EXPERIMENT #2
CHEMICAL RESOLUTION OF (±)-PHENYLSUCCINIC ACID
WITH (-)-PROLINE
Introduction: As you know from class discussion, enantiomers can be difficult to
separate because they have identical physical properties, except for the rotation of planepolarized light. You know too, that enantiomers can be distinguished by other chiral
compounds. You will use this concept to resolve a racemic (±) mixture of phenylsuccinic
acid, using the naturally occurring amino acid, (-)-proline, as the resolving agent. The
success of the reaction depends on the fact that (-)-proline reacts preferentially with (+)phenylsuccinic acid to form and precipitate its bis proline salt (in the bis salt both
carboxyl groups of (+)-phenylsuccinic acid are deprotonated by proline molecules.). This
solid precipitate can be collected by filtration. If desired, the other enantiomer, (-)phenylsuccinic acid, can be isolated as its mono proline salt if the filtrate is treated with
additional (-)-proline. Recovery of the individual enantiomers is accomplished by
acidification of the corresponding salts. In this experiment, however, you may simply
confirm that the filtrate is enriched in the (-)-enantiomer by evaporating the filtrate and
acidifying the residue. You will determine the efficiency of the resolution by
determining the specific rotation of the two enantiomers.
Ph
*
CO2H
N
CO2H
H
CO2H
H
(±)-phenylsuccinic acid
(-)-Proline
Week 1: Resolution of (±)-phenylsuccinic acid
Procedure
•
In a clean, dry, 250 mL round-bottomed flask, dissolve 2.9 g of (±)-phenylsuccinic
acid in 75 mL of 2-propanol. Add 1.73 g of (-)-proline.
•
Add a magnetic stir bar and gently mix the contents of the flask until you achieve a
homogeneous cloudy solution; the reactants will not dissolve. Place the flask into a
heating mantle (Do not plug the heating mantle directly into a regular outlet; only use
an outlet that has a voltage regulator dial) and attach a condenser with circulating
cold water. Heat the flask and let its contents reflux for 30 minutes. During this time
the bis proline salt of (+)-phenylsuccinic acid should begin to precipitate, and the
stirring bar may not longer be useful.
•
Stop heating and carefully remove the flask from the heating mantle. Let the flask air
cool for about 15-20 minutes until it is close to room temperature. While you are
waiting for the flask to cool, set up for vacuum filtration.
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•
Gently break up the solid in the flask with a spatula, and transfer as much as possible
to the filter. Rinse the flask with 15 mL of acetone and filter to recover as much of the
remaining solid in the flask as possible. Wash the residue on the filter paper with 15
mL of acetone and air-dry with occasional stirring of the product. Then dry the solid
between a few circles of filter paper (This is the bis proline salt of (+)phenylsuccinic acid). Transfer the filtrate to a 250 mL round-bottomed flask and
remove the solvent using a rotary evaporator.
•
In an ice-water bath, cool 15 mL of 6M HCl in a 50 mL beaker. Add the solid
obtained by vacuum filtration in the above step and stir the mixture for 5 minutes.
Collect the product by vacuum filtration. Rinse the beaker once with 15 mL of cold
water and filter to recover as much of the remaining solid as possible. Wash the solid
residue on the filter paper with 15 mL of cold water and let air dry a few minutes.
This is (+)-phenylsuccinic acid. Weigh your product and save enough for a melting
point. Recrystallize the rest of your product from water, collect, and dry until the
next lab period.
•
To recover phenylsuccinic acid enriched in the (-) enantiomer, add 15 mL of cold 6M
HCl to the residue in the recovery flask, obtained after evaporating the solvent at the
rotary evaporator. Scrape down the sides of the flask, and stir the mixture for at least
5 minutes. Collect the product by vacuum filtration. Rinse the beaker once with 15
mL of cold water and filter to recover as much of the remaining solid as possible.
Wash the solid residue on the filter paper with 15 mL of cold water and let air dry a
few minutes. This is enriched (-)-phenylsuccinic acid. Weigh your product and
save enough for a melting point. Recrystallize the rest of your product from water,
collect, and dry until the next lab period.
Prelab – Week 1
1. Read the laboratory procedure and sections 4.1 through 4.9 in your text.
2. Provide the structures for the group abbreviations “Ph” and “CO2H” (these will be
important throughout both semesters of organic chemistry!).
3. The (-) form of phenylsuccinic acid has the R-configuration and the (+) form has
the S-configuration. Draw the two enantiomers in your notebook, clearly showing
the configuration around the stereogenic atom.
4. Determine the configuration of (-)-proline whose structure is given on the first
page of this handout.
5. Draw the structure of the bis proline salt of (+)-phenylsuccinic acid. Remember
that the salt formation is just an acid-base reaction (Hints: reread the introduction.
Which atom in (-) proline can abstract the acidic proton from (+)-phenylsuccinic
acid?).
6. Prepare a flow chart that summarizes the resolution of racemic phenylsuccinic
acid.
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Week 2: Polarimetry and
Spartan Tutorial
Procedure
•
Determine the melting points of both the crude and recrystallized samples of both
enantiomers.
•
Prepare 10 mL solutions of each enantiomer in acetone; you will be provided with 10
mL volumetric flasks for the preparation. Precisely weigh each sample into a 20 mL
beaker and dissolve the sample in a minimum of acetone (Use your entire sample,
except what you saved for melting points). Quantitatively transfer the solution to the
volumetric flask, and bring to volume. The technique of quantitative transfer will be
explained to you if you have never used volumetric flasks before.
•
Use the two solutions prepared above to determine the optical rotation. Instructions
for the use of the polarimeter will be provided by the lab instructor. (NOTE: The
polarimeter cells are expensive and fragile. Please handle them with utmost care.)
You can calculate the specific rotation [α]D from your observed rotation αobs using the
formula given below.
[α]D = αobs
lxc
where l is the length of the cell in decimeters and c is the concentration of the sample
in g/mL.
•
While you are waiting for your turn to use the polarimeter, go up to the Schupf Lab
(Room 404) and go through the first two chapters of the Spartan Tutorial in
preparation for next week’s Molecular Modeling I lab. There are two types of
computers in the lab, the Unix (Silicon Graphics) and the PCs. The versions of
Spartan that run on the two types are slightly different, but the capabilities are
essentially the same.
Prelab – Week 2
1. Find out the expected specific rotation for (+)-phenylsuccinic acid and its
enantiomer.
2. If you measured the optical rotation of racemic phenylsuccinic acid, what would
you expect to find? Explain briefly.
3. Could you use the melting point data of the two enantiomers to tell them apart?
4. Read at least the introduction to the Molecular Modeling I lab, and bring a copy
of the handout with you to lab.
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Lab Report
•
Report the yields, melting points, observed and specific rotation values for the two
enantiomers. Be sure to analyze and discuss your results and include all calculations.
•
It is possible to calculate a quantity known as percent optical purity (or enantiomeric
excess) according to the equation shown below.
% Optical purity =
specific rotation of sample
specific rotation of pure enantiomer
x 100
Assuming that the literature value for the pure enantiomer was obtained
under the same conditions as your sample, calculate the % optical purity
of your two enantiomers.
•
Calculate the % of the (+)-phenylsuccinic acid enantiomer in each solution (include
calculations, of course).
Answer the following questions.
1. Had you used the base shown below instead of (-)-proline, could you have
successfully resolved the racemic mixture in this experiment? Explain.
N
H
2. Based on the knowledge gained in the experiment, do you think it might be
possible to resolve a racemic mixture of 1-phenylethylamine with (-)-malic acid?
Explain your reasoning in just a sentence or two.
3. A student was given a solution labeled only as 2,3-dihydroxybutane. She used the
polarimeter to determine its optical rotation, and found that the solution was not
optically active. Draw the absolute configuration of one isomer of 2,3dihydroxybutane that would give zero optical rotation.
Reference:
Stephani, R.; Cesare, V. J. Chem. Ed, 1997, 74, 1226.
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