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Organic chemistry i laboratoryOrganic Chemistry I Laboratory
Chirality and Optical Rotation:
Determination of the Enantiomeric Purity of Naproxenand Ibuprofen
Zubrick, J. W. The Organic Chem Lab Survival Manual, 5th edition, Wiley & Sons, Inc., New York, 2000.
Chromatography, TLC & GC Pg 185-198; 213-221.
Alleve is the trade name for Proctor-Syntex Health Products' brand of naproxen (6-methoxy-α-methyl-2- naphthaleneacetic acid). Naproxen is the active component of these tablets and as a chiral molecule, is one of agrowing class of enantiomerically pure drugs. Only the (S) stereoisomer of naproxen has the desiredantiimflammatory activity and is safe to use. The (R) stereoisomer is reported to be a liver toxin. Since it is muchmore expensive to manufacture enantiomerically pure compounds, there is the potential for counterfeit supplies toenter the market which are not enantiomerically pure. In other words, the compounds would contain somemixture of the two enantiomers (R & S) instead of being 100% of the (S) stereoisomer. In this lab we will analyzeboth naproxen and ibuprofen (α-methyl-4-(2-methylpropyl)benzeneacetic acid) with regards to theirstereochemical purity.
Polarimetry and Optical Rotation- The observed angle of rotation of the plane of polarization by an
optically active liquid, solution, or (more rarely) gas or solid is usually denoted by the symbol α. The angle maybe either positive (+) or negative (-) depending on whether the rotation is clockwise, to the right (dextro) orcounterclockwise, to the left (levo) as seen by an observer towards whom the beam of polarized light travels. Itmay be noted that no immediate distinction can be made between rotations of α ± 180no (n = interger), for if theplane of polarization is rotated in the field of the polarimeter by ± 180o, the new plane will coincide with the oldone. In fact α, as measured, is always recorded as being between -90o and +90o. Thus, for example, nodifference appears between rotations of +50o, +230o, +410o, or -130o. To make the distinction, one must measurethe rotation at least at one other concentration. Since optical rotation is proportional to concentration (see below),if solutions of the above rotations were diluted to one-tenth of their original concentrations, their rotations wouldbecome +5o, +23o, +41o, and -13o, values that are all clearly distinct. Readings taken at two differentconcentrations almost always determine α unequivocally.
Biot discovered that the observed rotation is proportional to the length of the cell or tube containing the optically active liquid or solution and the concentration c (or density in the case of a pure liquid): α = [α]cl (Biot'slaw). The value of the proportionality constant [α] depends on the units chosen; in polarimetry it is customary toexpress l in decimeters, because the cells are usually 0.25, 0.5, 1, or 2 dm in length, and c in grams per milliliter(g mL-1) or (and this is preferred for solutions) in g 100 mL-1. Thus: [α] = α/l (dm) c(g mL-1) = 100 α/l (dm) c(g 100 mL-1) The value of [α], the so-called specific rotation, depends on wavelength and temperature which are usually indicated as subscripts and superscripts, respectively. Thus, [α] 25 denotes the specific rotation for light of the wavelength of the sodium D-line (589 nm) at 25oC. In addition, [α] also depends on solvent and concentration. This is usually done by adding such information in parentheses, thus [α] 95% ethanol) denotes the specific rotation at 20oC for light of wavelength 546 nm in 95% ethanol solution at aconcentration of 5.77 g 100 mL-1. Technically the units for [α] will be deg cm2 g-1. However, the formal units arealmost never explictly given in the literature. Instead, [α] will be reported in degrees.
1 Walsh, T. D.; Koontz, C. S. J. Chem. Educ. 1997, 74, 585-586.
2 Sen, S. E.; Anliker, K. S. J. Chem. Educ. 1996, 73, 569-572.
3 Stinson, S. C. Chem. Eng. News 1995, 73(41), 44-74.
4 Caron, G.; Tseng, G. W.-M.; Kazlauskas, R. J. Tetrahedron: Asymmetry 1994, 5, 83-92.
Both Ibuprofen and Naproxen are chiral molecules. We will analyze the optical rotation of these compounds isolated from commercial tablets to determine the optical purity of the active component. This will beaccomplished by comparing our experimental optical rotations with the specific rotations for these compoundspublished in the literature. From the specific rotation, we can calculate back to predict an αobs for theconcentration of our samples if they are optically pure. Significant deviations from these calculated values wouldsignify that our isolated compound was not optically pure. In this case we will be able to determine anenantiomeric excess (%ee) of the two enantiomeric forms.
Ibuprofen is present in the tablet as the free acid (the carboxylic acid functionality is protonated as shown in the above structure). Naproxen, on the other hand, will be present as its sodium salt. You should be able toidentify the carboxylic acid functional group (this is from CHM 114) and draw the structure of the sodium salt.
Notice that the structure for naproxen shown above is the free acid. The solubilities of carboxylic acids willchange depending upon the form of the functional group. When the acid functionality is protonated, thesecompounds will be soluble in nonpolar solvents because the majority of the molecule is a nonpolar hydrocarbon.
However, formation of the sodium salt gives the molecule enough ionic character to allow it to dissolve in aqueoussolution. The initial isolation of each compound will make use of these differences. Supplemental reading fromZubrick includes the techniques of extraction, chromatography, thin layer chromatography, and filtration.
• Caution: Individuals with known sensitivity to aspirin, ibuprofen and related drugs should use extra caution
when performing this experiment, avoiding any unnecessary chemical contact or exposure.
Isolation of Naproxen. Swirl 6 Aleve tablets in 15-20 mL of methanol in a small (30 mL) beaker. Make sure themethanol covers the tablets. In a couple minutes, the outer layer of the tablets will begin to peel away from theinner tablet. As soon as this happens, carefully but quickly remove the inner tablets from the solution withforceps. Leave all of the outer coating in the methanol solution, which will be discarded. Place the 6 inner tabletsin 25 mL of fresh methanol in a 50 mL flask. Swirl to dissolve the naproxen sodium salt. Dissolution is ratherslow, requiring about 20 minutes at room temperature. While waiting for the naproxen to dissolve, isolate theibuprofen and/or take a zero reading from the polarimeter with just methanol in the cell. When the naproxen iscompletely dissolved, there may be some flaky material that still has not dissolved. Filter the solution using aHirsch funnel and pre-cut filter paper (this will be a vacuum set-up using the aspirator). Rinse the filter paper(twice) with a small amount of fresh methanol after the filtration.
Evaporate the solution on the rotovap in a suitable flask to give a volume which will fit into a polarimetertube.Transfer all of the solution to the polarimeter tube using a pasture pipet You may need to add moremethanol to the tube to fill it up. Measure the optical rotation of the solution (three times).
Transfer the naproxen solution back to a round bottom flask and rotovap off the methanol. Weigh the solidremaining to determine a percent isolated yield from the tablet and the concentration in the polarimeter tube.
Remember to use the molecular weight of the sodium salt of naproxen.
Dissolve your solid naproxen sodium salt in water and add 3 M HCl to precipitate the free acid of naproxen.
Dissolve the free acid in chloroform and extract the organic solution from the water using a separatory funnel.
Wash the organic layer once with water, dry with MgSO4 and decant off organic phase. Rinse the solids oncewith fresh chloroform and add to the organic layer. Transfer to a polarimeter tube and measure the opticalrotation (three times). Evaporate off the solvent to determine the weight of naproxen in the polarimeter tube.
Isolation of Ibuprofen. Crush 5 Ibuprofen tablets using a mortar and pestle. Transfer the resulting powder to asmall flask and add 10 mL of 3M HCl. Agitate the contents gently. Extract the aqueous solution three times with dichloromethane (3x5mL). Remember which layer is the dichloromethane so you do not remove the wrong layer.
Combine the organic extracts and dry with a small amount of sodium or magnesium sulfate. Transfer the dryorganic extract into a 25 mL round bottomed flask and concentrate under reduced pressure (using the rotovap)until all of the solution can be placed in a polarimeter tube. Measure the optical rotation three times thenevaporate the solvent to get the solid ibuprofen. Weigh the crude product and calculate a crude yield.
At this time, recrystallization of the ibuprofen may not be required. Check with instructor! Recrystallization of crude Ibuprofen. Transfer the crude ibuprofen to a centrifuge tube and dissolve in a minimumquantity of hot isopropyl alcohol. Cool the solution to room temperature. Use this solution to spot on your TLCplate. Add water dropwise, with swirling, until the solution becomes cloudy. Cool on ice until precipitation iscomplete. Use the centrifuge to separate the solid precipitate from the solution. Carefully remove the solutionfrom the recrystallized ibuprofen. Transfer the ibuprofen from the centrifuge tube to a watchglass to dry. Weighthe dry, recrystallized ibuprofen and calculate the isolated percent yield. How does this compare with the amountreported on the label of the bottle? The dry ibuprofen should be transferred to a vial for storage. It may takeovernight to completely dry.
Zero Reading of the Polarimeter:• Record the 10 independent readings for the zero reading of the instrument (only methanol in the tube).
• Calculate a mean and standard deviation for this “zero” value.
1. What is the measured rotation for naproxen sodium salt, naproxen free acid, and ibuprofen? 2. How do these values compare with the literature values for the specific rotation of these compounds? 3. Looking at your data for the two forms of naproxen, does it appear that molecular structure can be used to determine whether compounds will have (+) of (-) rotations? 4. Where do you believe more error is introduced into the specific rotation measurements: measuring the actual rotation reading or calculating the concentration of material in the polarimeter tube?
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