Recrystallization

1

• Chapter 4
• Recrystallization Melting Point
• Recrystallization
• A purification technique for impure solid
  compounds
• A several-step process
• Can be on on a microscale or macroscale
• Melting Point
• Verifies the purity of a compound
• Aids in the identification of an unknown
• Typically reported as a range
• An impure solid will have a lower melting point
  than the
• pure solid - less attractive forces within the
  solid, thus less
• energy to break up those forces, thus lower mp

2
The Experiment

• You will be given an impure Recrystallization
  Unknown.
• Recrystallize/purify this solid
• Take a melting point of the pure impure solid
• Calculate the percent recovery from the
  recrystallization
• You will be given a Spectral Unknown (Ch 12) -
  take its melting point.
• Once you have the mp, go to the course website
  click on the
• Spectral Unknown link.
• List all compounds that have melting points from
  5C and -5C of your observed melting point.
  One of these compounds will be
• your spectral unknown.

3
Possible Recrystallization Unknowns
Use your solubility data to help explain the
identification of your unknown! Remember the
like dissolves like concept.
4
Recrystallization

• The Experimental Process
• Find a suitable recrystallization solvent for
  your solid (solute) - solubility refer to Table
  4.1
• First try hexanes (nonpolar), if needed try water
  (polar)
• Trial and error
• Choice of solvent will give a clue as to the type
  of compound you have - either nonpolar or polar
• Ideal solvent solute insoluble at rt, solute
  soluble with heat
• Dissolve the compound in a minimal amount of the
  chosen solvent
• Remove insoluble impurities (may skip)
• Pipet Filtration, hot filtration
• Crystallize your compound - slow cooling of
  crystals may need to scratch tube to induce
  nucleation
• Collect and wash the crystals
• Pipet filtration or vacuum filtration
• Dry the crystals

5
Recrystallization
Example Recrystallize an impure sample of
benzamide
Benzamide mp 127-130C

6
Recrystallizing Benzamide

• Find a recrystallizing solvent
• Consider the polarity of benzamide
• Like dissolves like concept
• Possible solvents (Table 4.1)
• Hexanes Least Polar
• Toluene
• Benzene
• Ether
• Chloroform
• Dichloromethane
• Acetone
• Ethanol
• Methanol
• Acetic acid
• Water Most Polar

• Polar
• H-bonding

Test polar solvents Good solvent Solute should
be insoluble at room temp and soluble at high
temps impurities should be soluble at all temps.
Water is a good solvent for recrystallizing
benzamide.
7
Recrystallizing Benzamide
2. Crystallize a larger amount of benzamide in
water. 3. Remove impurities by hot filtration or
with Norit (macroscale). 4. Allow the hot
filtrate to cool slowly! Slow cooling allows
better crystal growth - better crystal growth
means higher purity of crystals. Slow cool by
allowing the filtrate to cool to room temperature
then cool in an ice-water bath. 5. Collect and
wash crystals. Collect crystals by
filtration. Be sure to wash crystals with cold
solvent. Example For benzamide, wash with cold
water. 6. Dry crystals air dry or pat crystals
with filter paper.
8
Purification by Recrystallization
Four main principles Solubility
Saturation Level The concentration of the
desired solute is significantly higher than
the concentration of the impurity. When solution
cools, the impurities will remain in solution,
and the desired solute will crystallize out of
solution.

• Exclusion
• Every solid has a defined crystal
  structure/lattice.
• As the solution cools, crystals form into their
  well-defined lattice.
• Impurities cannot fit inside these lattices.
• The desired crystal solute will be pure since
  impurities stay in
• solution.

Nucleation
9
Recrystallization
What if a suitable solvent isnt found? You can
try a two-solvent system two solvents that
are miscible with each other. Table 4.2 lists the
miscibility of common organic solvents. Examples
of two-solvent systems Water acetone Hexanes
acetone Dichloromethane ether Methanol
acetic acid
10
Melting Points

• Load a small amount of sample into a capillary
  tube.
• Use a Mel-Temp or Thomas-Hoover and to obtain a
  melting point.
• These are located throughout the lab limited
  number.
• Soluble impurities in a compound will cause the
  compounds
• melting point to be lowered. Insoluble
  impurities have no effect
• on a compounds melting point.
• Report melting point as a range. Record the
  temperature at which you start to see the
  compound melt (even if its a drop at
• first) then record the temperature at which all
  of the sample becomes liquid.

11
Determining the Unknown

• Mixed Melting Point
• To verify the proposed identification of unknown
• Mix your purified unknown with a known sample
• If the two compounds are the same, the melting
  point
• will match the melting point of your purified
  unknown.
• If the two compounds are not the same, the
  melting
• point will be lower than the melting point of
  your
• purified unknown.
• Consider and discuss in your final report
• Does the choice in recrystallization solvent make
  sense
• in terms of like dissolves like?
• Compare the experimental mp to the actual,
  reported mp - do they
• match?

12
Recrystallization Lab

• Next Week in Lab
• PreLab for Chapter 4 is due.
• Quiz 1 on Chapter 4