Stereochemical Resolution of Ibuprofen
1. Read Mohrig 2nd edition Techniques14.1-14.5, pp. 165-172 or 3rd edition Techniques 16.5-16.7, pp. 203-211and Chapters 4.2, 7.4, 7.7 in Hornback.
2. Review Polarimetry in the Appendix.
(Please turn in the answers to these questions when you arrive to lab.)
1. Please complete the Table of Physical Constants below.
2. Please show the balanced chemical reaction that is taking place between potassium hydroxide and ibuprofen. Please use ChemDraw to show the overall structures of the reactants and products.
3. Use the Physician’s Desk Reference or another cited reference to find out which enantiomer of ibuprofen, R or S, is the biologically active form in our body. Please use ChemDraw to draw the enantiomer of the active form. Make sure to use “wedges/dotted lines” to show the correct stereochemistry.
4. Please give a detailed procedure for how to precisely create a 2.5% w/v solution of your solid enantiomer dissolved in ethanol using a 25-mL volumetric flask for your solution.
5. A 0.0109 g/mL solution from a student’s sample in this lab gave an observed optical rotation of +0.35. What would be the percent enantiomeric excess of (S)-(+)-ibuprofen in this student’s sample? What percentage of her sample is (+)-ibuprofen and what percentage of her sample is (-)-ibuprofen? (Note: pure (+)-ibuprofen has a specific rotation of +57.)
The process by which the enantiomers of a racemic compound are separated is called stereochemical resolution. In 1848, Louis Pasteur performed the first stereochemical resolution of an organic compound. While observing crystals of tartaric acid under a microscope, he noted that they appeared to be mirror images of each other. By looking at the crystals under a microscope and physically separating the crystals which looked different, Pasteur isolated (+) and (-) tartaric acid from the racemic mixture of the two, and showed that the two samples rotated plane-polarized light in opposite directions. Today, chiral chromatography, in which the stationary phase preferentially retains one enantiomer over the other, is a preferred method of stereochemical resolution. Enzymatic resolution is another method of stereochemical resolution. Certain enzymes will preferentially react with one enantiomer in a racemic mixture, creating a new product through functional group conversion; the new chiral molecule can then be separated by conventional chromatography from the starting material.
A racemic carboxylic acid can be resolved by reacting it with one enantiomer of a chiral amine. The resulting diastereomers can be separated based on their physical properties (solubility, for example). In this lab, ibuprofen (a carboxylic acid) will be reacted with (-)-1-phenylethanamine, as shown in Scheme 1, below, to produce two diastereomeric salts, only one of which is water soluble. The salts will be separated by filtration and the enantiomers of ibuprofen will be recovered by an acid base reaction, as indicated in Scheme 2.
Scheme1. Formation of diastereomeric salts from racemic ibuprofen.
Scheme 2. Regeneration of ibuprofen from its (-)-1-phenylethanamine salt.
Table of Physical Constants (TPC)*
||Grams or mL Used
MP or BP °C
|| 76 (m.p.)
||sl H2O, s os
||KOH, .25 M
||s H2O, chl; msc EtOH, eth
|Hydrochloric acid, 5%
* CRC Online (87th edition)
Formation of the diastereomers:
- Work in teams of two people. Weigh 2.4 g of racemic ibuprofen and transfer to a 100 mL round bottom flask. Add 24 mL of 0.25 M KOH. Set up a reflux apparatus, using an air condenser Why don't you need to run water through your condenser?, a hard ceramic heating mantle connected to a Variac, and a magnetic stirrer. Place a magnetic stir bar in the flask and begin stirring the reagents. Heat the reagents to 75-85°C and adjust the Variac to maintain this temperature range. The temperature can be measured by removing the condenser and placing a thermometer into the reaction mixture. You can do this occasionally, but it is not recommended or necessary that you monitor the temperature continuously. There will be a small amount of vapor in the condenser but the reagents should not be allowed to boil. What is the reaction that is taking place between the ibuprofen and potassium hydroxide?
- When the temperature of the reagents has reached 75-85°C Why 75-85°C? and most of the ibuprofen has dissolved, remove the condenser and add 0.75 mL (-)-1-phenylethanamine, dropwise, using a plastic measuring pipet. (Adding this reagent slowly prevents the mixture from cooling so much that the ibuprofen solidifies.) Replace the condenser and continue to heat and stir for 45 minutes, maintaining a temperature of 75-85°C.
- Cool the mixture to room temperature and vacuum-filter, using a Buchner funnel to collect the insoluble phenylethanamine salt of ibuprofen. Use a 500 mL filter flask as there will be a large volume of foam produced.
Regeneration of (+)-and (-)-ibuprofen:
- To recover a sample of (+)-ibuprofen, transfer the insoluble salt from the Buchner funnel to a 150 mL beaker. Add 18 mL of 5% HCl solution and stir with a glass rod for at least 15 minutes, breaking up the salt to insure that all of the solid reacts with the acid. What is the reaction taking place with the HCl? Chill the mixture in an ice bath Why?, and vacuum filter, collecting the solid on a Hirsch funnel. Wash with ice water and let dry for 10-15 minutes. Alternatively, you can dry the solid by spreading it on an unglazed porcelain tile. Use a metal spatula to squeeze the solid into the tile over and over again. After doing this several times, the solid should look flaky and dry. Use all of your solid to prepare a solution in absolute ethanol. Calculate the concentration of your solution in g/mL and determine its absolute observed rotation by polarimetry.
- Optional: To recover (-)-ibuprofen from the soluble phenylethanamine-ibuprofen salt in the filtrate, gravity filter into a 100 mL beaker, using a stemless funnel and fluted filter paper. What is the purpose of this filtration step? Add 18 mL of 5% HCl solution, and stir with a glass rod for at least 15 minutes. Chill the mixture in an ice bath, and vacuum filter, collecting the solid on a Buchner funnel. Wash with ice water and let dry for 15 minutes. Alternatively, you can dry the solid by spreading it on an unglazed porcelain tile. Use a metal spatula to squeeze the solid into the tile over and over again. After doing this several times, the solid should look flaky and dry. Use all of your solid to prepare a solution in absolute ethanol. Calculate the concentration of your solution and determine its absolute rotation by polarimetry.
* Adapted from McCullagh, James V., J. Chemical Education, Vol. 85, No. 7, July 2008, pp. 941-943.
Waste Disposal and Clean-up
- Dispose of all organic wastes and solvents in the flammables waste container.
- Dispose of the liquid filtrate in the aqueous acid waste container.
- Rinse your glassware with acetone and dispose of the acetone rinse in the flammables waste bottle.
- Wash your glassware in hot soapy water and perform a final rinse with acetone before returning glassware to the appropriate storage area.
(Please turn in the answers to these questions when you arrive at your NEXT lab.)
- Write out equations for all of the acid-base reactions you and your partner carried out.
- What is your observed specific rotation for your isolated (S)-(+)-ibuprofen that you will be comparing to the literature specific rotation for (S)-ibuprofen?
- What is your percent enantiomeric excess for your resolved sample of (S)-(+)-ibuprofen?
Pure (S)-(+)-ibuprofen has a specific rotation of +57.
- What percentage of your isolated (S)-(+)-ibuprofen is pure (S)-(+)-ibuprofen?
- Save the calculated models of the two diastereomers, ibuprofen-salt-1 and ibuprofen-salt-2, to your computer (right-click and Save link as,,,), then open the models with ChemDraw 3D, and answer the following questions.
- Identify the two salts by their stereochemistry. Which one is the (S,S) salt and which is the (R,S) salt?
- For one of the models, find the chiral carbon atom in the ibuprofen portion of the salt. Draw the Newman projection down the bond from the chiral carbon atom to its adjacent methyl group. Repeat for the other salt. How are these two Newman projections similar or different?
- Ibuprofen takes part of its name from the iso-butyl group, the 4-carbon chain at one end of the molecule. The condensed structure for the iso-butyl group is (CH3)2CH—CH2—. For each of the models, draw the Newman projection down the methine (CH)—methylene (CH2) bond. Are the two Newman projections the same or are they different? Do they represent the lowest energy conformation? Explain your answer.