Today:
• Your experiences with recrystallization?
• Mixed-solvent recrystallization
MiniQuiz
Table for Part A
Solubility of Pure Benzoic Acid
Which solvent(s) is/are suitable for recrystallization?
Rt
H2O Hot
Cooled
Rt
95% EtOH Hot
Cooled
Rt
Pet Ether Warm
Cooled
Rt
Acetone Warm
Cooled
Part B Recrystallization of
Impure Benzoic Acid
Your sample contained benzoic acid (70%), oxalic acid
(30 %) and a trace of a dye.
Role of “active charcoal”?
Today:
Obtain mp. of purified benzoic acid (range!) and of
your original impure benzoic acid.
Questions:
1. What do you expect the purities of Crop 1 and Crop 2
to be? Same? Different? What melting point ranges do
you expect for Crop 1, Crop 2, original mixture?
Explain.
2. What is the “mother liquor”?
3. What compound(s) do you expect to be in the mother
liquor after filtering off your first crop of benzoic acid
crystals?
4. Is it possible to retrieve 100% of solid sample in a
recrystallization? (Assume that you are a very
experienced chemist.)
Part C Recrystallization of Naphthalene
from a solvent pair
melting point?
Often it is impossible to find one good solvent for
recrystallization. Then a solvent pair has to be
used:
1) Solvent (#1) should dissolve compound at RT.
2) Solvent (#2) should NOT dissolve compound at
RT.
3) Both solvents must be miscible!
Common Solvent Pairs
for Crystallization
• Methanol-Water • Ether-Acetone
• Ethanol-Water • Ether-Petroleum
• Acetic acid-Water ether
• Acetone-Water • Toluene-Ligroin
• Ether-Methanol • Methylene chloride-
Methanol
Procedure:
Part C:
1) Do solubility tests for Naphthalene:
H2O, 95% EtOH, Pet Ether, and Acetone
Goal is to find a good “solvent pair”: Solvents #1 and #2
2) Once you have chosen the solvent pair, then use ~ 0.5 g
of Naphthalene in 50-mL E-flask.
(Record the exact weight to three decimal places 0.000 g)
3) Dissolve compound in minimum amount of warm solvent
#1(“good solvent”), start by using 10 mL of solvent #1.
Record additional mL of solvent #1 used.
Use 20 drops per 1 mL, when using a disposable glass pipette and a
small rubber bulb.
Crystals must be just dissolved; do not use a great excess!
4) Solvent #2 is added dropwise to the warm mixture, until
mixture just becomes cloudy. Cloudiness indicates
precipitation. At this time, add solvent #1, just until the
cloudiness disappears. The solution is now saturated. Cool
solution to obtain crystals.
Reading Solubility Data
Example:
Solubility of Benzoic Acid
Solubility in water(g/l)
at 0o: 1.7
at 95o: 68.0
Next time:
Molecular Modeling
(no Notebook Preparation required,
but do preview/prestudy
Experiment)