1. Recrystallization & Melting Point
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 +5°C and -5°C 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 °C

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 isn’t 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 compound’s
melting point to be lowered. Insoluble impurities have no effect
on a compound’s melting point.
Report melting point as a range. Record the temperature at which you start to see the compound melt (even if it’s 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