Exercise F2 Recrystallization
and Vacuum Filtration
Organic Chemistry Lab I
Fall 2009
Dr. Milkevitch
September 21 & 23, 2009
Steps in a Chemical Synthesis
Determine target compound
Design synthesis (research reactions)
Obtain starting materials & equipment
Assemble apparatus, begin synthesis
Monitor & complete synthesis
Extract and isolate product
Purify product
Identify product (spectroscopic methods, physical
properties)
Assess success of synthesis
Modify synthesis, continue work if necessary
Most Important Steps??
Determine target compound
Design synthesis (research reactions)
Obtain starting materials & equipment
Assemble apparatus, begin synthesis
Monitor & complete synthesis
Extract and isolate product
Purify product
Identify product (spectroscopic methods, physical
properties)
Assess success of synthesis
Modify synthesis, continue work if necessary
Introduction to Recrystallization
Exercise: Recrystallization
Recrystallization: “A purification
method in which a desired product is
separated from impurities through
differential solubility in a hot solvent.”
– Reaction Result: impurities may become trapped in
crystal lattice or adsorbed onto crystal lattice
Can remove adsorbed impurities by washing
How do you remove trapped impurities?
Recrystallization!
Theory
Organic solids usually more soluble in hot solvent
– Than a comparable volume of cold solvent
Make a saturated solution by adding just enough hot
solvent
– To a given amount of impure solid to just dissolve it
When solution cools, solubility of solid decreases
Solid crystallizes (“crashes out”) of solution
Impurities are left behind in solvent
Requirements for Recrystallization
Solvents
Dissolve all of the compound at the boiling point of the
solvent
Dissolve very little or none of the compound when the
solvent is at room temperature (RT)
Have different solubilities for the compound and the
impurities
Have a boiling point below the melting point of the
compound
Compound dissolves, not melts into the solvent
Relatively low boiling point
Be non-reactive with the compound, nontoxic, low odor
Cheap
Which Solvent to Choose?
Compound should dissolve in a reasonable amount of hot
solvent, but not in cold solvent
“Like dissolves like”
Polar in polar solvents
Non-polar in non-polar solvents
Polarity determined by dipole moment
Polarity and solubility of an organic compound
Molecular weight and proportion of hydrocarbon in molecule
– Higher molecular weight less soluble than lower molecular weight
– Higher proportion hydrocarbon, less polar
Try a solvent and see how it does
– Choose an alternate one if it doesn’t work
Tips on Performing a
Recrystallization
Dissolve compound in minimum amount of
hot solvent
After compound dissolves, leave
undisturbed, followed by placing in ice
– Cool too quick, crystals too small and they will
adsorb impurities
– Crystals too large, impurities get trapped inside
No crystals form?
– Scratch inside of flask with glass rod
– Add a seed crystal
– Evaporate off some solvent in the hood
Measuring Success of
Recrystallization
Check purity of crystals
Perform a melting point determination
Determine % recovery
Mass of pure crystals
Mass of impure crystals
X
100
=
% Recovery
Performing the Recrystallization
1. Weigh out 500 mg of impure
compound, place in 50 ml Erlenmeyer
• Add a stir bar
2. Set up hot plate.
• Place 50 ml of ddH2O in a 100 ml erlenmeyer
• Add a stir bar
• Bring to boil
3. Add small amounts of boiling water
to the impure compound
4. Place erlenmeyer + compound
back on hot plate to keep hot
5. Continue to add hot ddH2O
until compound dissolves
completely
6. Remove from heat, place
on bench top to cool to
RT.
7. After the erlenmeyer is at RT,
place in ice bath for full
recrystallization
ice
8. After about 10 min, remove
from ice and vacuum filter
Vacuum Filtration
1. Set up apparatus
Filter paper
Buchner funnel
venturi
Sidearm flask
2. Turn on water
Pour solution +
crystals into
Buchner funnel
venturi
venturi
3. Continue to filter until all your crystals are in the funnel
•Continue to draw air through crystals until they are
dry
4. Remove crystals, weigh them and record weight
Final Notes
Let solid dry in Buchner Funnel
– Place in crystallizing dish, place in hood
to dry
Weigh, calculate % recovery
– Next week
For your report, complete:
– Questions on the following pages
– Place in your results section
Questions
1) List the more important criteria for selecting a recrystallization solvent
2) Why should a recrystallization solvent have a fairly low boiling point?
3) What problems may occur if a recrystallization occurs too rapidly?
4) Why should the boiling point of the solvent be lower than the melting pint
of the compound being recrystallized?
5) Sometimes crystallization does not occur even after cooling the solution
in an ice bath. Describe the probably cause and explain how you could
induce crystallization.
6) During a suction filtration, what are the consequences of using a filter
paper that is larger than the funnel diameter and one that is too small? What
are the consequences for the use of each?
7) A compound has a solubility in ethanol of 4.24 g / 100 ml at 78°C, and
0.86 g /100 ml at O°C.
–
–
A) What volume of hot ethanol will be necessary to dissolve 50 mg of the compound
(assume no impurities) ?
How much compound will remain dissolved in the solvent after recrystallization is complete?