixed. Addition of the poorer
solvent is stopped while the solute is still completely in solution. The solution is then warmed so
that the more volatile solvent preferentially evaporates. Crystallization will begin when the
solubility limit is reached. If the compound does not crystallize but produces insoluble oil (as
indicated by the onset of cloudiness rather than the production of crystals) a drop of "good" solvent
is added and the flask set aside to cool. This technique is especially useful in the recrystalliza tio n
of small amounts (< 100 mg) of material.
The Erlenmeyer flask
Erlenmeyer flasks are ideal for recrystallization since they prevent too rapid an evaporation of
solvent upon heating. The narrow mouth acts as an air - cooled condenser for solvent vapor. The
contents of the flask can also be agitated easily without spillage. The ratio of the size of Erlenme yer
flask (in mL) to the substance (in g) should be approximately 50:1 to 100:1.
Induction of Crystallization
When allowing the solution to cool, it is best to allow crystallization to proceed slowly by not
moving the flask and letting the solution cool at room temperature. Rapid cooling (such as running
cold water down the outside of the flask) will frequently lead to the formation of small crystals
containing occluded impurities. The solution should be allowed to reach room temperature slowly
without being disturbed. Additional cooling in ice/water, after room temperature has been reached,
may be necessary to maximize recovery.
Supersaturation may occur during the cooling process, preventing crystallization. Under these
circumstances, scratching the inside of the flask with a spatula or glass rod will often cause
crystallization by providing points of nucleation. If crystals of the pure compound are available,
these can also be used sparingly (one crystal should do) to act as seed crystals when the solution
has reached room temperature.
Wet solids
"Wet" in organic chemistry usually means "wet with water". Crude product should always be free
of water before attempting recrystallization with solvents which are immiscible with water. (A
table of solvent miscibility is shown on page 33.) Solids may be air dried on filter paper. If it is
known that the solid will not decompose on heating, the drying oven may be used.
Oils
Formation of oils rather than crystals occurs occasionally, especially if the compound to be
recrystallized has a very low melting point or if the material is highly impure or wet. This is a very
undesirable situation since oils are much harder to handle than crystals. If oiling out occurs, this is
an indication that a number of recrystallizations may be necessary in order to obtain a pure
compound or that the compound has come out of solution too fast, i.e. the solution was too
concentrated, the temperature was too low or the solvent (mixture) was too 'poor'.
A number of techniques may be used to solidify an oil. These include freezing the solvent,
scratching the inside of the flask vigorously or adding seed crystals. Sometimes crystalliza tio n
may be induced by freezing the solution to a viscous mass and slowly reheating until the mixture
passes through an optimum temperature range for crystallization. The crystals which form should
then be recrystallized using fresh solvent.
It should not be forgotten that crystallization is not the only technique available for purifica tio n
and chromatography will commonly be used in cases where the desired compound is highly
impure.
Decolorization with Activated Charcoal
Solutions of reaction products at times contain soluble impurities such as pigments or colored
oxidation products which cause the solutions and the resultant crystals to be colored. Under these
circumstances the use of activated charcoal is often very useful in adsorbing the colored impurities
and producing non-colored crystals.
The correct method of using activated charcoal is to add the proper amount to a cool solution, heat,
and then agitate the solution. Charcoal should never be added to a solution at the boiling point;
the solution will almost certainly foam up with a resultant loss of product.
The approximate amount of activated charcoal to be used may be determined by taking into
consideration the surface area of the specific charcoal used. The actual weight of charcoal used
should be increased by 50-100% over the calculated amount. This is because the charcoal may
well adsorb other molecules besides the pigment and thus not all of the adsorption sites on the
charcoal will be available.
Boiling of the solution containing charcoal is not required since adsorption occurs rapidly. In fact,
decolorizing carbon is quite often less effective at high than at low temperatures , and the sole
reason for carrying out this decolorization technique at the higher temperature is to keep the
substance being crystallized in solution. For the same reason filtering of the charcoal must also be
completed while the solution is hot.
While pigments, which are highly colored large, planar molecules, may generally be removed by
treating the solution with activated charcoal, colored oxidation products (often yellow or brownish)
are often not appreciably adsorbed. Fortunately, however, these compounds tend to remain in
solution and are eliminated when the crystals are filtered and washed. Consequently, if treatment
with activated charcoal is not, or only partially, effective , the process is generally not
repeated since an excess of charcoal could adsorb too much of the required sample and result
in a significantly reduced yield. Further purification should therefore be accomplished through
regular recrystallizations.
TABLE OF COMMONLY USED SOLVENTS
IN ORDER OF INCREASING POLARITY
Name (Molecular Weight - g/mol)
M.P.
(C)
B.P.
(C)
Density
(g/cm3)
Azeotropes with Water
(C [% comp.])
n-Pentane (72.15)
-129.7
36.1
34.6 [98.6/1.4]
n-Hexane (86.17)
Cyclohexane (84.16)
Carbon tetrachloride (153.84)
-95
-6.47
-23
69.0
80.7
76.7
0.6263820
0.66020
0.778120
Toluene (92.13)
Benzene (78.11)
-95
5.5
110.6
80.1
Diethyl ether (74.12)
Chloroform (119.39)
Methylene chloride (84.94)
-116.3
-63.5
-95
34.6
61.2
39.75
Acetone (58.08)
Ethyl acetate (88.10)
Isopropanol (60.11)
-94
-83
-88.5
56.5
77
82.5
Ethanol (46.07)
-114.1
78.5
Methanol (32.04)
Glacial acetic acid (60.05)
-97.8
16.7
64.7
117.9
0.0
100.0
Water (18.016)
1.58925
0.86620
0.878715
0.713420
61.6 [94.4/5.6]
69.8 [91.5/8.5]
66.8 [95.9/4.1]
85.0 [79.8/20.2]
69.4 [91.1/8.9]
1.48420
1.325520
0.78825
0.90220
07850520
56.3 [97.0/3.0]
0.78920
0.791520
1.04925
1.00004
78.2 [95.6/4.4]
70.0 [91.9/8.1]
80.4 [87.8/12.2]
TABLE OF COMMON POLAR SOLVENTS
USED IN ORGANIC SYNTHESIS
Name (Molecular Weight - g/mol)
t-Butyl alcohol (74.12)
Pyridine (79.10)
Dioxane (88.10)
1,1,1-Trichloroethane (133.42)
1,2-Dimethoxyethane (90.12)
n-Butyl alcohol (74.12)
Tetrahydrofuran (72.10)
Dimethyl sulfoxide (78.13)
Pyrrolidine (71.12)
N,N-Dimethylformamide (73.90)
M.P.
((C)
25.6
-42
11.8
B.P.
((C)
82.41
115
101.1
74.1
82
-90
117
-108.5 66
18.4
189
88.5
-61
Density
(g/cm3 )
0.785720
0.978025
1.032920
1.337920
0.862520
0.80120
0.88922 0
1.10020
0.852022.5
153
Azeotropes with Water
((C [% comp.])
79.9 [88.2/11.8]
92.6 [57.0/43.0]
87.8 [81.6/18.4]
93.0 [55.5/44.5]
0.94525
Hexamethylphosphoric
(179.20)
Acetonitrile (41.05)
triamide
-45
81.6
230
1.0320
0.7874515
76.5 [83.7/16.3]
MISCIBILITY TABLE
The dark squares indicate that the pair of solvents is immiscible. Immiscible pairs of solvents can
however dissolve in each other to an appreciable extent. An example of this phenomenon is the
diethyl ether/water pair. This is why it is necessary to “dry” ether which has come into contact
with water. Conversely, small volumes of ether will “disappear” into large volumes of water.