Experiment:
Resolution of a Racemic Mixture
A mixture of equal numbers of enantiomeric molecules is called a racemic mixture. The process
of separating the enantiomers is referred to as “resolution”. Resolution is frequently
accomplished by chemical conversion of the enantiomers into a pair of diastereomers through
reaction with one enantiomer of another chiral compound. For example, if you react a mixture of
R and S isomers with another chiral compound that is pure R isomer, you will obtain RR and SR
products, which are diastereomers. These diastereomers are not enantiomers, and therefore have
different physical and chemical properties. Thus they can be separated by physical means, often
by fractional crystallization. The separated enantiomers must then be chemically regenerated
from the diastereomeric compounds.
In this experiment, the racemic amine, α- methylbenzylamine, a base, is easily converted to a
mixture of diastereomeric salts by treatment with R,R-tartaric acid, which is readily available
from grapes and other fruit.
racemic mixture
H
CH3
CH3
NH2
H
NH2
S-isomer
R-isomer
The lone pair of electrons on
the amino group (-NH2) acts
as a base and removes a proton
from one of the carboxyl
groups (-COOH), forming the
diastereomeric salts shown below.
O
+
HO
H
OH
HO
HO
H
O
R,R-tartaric acid
CH3
H
H
NH3
O
HO
H
OH
O
CH3 O
NH3
HO
H
OH
O
HO
R,R,R-salt
H
O
HO
H
O
S,R,R-salt
Crystallization from methanol
favors formation of solid SRR-salt, which
is isolated by filtration, leaving the RRR-salt
in solution (discarded).
In this acid-base reaction, both diastereomeric products form quickly. One of them, the RRR
salt, is more soluble in methanol than the SRR salt. Consequently, the crystals that form have a
higher percentage of the less soluble SRR diastereomer than the more soluble RRR diastereomer.
Sometimes these diasteromeric salts may begin to crystallize within 24 hours, but often they will
not crystallize even after a week of sitting in solution. You will be mixing the reagents for this
reaction during one lab period and coming back the following week to collect the diastereomeric
crystals and isolate the partially resolved amine from them. Your instructor will be monitoring
the formation of crystals, and if your crystals have not begun to form a day or so before your lab,
he or she will “seed” them for you. Seeding a solution induces crystallization by adding a small
amount of the desired crystals, in this case from another pair of students whose crystals have
formed well, to the solution which has not yet begun to crystallize. The seed crystals provide a
surface on which the molecules which are in solution can crystallize. Sometimes dust particles
or scratches on the inside surface of a glass container can serve as crystallization sites. Just
because your crystals did or didn’t form says nothing about how correctly you carried out the
resolution process. All of your reactions produced good yields of diastereomeric salts, but for
some of you, your salts did not crystallize. They all will eventually, with or without seeding.
At the beginning of the second week of this resolution experiment, you will isolate the
diastereomeric salt crystals by filtration. Remember, this salt mixture will have more of the SRR
salt than the RRR salt. Then you will recover the mixture of S and R amines. But now, instead
of being a racemic mixture with 50% each of the R and S amines, your mixture will now contain
more of the S than the R. The racemic mixture has been partially resolved. To isolate pure S,
the resolution process must be carried out again on this newly formed partially racemized amine.
Recovery of Amines
Vacuum filtration is used to collect SRR salt (major product) and RRR salt (minor product). The
equations and flow charts below are for the recovery of the S-amine from the salt. You should be
able to write identical equations for the recovery of the R-amine (minor product) from the same salt.
H
CH3 O
NH3
HO
H
S,R,R-salt (major product)
OH
O
HO
O
H
NaOH (aq)
H
CH3
NH2
+
sodium tartrate
S-isomer
add ether to aqueous solution
ether layer
water layer
H
O
CH3
NH2
Na
S-isomer (major product)
+-
HO
H
-
O Na
+
O
HO
H
O
sodium tartrate (discarded)
(Further purified by washings and finally distillation.)
The optical purity of the resolved amine can be determined by comparing its specific rotation
with the known value for S--methylbenzylamine.
 Pre-lab Preparation
Before coming to lab next week, you must do the following:
Carefully review the reactions shown in the diagrams above and the procedures. Be prepared for
a possible quiz on this material.
Review material that you have learned about washing and extraction.
In your lab notebook, write the physical constants for the following compounds:
1-phenylethanamine
(This is a tough one. Look under benzene, (1-aminoethyl) dl or
use the formula: C8H11N. The dl indicates that it is the racemic
mixture.)
(+)-tartaric acid
(This may be listed as L-tartaric acid. L here does not refer to
levorotatory tartaric acid. It refers to a structural characteristic of
chiral compounds. It is confusing, I know. The L-tartaric acid is
dextrorotatory. You'll see it in the Handbook.)
Methanol
Experimental Procedure
! Safety Considerations
! Methanol is extremely toxic and can cause blindness if consumed. If you spill some on your
hands, wash it off with water. If you spill any on your clothing or large quantities on your
skin, contact your instructor.
! Ether is highly flammable. Keep all ether in the fume hoods.
! Use boiling stones for your distillation.
! Don’t plug sand baths directly into the wall; use Variac controllers.
! Keep unknown liquids off of skin and avoid breathing the vapors.
! Dispose of waste organics in the waste container provided, not in the sink.
1. Place 100 mL of methanol and 7.6 g of (+)-tartaric acid (this is the pure RR enantiomer) into
a 250-mL Erlenmeyer flask. The tartaric acid should dissolve in the methanol with swirling,
but if it doesn’t, gentle heating on a hot plate should dissolve it. If heating is required, do it
in the hood because methanol vapors are toxic.
2. Remove from the heat and cautiously, over a period of a minute or so, add 6.1 mL of the
racemic -methylbenzylamine. The reaction mixture should give off heat. Then place a
cork in the flask and allow the solution to stand undisturbed at room temperature until the
next lab period. By then, prism-shaped crystals of the diastereomeric salt mixture will have
separated from the solution.
3. During the following lab period, you will collect the crystals by suction filtration and wash
them with a small amount of ice-cold methanol.
4. In a beaker, partially dissolve the salt mixture in 20 mL of water. The crystals will not
dissolve completely in the water, so don’t spend too much time trying to get them into
solution. Slowly add 3 mL to 4 mL of 50% by mass sodium hydroxide solution. Add
enough to completely dissolve the salt. (The base will react with the diastereomeric salts to
reform the free amines that are now partially resolved. These amines are insoluble in water
and will form a thin layer of organic liquid on the top of the water. The extraction procedure
described below will separate the amine from the aqueous layer.)
5. Cool the solution to room temperature and then, working in the hood, extract the liberated
amine using your separatory funnel and three 10 mL portions of diethyl ether. As you recall,
ether is less dense than water, and therefore it will be the top layer and water the bottom
layer. You will need to remove the aqueous layer after each washing and retain it in a flask.
Pour the ether out of the top of the separatory funnel into a 50-mL Erlenmeyer flask and then
place the aqueous layer back into the separatory funnel for the next washing. Repeat this
until you have extracted the aqueous layer with three portions of ether, combining the three
ether layers in the same Erlenmeyer flask. Be very careful not to throw anything away, in
case you get confused as to which layer contains your product.
6. Take your combined ether washes, which now contain the amine, and dry it with anhydrous
magnesium sulfate. Add a full spatula of the MgSO4, stopper and swirl for two minutes. If
the solid does not flow freely, then you need to continue to add MgSO4 until it does flow
freely when the flask is swirled.
7. While the ether layer continues to dry for a few minutes, begin assembling a simple
distillation apparatus. See p. 25 for a diagram. You instructor may have you lightly grease
all joints to prevent leaks.
8. Decant the ether layer from the MgSO4 into a 50-mL round bottom flask. Rinse the
magnesium sulfate with 2-3 mL of ether and decant this ether carefully into the flask with the
rest of the ether extracts. Add boiling stones and begin to distill off the ether. Collect the
ether in a 25-mL r.b. flask. After most of the ether is gone, wrap the distilling flask with foil
to keep the heat in the system to allow for smoother distillation of the product. You should
also increase the setting on your Variac. The temperature recorded at the distilling head will
continue to rise, slowly at first with only a few drops of distillate collected. It will look like
you have no product left, but don’t worry, there will still be enough amine to distill. Once
the temperature reaches 100ºC you should no longer see ether distilling off. This is a good
time to change receivers to a tared 15-mL round bottom flask. Collect all material in the
range of 182 - 186ºC.
9. Determine the optical rotation of the product. Weigh the tared flask containing the amine. If
the mass of your amine product is less than 0.8 g, you may want to consider combining your
sample with another student also having less than 0.8 g of product in order to insure that you
get a measurable optical activity. Then carefully transfer the amine to a 25-mL volumetric
flask. Use an eyedropper to transfer the amine. Rinse the distillation receiver with small
amounts of methanol and transfer it with an eyedropper into the volumetric flask in an effort
to get all of the product you collected into the volumetric flask. Fill the volumetric flask to
the engraved mark on the neck with methanol. Invert the flask and shake. Repeat this
inversion and shaking at least ten times to insure that the solution is homogeneous.
10. Pour the solution carefully into one of the 2 dm polarimeter tubes. Tip up the end of the tube
in each direction to release any air bubbles. Place the tube in the polarimeter and take a
reading. Using the concentration of your sample and the reading you measure with the
polarimeter, you will be able to calculate the optical purity of your sample.
 Post-Lab and Report Requirements
1.
In a Results Section of your report, write the mass of your product transferred into the
volumetric flask and the experimental value of  measured on the polarimeter.
2.
Use these values to calculate the specific rotation [] of your product. Show all
calculations.
[]
T
D
=
 measured
C
(for a solution)
Where,
measured = the degree of rotation measured with the polarimeter
(+) for dextrorotatory and (-) for levorotatory
ℓ = the path length of the sample holder in decimeters (2.00 dm in this case)
C = the concentration of the sample in grams solute/mL of solution
3.
Calculate the Optical Purity of your product and the percent purity of the levorotatory
isomer. Show all calculations.
Optical purity =
obs
 100
ref 
Where,
T
obs  = the value of []
calculated in question 2 above
D
ref  = the value of []
T
obtained from the literature for a pure sample. This
D
value is -38º for a methanol solution of this enantiomer.
4.
Percent Major Isomer = 50 +
opt. purity
2
Percent Minor Isomer = 50 -
opt. purity
2
Describe strategies that you could use to increase the optical purity of this compound.
Don’t forget to write a summary/conclusion.