This document provides microscale procedures for crystallizing various organic compounds including phthalic acid, naphthalene, and anthracene. It describes dissolving the compounds in solvents like water or alcohols, heating the solutions, adding decolorizing charcoal if needed, cooling to allow crystallization, collecting the crystals through filtration or centrifugation, and calculating percent recovery. Safety instructions are included for proper disposal of solvents and other waste.

1. 58 Macroscale and Microscale Organic Experiments
These compounds can also be
isolated using the Wilfilter.
Microscale Procedure for Phthalic Acid
Crystallize 60 mg (0.060 g) of phthalic acid from the mínimum volume of water,
using the above data to calcúlate the required volume. First, tum on the electri-
cálly heated sand bath. Add the solid to a 10 X 100 mm reaction tube, and then,
using a Pasteur pipette, add water dropwise. Use the calibration marks found in
Chaper 1 (see Fig. 1.18) to measure the volume of water in the pipette and the
reaction tube. Add a boiling stick (a wooden applicator stick) to facilitaté even
boiling and prevent bumping. After a portion of the water has been added, gen-
tly heat the solution to boiling on á hot sand bath in the electric heater. The
deeper the tube is placed in the sand, the hotter it. will be. As soon as boiling
begins, continué to add water dropwise until all the solid just dissolves. Cork'the
tube and clamp it as it cools, and observe the phenomenon of crystallization.
After the tube reaches room temperature, cool it in ice, stir the crystals
with a Pasteur pipette, and expél the air from the pipetuSasJhe tip is pushed to
the bottom of the tube. When the tip is firmly and squarely seated in the bot-
tom of the tube, release the bulb and withdraw the water. Rap the tube sharply
on a wood surface to compress the crystals and remove as much of the water
as possible with the pipette. Then cool the tube in ice and add a few dróps of
ice-cold ethanol to the tube in order to remove water from the crystals. Connect
the tube to a water aspiratór and warm it in a beaker of hot water (Fig. 3.13).
Once all the solvent is removed, using the stainless steel spatula, scrape the
crystals onto a piece of filter paper, fold the paper over the crystals, and squeeze
out excess water before allowing the crystals to dry to constant weight. Weigh
the dry crystals, and calcúlate the pércent recovery of product.
Microscale Procedure for Naphthalene and Anthracene
Following the procedure outlined above, crystallize 40 mg of naphthalene from
80% aqueous methanol or 10 mg of anthracene from ethanol. You may find it more
convenient to use a hot water bath to heat these low-boiling alcohols. These are
more typical of compounds to be crystallized in later experiments in that they are
□ ü
D
Set the heater control to about
20% of the máximum.
Altérnate procedure: Dry the
crystals under vacuum in a
steam bath in the reaction tube.
“Physical Constants of Organic Compounds,” the entry for phthalic acid gives
the following solubility data (in grams of solute per 100 mL of solvent). The
superscripts refer to temperature in °C:
Water Alcohol Ether, etc.
0.5414 11.7118 0.6915 eth., i. chl.
18"
The large difference in solubility in water as a function of temperature suggests
this as the solvent of choice. The solubility in alcohol is high at room tempera-
ture. Ether is difficult to use because it is so volatile; the compound is insoluble
in chloroform (i. chl.).

2. 3. Decolorizing Solution with Decolorizing Charcoal
MACROSCALE
i
r
fe
Cleaning Up Place the Norit in the nonhazardous solid waste container.
4. Decolorization of Brown Sugar (Sucrose, C12H22OT1)
Raw sugar is refined corrimercially with the aid of decolorizing charcoal. The
clarified solution is seeded generously with small sugar crystals and excess
water removed under vacuum to facilítate crystallization. The puré white
c? E
□ 6I
Into a reaction tube place 1.0 mL of a solution of methylene blue dye that has
been made up at a concentration of 10 mg per 100 mL of water. Add to the
tube about 10 or 12 pieces of decolorizing charcoal, shake, and observe the color
over a period of 1-2 min. Heat the contents of the tube to boiling (reflux), and
observe the color by holding the tube in front of a piece of white paper from
time to time. How rapidly is the color removed? If the color is not removed in
a minute or so, add more charcoal pellets.
Wl' ’i-.l
fQ^colorizing using pelletized
Iñforií
| MICROSCALE
ggt-
Chapter 3 Crystallization 59
soluble in organic solvents. It will be much easier tó remove these solvents from
the crystals under vacuum than it is to remove water from phthalic acid. You will
seldom have occasion to crystallize less than 30 mg of a solid in these experiments.
Cleaning Up Dilute the áqueous fíltrate with water, and flush the solution
down the drain. Phthalic acid is not considered toxic to the environment.
Methanól and ethanol filtrates go into the organic solvents container.
Macroscale Procedure
Using the above data to calcúlate the required volume, crystallize 1.0 g of phthalic
acid from the mínimum volume of water. Add the solid to the smallest practical
Erlenmeyer flask and then, using a Pasteur pipette, add water dropwise from a full
10-mL graduated cylinder. A boiling stick (a stick of wood) facilitates even boiling
and will prevent bumping. After a portion of the water has been added, gently heat
the solution to boiling on a hot píate. As soon as boiling begins, continué to add
water dropwise until all the solid dissolves. Place the flask on a cork ring or other
insulator, and allow it to cool undisturbed to room temperatura, during which time
the crystallization process can be observed. Slow cooling favors large crystals. Then
cool the flask in an ice bath, decant (pour off) the mother liquor (the liquid remain-
ing with the crystals), and remove the last traces of liquid with a Pasteur pipette.
Scrape the crystals onto a filter paper using a stainless steel spatula, squeeze the
crystals between sheets of filter paper to remove traces of moisture, and allow the
crystals to dry. Alternatively, the crystals can be collected on a Hirsch funnel. Com­
pare the calculated volume of water to the volume of water actually used to dis-
solve the acid. Calcúlate the percent recovery of dry, recrystallized phthalic-acid.
Cleaning Up Dilute the fíltrate with water, and flush the solution down the
drain. Phthalic acid is not considered toxic to the environment.

3. Cleoning Up The aqueous solution, after dilution with
down rfie drain.
Naphthalene
COOH
COOH
Phthalic acid
COOH
Benzoic acid
be flushed^
1
MACROSCALE
Do not try to grasp Erlenmeyer
flasks mth a test tube holder.
6. Recrystallization of Naphthalene |
from a Mixed Solvent
Add 2.0 g of impure naphthalene5 to a 50-mL Erlenmeyer flask along with 3 mL|
of methanol and á boiling stick to promote even boiling. Heat the mixture to|
boiling over a steam bath or hot píate, and then add methanol dropwise untilf
the naphthalene just dissolves when the solvent is boiling. The total volume of|
methanol should be 4 mL. Remove the flask from the heat, and cool it rapidlyj
in an ice bath. Note that the contents of the flask set to a solid mass, whicH
would be impossible to handle. Add enough methanol to bring the total volumel
to .25 mL, heat the solution to the boiling point, remove the flask from the heat,?
allow it to cool slightly, and add 30 mg of decolorizing charcoal pellets toj
remove the colored impurity in the solution. Heat the solution to the boiling'
point for 2 min; if the color is not gone, add more Norit and boil again, then’
- ■ . . - j
* • • • ... í
5. A mixture of 100 g of naphthalene, 0.3 g of a dye such as congo red, and perhaps sand, mag-
nesium, sulfate, dust, etc. «
Crystallize 50 mg of benzoic acid from water in the same way phthalic acidl
was crystallized. Then in a dry reaction tube dissolve another 50-mg sample of|
benzoic acid in the minimum volume of hot methanol, and add water to the houj
solution dropwise. When the hot solution becomes cloudy and crystallizationj
has begun, allow the tube to cool slowly to room temperature; then cool it in|
ice and collect the crystals. Compare crystallization in water to that in the|
solvent pair. i
MICROSCALE 5. Crystallization of Benzoic Acid from
Water and a Solvent Pair
60 Macroscale and Microscale Organic Experiments
crystalline product is collected by centrifugation. Brown sugar is partially
refined sugar and can be decolorized easily using charcoal. ¡
In a 50-mL Erlenmeyer flask, dissolve 15 g of dark brown sugar in 30 mL |
of water by heating and stirring. Pour half the solution into another 50-mL flask. |
Heat one of the solutions nearly to the boiling point, allow it to cool slightly, and |
add to it 250 mg (0.25 g) of decolorizing charcoal (Norit pellets). Bring the solu-1
tion back to near the boiling point for 2 min; then filter the hot solution into an |
Erlenmeyer flask through a fluted filter paper held in a previously heated funnel. |
Treat the other half of the sugar solution in exactly the same way, but use only 50S
mg of decolorizing charcoal. In collaboration with a fellow student, try heating thejl
solutions for only 15 s after addition of the charcoal. Compare your results. 3
Cleaning Up Decant (pour off) the aqueous layer. Place the Norit in the non-¡|
hazardous solid waste container. The sugar solution can be flushed down the drain. j
o■=3

4. Chapter 3 Crystallization 61
50-
filter through a fluted filter paper in a previously warmed stemless funnel into
mL Erlenmeyer flask. Sometimes filtration is slow because the funnel fits
so snugly into the mouth of the flask that a back pressure develops. If you note
that raising the funnel increases the flow of fíltrate, fold a small strip of páper
two or three times and inserí it between the funnel and flask. Wash the used
flask with 2 mL of hot methanol, and use this liquid to wash the filter paper,
transferring the solvent with a Pasteur pipette in a succession of drops around
the upper rim of the filter paper. When the filtration is complete, the volume of
methanol should be 15 mL. If it is not, evapórate the excess methanol.
Because the fíltrate is far from being saturated with naphthalene at this
point, it will not yield crystals on cooling; however, the solubility of naphthalene
in methanol can be greatly reduced by addition of water. Heat the solution to
the boiling point and add water dropwise from a 10-mL graduated cylinder using
a Pasteur pipette (or use a precalibrated pipette). After each addition of water
the solution will tum cloudy for an instant. Swirl the contents of the flask, and
heat to redissolve any precipitated naphthalene. After the addition of 3.5 mL of
water the solution will almost be saturated with naphthalene at the boiling point
of the solvent. Remove the flask from the heat, and place it on a cork ring or
other insulating surface to cool, without being disturbed, to room temperature.
Immerse the flask in an ice bath along with another flask containing
methanol and water in the ratio of 30:7. This coid solvent will be used for wash-
ing the crystals: The coid crystallization mixture is collected by vacuum filtra­
tion on a small Büchner funnel (50-mm) (Fig. 3.23). The water flowing through
the aspiiator should always be tumed on full forcé. In collecting the product by
suction filtration, use a spatula to dislodge crystals and ease them out of the flask.
If crystals still remain in the flask, some fíltrate can be poured back into the crys­
tallization flask as a rinse for washing as often as desired because it is saturated
with solute. To free the crystals from contaminating mother liquor, break the suc­
tion, pour a few milliliters of the fresh coid solvent mixture into the Büchner
funnel, and immediately reapply suction. Repeat this process until the crystals
and the fíltrate are free of color. Press the crystals with a clean cork to elimínate
excess solvent, pulí air through the filter calce for a few minutes, and then put
the large fíat platelike crystals out on a filter paper to dry. The yield of puré white
crystalline naphthalene should be about 1.6 g. The mother liquor contains about
0.25 g, and about 0.15 g is retained in the charcoal and on the filter paper.
Cleaning Up Place the Norit in the nonhazardous solid waste container. The
methanol fíltrate and washings are placed in the organic solvents container.
7, Purification of an Unknown
Bear in mind the seven-step crystallization procedure:.
1. Choose the solvent. . .
2. Dissolve the solute. :
3. Decolorize the solution (if necessary).
Suppo.rt the funnel in a ring
stand.
|IG. 3.23 Suction filter
assembly clamped to provide
flrm support. The funnel must
fie pressed down on the
Ijltervac to establish reduced
pressure in the flask.
i' ■ •
Ki •
¡fe
R. ’
B

5. 62 Macroscale and Microscale Organic Experiments
7. Dry the product. ®
You are to purify an unknown provided by the instructor, 2.0 g if working on a ®
macroscale and 100 mg on a microscale. Conduct tests for solubility and ability S
to crystallize in several organic solvents, solvent pairs, and water. Conserve your B
unknown by using very small quantities for solubility tests. If only a drop or two
of solvent is used, the solvent can be evaporated by heating the test tube on the B
steam bath or sand bath, and the residue can be used for another test. If decol- B
orization is necessary dilute the solution before filtration. It is very difficult to fil- B
ter a hot, saturated solution from decolorizing carbón without crystallization '®
occuring in the filtration apparatus. Evapórate the decolorized solution to the point B
of saturation and proceed with the crystallization. Submit as much puré product B
as possible with evidence of its purity (i.e., the melting point). From the posted B
list identify the unknown. If an authentic sample is available your identification 'B
can be verified by a mixed melting point determination (see Chapter 4). B
Cleaning Up Place decolorizing charcoal, if used, and filter paper in the non- B¡
hazardous solid waste container. Put organic solvents in the organic solvents <B
container, and flush aqueous solutions down the drain. H
Crystallization Problems and Their Solutions S
Induction of Crystallization "
Occasionally a sample will not crystallize from solution on cooling, even thoughíB
the solution is saturated. with the solute at elevated temperatura. The easiestB?
method for inducing crystallization is tó add to the supersaturated solution a®
seed crystal that has been saved from the crude material (if it was crystalline ®|
before recrystallization was attempted). In a probably apocryphal tale, the great Ki
sugar chemist Emil Fischer merely had to wave his beard over a recalcitran! B
solution and the appropriate seed crystals. would drop out, causing crystalliza-B
tion to occur. In the absence of seed crystals, crystallization can often be inducedB
by scratching the inside of the flask with a stirring rod at the air/liquid ínter-J
face. One theory holds that part of the freshly scratched glass surface has anglesB
and planes corresponding to the crystal structure, and crystals start growing onB
these spots. Crystallization is often very slow to begin. Placing the sample in aB
refrigerator ovemight will bring success. Other expedients are to change the sohB
vent (usually to a poorer one) and to place the sample in an open container.®
where slow evaporation and dust from the air may help induce crystallization.®
Oils an^t ''Qjling Out^
J Some saturated solutions—especially those containing water—when coole^B
deposit not crystals but small droplets referred to as oils. “Oiling out” occu^^B
Filter suspended solids (if necessary).
Crystallize the solute.
Collect and wash the crystals.
i
■
Seeding
Scratching
CLA.4^

6. Chapter 3 Crystallization 63
4.5, H
8, D
11, 4, F, G
7,D
k
Choosing the solvent. “Like dissolves like. ’ Some common solvents are
water, methanol, ethanol, ligroin, and toluene. When you use a solvent pair,
dissolve the solute in the better solvent, and add the poorer solvent to the
hot solution until saturation occurs. Some common solvent pairs are
ethanol-water, ether-ligroin, and toluene-ligroin.
Dissolving the solute. In an Erlenmeyer flask or reaction tube, add sol­
vent to the crushed or ground solute and heat the mixture to boiling. Add
more solvent as necessary to obtain a hot, saturated solution.
Decolorizing the solution. If it is necessary to remove colored impurities,
cool the solution to near room temperatura and add more solvent to pre-
vent crystallization from occurring. Add decolorizing charcoal in the form
of pelletized Norit to the cooled solution, then heat it to boiling for a few
minutes, taking cara to swirl the solution to prevent bumping. Remove the
Norit by filtration, then concéntrate the fíltrate.
Filtering suspended solids. If it is necessary to remove suspended
solids, dilute the hot solution slightly to prevent crystallization from
occurring during filtration. Filter the hot solution. Add solvent if crys­
tallization begins in the funnel. Concéntrate the fíltrate to obtain a sat­
urated solution.
Crystallizing the solute. Let the hot saturated solution cool to room tem­
peratura spontaneously. Do not disturb the solution. Then cool it in ice. If
crystallization does not occur, scratch the inside of the container or add
seed crystals.
Collecting and washing the crystals. Collect the crystals using the
Pasteur pipette method, the Wilfilter, or by vacuum filtration on a Hirsch
funnel or a Büchner funnel. If the latter technique is employed, wet the
filter paper with solvent, apply vacuum, break vacuum, add crystals and
liquid, apply vacuum until solvent just disappears, break vacuum, add coid
wash solvent, apply vacuum, and repeat until crystals are clean and fíltrate
comes through clear.
Drying the product. Press the product on the filter to remove solvent.
Then remove it from the filter, squeeze it between sheets of filter paper to
remove more solvent, and spread it on a watch glass to dry.
Crystallize at a lower
|| temperature
of necessity, when the temperature of the solution is above the melting point of
the crystals. If these droplets solidify and are collected they will be found to be
quite impura. Similarly, the melting point of the desired compound may be
depressed to a point such that a low-melting eutectic mixture of the solute and
the solvent comes out of solution. The simplest remedy for this problem is to
lower the temperature at which the solution becomes saturated with the solute
by simply adding more solvent. In extreme cases it may be necessary to lower
this temperature well below room temperature by cooling the solution with dry
ice. If oiling out persists use another solvent.
Crystallization Summary
V-
LObJ
aw

7. Macroscale and Microscale Organic Experiments
A sample of naphthalene, which should be puré white, was found to have|
a grayish color after the usual purification procedure. The melting pointj
was correct and the melting point range small. Explain the gray color. I
1
How many milliliters of boiling water are required to dissolve 25 g of|
phthalic acid? If the solution were cooled to 14°C, how many grams of|
phthalic acid would crystallize out? |
What is the reason for using activated carbón during a crystallization? |
1
If a little activated charcoal does a good job removing impurities in a crys-|
tallization, why not use a larger quantity? I
; I
Under which circumstances is it wise to use a mixture of solvents to carryf
■ .
out a crystallization? |
Why is gravity filtration and not suction filtration used to remove sus­
pended impurities and charcoal fróm a hot solution?
4.
7. Why is a fluted filter paper used in gravity filtration? |
Why are stemless funnels used instead of long-stem funnels to filter hot i
solutions through fluted filter paper? Jí
IWhy is the final product from the crystallization process isolated by vac- |
uum filtration and not by gravity filtration? |
LA