Fall 2007
CH241L
PCC Sylvania
Maazouz
Exp #2 Recrystallization
Pre-Lab
Complete items numbered 1-6 (omit number 3) as described in your lab syllabus. In
addition to the known compounds, make sure your data table includes the appropriate
physical properties for all the possible unknowns for each experiment. Remember to
write in pen in your lab notebook, and to turn in your carbon copies before you start the
lab.
Given that the solubility of benzoic acid in water is approximately 68.0 g per liter at 95
°C; and 1.7 g per liter at 0 °C, calculate the amount of hot water needed to dissolve a 50
mg sample of benzoic acid.
Watch a 16 min 40 second movie posted on Blackboard!!!
Questions of the Day: How can a solid be purified? Specifically, how is benzoic acid
recrystallized? What is the difference between single solvent and mixed solvent
recrystallization? At your house which items contain benzoic acid or sodium benzoate?
When organic chemists synthesize new compounds in the lab, they frequently need to
remove any possible contaminants in the newly formed crystalline products.
Recrystallization is a useful purification method for most organic compounds that are
solids at room temperature. In addition to purification, recrystallization is also useful to
isolate an extremely some amount of a particular compound from natural products, (i.e.
isolate taxol out of the bark of a yew tree).
To purify a solid organic compound, chemists use the differing solubilities of compounds
in different solvents, as well as the fact that the impurity will be present in a smaller
amount than the desired product. The general technique involves dissolving the organic
material to be crystallized in a hot solvent (or solvent mixture) and cooling the solution
slowly. The solubility of a dissolved material decreases with decreasing temperatures;
therefore the solution will become oversaturated and the solid will separate from the
solution as it is cooled. If the two compounds dissolved in the solvent have differing
solubilities, it becomes possible to separate these two compounds. In other words,
many impurities will stay in solution and your target compound is purified. This process
is called “Recrystallization,” because it relies on dissolving the crystals, which are then
regenerated by crystallization. However, it is not possible to recover your entire
compound following a recrystallization. To maximize your yield, it is very important to
only use a minimum amount of hot solvent.
Choosing the appropriate solvent is the most challenging part of the recrystallization. In
choosing a solvent, remember that “like dissolves like”, which means that a nonpolar
compound will dissolve well in a nonpolar solvent, and a polar compound will dissolve
well in a polar solvent. Also note whether a solvent is flammable or not and find its
boiling point. This will enable you to select an appropriate heating device. Generally
speaking, solvents with boiling points below 100°C should be heated in a water bath (NO
OPEN FLAME); solvents with boiling points above 100°C can be heated directly on a hot
plate or a sand bath.
Fall 2007
CH241L
PCC Sylvania
Maazouz
A table of solvents and their dielectric constants can be found at the ACS website
http://organicdivision.org/organic_solvents.html
The technique of recrystallization involves the following steps:
1. Selection of an appropriate solvent or mixture of solvents (consult table)
2. Dissolution of the solute using a minimum amount of hot solvent
3. Decolorization of the solution if necessary (with an activated form of carbon)
4. Removal of suspended solids (through filtration of the hot solution)
5. Crystallization of the solid from the solution as it cools
6. Collecting the purified solid by filtration
7. Washing the crystals with an appropriate solvent to remove impurities
8. Drying the crystals
Objective: Your task will be to recrystallize benzoic acid from water and from a solvent
pair of your choice.
Benzoic acid is widely distributed in plants, such as anise seed, cranberries, prunes,
cherry bark, cinnamon, and cloves. In most plants, it acts as a natural preservative that
inhibits the growth of bacteria, yeasts, and molds. Its salt, sodium benzoate, is produced
by the neutralization of benzoic acid with sodium bicarbonate, sodium carbonate, or
sodium hydroxide. The salt is not found to occur naturally. It is used as a preservative in
a variety of food products, such as jams, soft drinks, and cereals. Benzoic acid is also
an effective food preservative, but its sodium salt is more popular, partly because it is
more soluble in water than benzoic acid. Benzoic acid is also detected in car exhaust
gases, presumably as an oxidation product of toluene.
Procedure
A. Single solvent recrystallization
Obtain the melting point of a few milligrams of impure benzoic acid. While you are
waiting for the MP, weigh out about 0.050 g of impure benzoic acid and record the mass
in your notebook. Transfer the solid to an Erlenmeyer flask containing a few milliliter of
distilled water (the solvent). Heat the mixture with a hot plate or sand bath. Add a
boiling stick (a wooden applicator) to the Erlenmeyer flask to prevent bumping. Heat the
mixture to reflux. Add more solvent to the
mixture in small portions using a Pasteur pipet
until the solid just dissolves (saturated solution).
Stir until the solid completely dissolves. Record
the total volume of water used.
Allow the flask to cool slowly until it is cool to the
touch. As the solution cools, the solubility of the
solid decreases and the solid crystallizes. Cool
the solution in an ice bath. Once the flask is cool
and crystals have precipitated out, collect by
vacuum filtration.
Learn more about filtration at http://orgchem.colorado.edu/hndbksupport/cryst/cryst.html
Fall 2007
CH241L
PCC Sylvania
Maazouz
Record the mass and obtain a Melting point of your recovered crystals, just in
case they dissolve and disappear in the next step!! Rinse the crystals with a very
small amount (a few drops) of an ICE COLD 50/50 by volume water/ethanol mixture.
Allow the crystals to air dry on the filter, then weigh them, and take a melting point.
Note: The products of a chemical reaction should always be dried to constant mass,
meaning its mass after drying should not change significantly between two successive
weighings.
B. Mixed solvent recrystallization
Background
For recrystallization sometimes it is difficult to find a solvent that will dissolve the
compound when it is hot, but will precipitate it out when it is cold, something necessary
for an ideal solvent. If no solvent can be found with the appropriate solubility
parameters, sometimes it is possible to create an appropriate solvent by mixing two
inappropriate solvents. By using two solvents, or a solvent pair, it is possible to
successfully complete the recrystallization. The solvent pair should be chosen such that
one solvent dissolves the solute at elevated temperature and the other one does not.
Refer to a table of dielectric constants for solvents to choose your appropriate solvent
pair but remember that the two solvents you select must be miscible.
Following is a great flow chart for mixed solvent recrystallization:
Sometimes it is necessary to induce crystallization. This becomes necessary, if a
solution does not crystallize upon continuous cooling. Several techniques may be used
to induce crystal formation. One technique involves scratching the inside surface of the
flask with a glass rod. The rod should not be fire-polished and is moved vertically in and
out of the solution. Enough force should be used so that an audible scratching is heard.
Seeding is the second technique. A small amount of the original crystal is saved, and
then placed in the cooled solution. This seed can promote crystal formation.
Fall 2007
CH241L
PCC Sylvania
Maazouz
Procedure
Recrystallize another accurately weighed sample (about 50 mg) of benzoic acid using a
solvent pair of your choice. A suggested solvent pair is hot toluene/cyclohexane.
However, you may choose a different solvent pair if you think you may get better results.
Consult the table of solvents and their dielectric constants for other possibilities and/or
practice beforehand with a small amount of benzoic acid. Start with the solvent in which
the substance is soluble. Always only use a minimum amount of solvent, but make sure
that the benzoic acid is completely dissolved!! Continue to add dropwise the hot solvent
in which the compound is more soluble to form a solution. (Be careful that the hot
solvent doesn’t evaporate away.) Then add the second solvent, dropwise again, to
induce crystallization. Collect, wash, and dry the crystals as above. You may want to
obtain a mass and melting point of your crystals before you wash them, in case they
disappear during the washing process. Weigh the dried, purified crystals and obtain a
melting point.
Hand in your combined purified samples of benzoic acid in a properly labeled vial.
(Name of the sample, combined mass, and your name.)
Cleaning Up. Place all organic solvents in the flammable waste container and record
the name and the amounts of waste on the sheet. Organic solids go in the solid waster
containers. Return any clamps that you used.
Analysis
Calculate the percent recovery for both benzoic acid recrystallization procedures and
compare the efficiency of each.
Percent recovery = weight of pure crystals recovered x 100
weight of original sample
Compare the melting points of the impure benzoic acid to the melting points of the pure
samples. Comment on the two methods of recrystallizations. Keep in mind that there
are many situations where a solvent pair is the only available option. Explain your
choice of solvents in the solvent pair recrystallization. Make suggestions for
improvements to the experimental methods for recrystallization in our chemistry lab.
Questions
1. Briefly describe how insoluble impurities are removed during recrystallization?
2. Briefly describe how soluble impurities are removed during recrystallization?
3. Both benzoic acid and 2-naphthol melt around 122 ºC. If you have an unknown
sample of either of these two solids, describe how you could determine if the solid was
benzoic acid or 2-naphthol.
Fall 2007
CH241L
PCC Sylvania
Maazouz
For more information about recrystallization see:
Everything you want to know about recrystallization and melting points:
http://www.chem.arizona.edu/courses/chem243/recrystallization.html
Great site with step-by-step information about recrystallization as well as calculations:
http://ice.chem.umbc.edu/Chem351l/mp_notes.html
http://orgchem.colorado.edu/hndbksupport/cryst/crystdiag.html
http://www.usm.maine.edu/~newton/Chy251_253/Lectures/Solvents/Solvents.html
Chapter 4
Sotvent
Water(Hr0)
Aceticacid(CH3C00H)
Point("c)
100
118
(DM50;
Dimethyt
sulfoxide
cH3s0cH3)
M e t h a n(oCt H r 0 H )
95%Ethano[(CH]CHr0H)
78
Acetone(CH3C0CH3)
56
2-Butanone;
alsoknownas
methylethyl ketone
MF t . r H - r n r H - r H - \
(CH3C00CrH5)
tthytacetate
Dichtoromethane;
atsoknown
(CH?Ctr)
asmethylene
chtoride
40
Djethyt
ether;atsoknownas
ether(CH3CHr0CHrCH3)
35
Methylt-butyl ether
(cH30c(cH3)3)
52
0ioxane(CaH60r)
(C6H5CH3)
Totuene
111
.
Reery3tallization
63
Remarks
It is thesotvent
of choice
it is cheap,
nonflammabte,
because
and
nontoxic
andwit[dissolve
a targevarietyof potarorganic
motecutes.
Its highboitingpointandhighheatof vaporization
make
it difficult
to remove
fromcrystats.
It lvittreadwithatcohots
andamines,
andit is difficultto remove
fromcrystals.
It is nota common
solventfor recrystattizations,
atthough
it is usedasa sotvent
whencar4ring
readions.
out oxidation
It is not a commonty
usedsolventfor recrystattization,
but it is
usedfor readions.
It is a verygoodsotventthat is oftenusedfor recrystaltization.
It
wittdissotve
molecutes
of higherpotarity
thanotheratcohols.
It is oneof the mostcommonty
usedrecrystattization
sotvents.
Its highboilingpointmakes
it a bettersolventfor lesspolar
motecules
readityfromcrystats.
than methano[.
It evaporates
groups
Esters
mayundergo
aninterchange
of atcohot
on
recrystattization.
It is an excellent
solvent,but its lowboitingpointmeans
thereis
notmuchdifference
in thesotubitity
of a compound
at its boi[ing
oointcomDared
to about22'C.
It is an exceltent
sotventthat hasmanyof the mostdesirabte
properties
of a goodrecrystaltization
sotvent.
It is an exceltent
sotventthat hasaboutthe rightcombination
of
moderatety
highboitingpointandthe votatitityneeded
to remove
it
fromcrystats.
Although
a common
extraction
sotvent,
dichtoromethane
boitstoo
to makeit a goodrecrystaltization
sotvent.It is usefulin a sotvent
Dairwithliqroin.
pointis too lowfor recrystathzation,
Its boil.ing
atthough
it is an
goodsotventandfairtyinert.It is usedin a solventpair
extremety
withtigroin.
It is a retatively
newandinexpensive
solventbecause
of iti large-scale
useasan antiknock
agentandoxygenate
in gasoline.
It doesnot
it is lessvolatitethan diethylether,but it
easityformperoxides;
(SeeolsoChapter
hasthe samesotventcharacteristics.
7.)
from
It is a verygoodsotventthat is not too difficultto remove
it is a mitdcarcinogen,
crystats;
andit formsperoxides.
It is an excellent
solventthat hasreplaced
the formertywidelyused
(a weakcarcinogen)
for the recrystattization
benzene
of aryl
compounds.
Because
of its boitingpoint,it is noteasilyremoved
fromcrystats.
(continued)