In the Laboratory
The Conversion of L-Phenylalanine to (S)-2-Hydroxy-3-phenylpropanoic Acid: A Simple, Visual Example of a Stereospecific SN2 Reaction Nanine A. Van Draanen*,† and Stephanie Hengst Department of Chemistry, California Lutheran University, Thousand Oaks, California 91360 *
[email protected] † Current address: Department of Chemistry and Biochemistry, California Polytechnic State University, San Luis Obispo, California 93407.
Good laboratory examples of a stereospecific SN2 reaction using a chiral substrate appropriate for the second-year organic chemistry lab are hard to find (1). Many SN2 reactions are plagued by problematic solvents such as dimethylformamide (DMF) or dimethylsulfoxide (DMSO), by competition with elimination reactions, or by noxious or dangerous nucleophiles or electrophiles. Also, many optically active substrates are prohibitively expensive, making a true demonstration of what is arguably the most important feature of the SN2 reaction, its stereospecificity, inaccessible to the undergraduate organic laboratory. We found that the conversion of L-phenylalanine to (S)-2-hydroxy3-phenylpropanoic acid (L-phenyllactic acid) is an outstanding candidate for a hands-on experience with stereospecific SN2 reactions. Experiment L-Phenylalanine (1, Scheme 1) is an abundant, inexpensive, nontoxic amino acid, and a good example of a member of the “chiral pool” (2). Diazotization of L-phenylalanine results in the unstable aliphatic diazonium salt 2, which is believed to undergo a rapid, intramolecular SN2 reaction to give the highly strained R-lactone (3) (3). In a second, slower, intermolecular SN2 reaction, 3 reacts with the solvent (water) to open the lactone and yield the final product, (S)-2-hydroxy-3-phenylpropanoic acid (4). Because this process occurs with two SN2 reactions, the final product has a net retention of configuration. This reaction has the added advantage of being environmentally friendly: the reaction is run in aqueous solution, using a safe amino acid and generates no hazardous waste requiring disposal. This experiment illustrates some important chemical concepts, including
• Water solubility dependence on the state of ionization of a compound • Stereospecificity of the SN2 reaction. • Measurement of optical activity. • Effect of diastereotopic protons in the 1H NMR spectrum.
The starting amino acid is highly soluble in the acidic solution of the reaction; the amphoteric nature of L-phenylalanine is apparent at the start of the reaction. The L-phenylalanine solution is cooled and then the aqueous NaNO2 solution is added with stirring. The reaction mixture begins to form tiny bubbles as the diazonium salt forms and nitrogen gas is liberated by the
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intramolecular reaction with the carboxylic acid. Students can see reaction occurring as the N2 bubbles form. This rapid intramolecular reaction reinforces the concepts that (i) N2 is an excellent leaving group and (ii) that intramolecular reactions are relatively rapid and easy. Once the addition of NaNO2 is complete, the reaction is allowed to warm to room temperature. The bubbling continues, but after approximately an hour at room temperature, the final product begins to crystallize from the solution, again giving a visual clue that another reaction is occurring. This second, intermolecular reaction is much slower and requires at least 24 h to complete. We allow the reaction to continue at room temperature in a lightly corked Erlenmeyer flask until the next lab period (2 days), by which time the reaction mixture appears solid with crystalline product. The difference in solubility between the starting amphoteric amino acid and the final R-hydroxy acid is worthy of discussion with the students; both compounds have similar molar masses and hydrogenbonding capability, so that the difference in solubility rests in the basicity of the amino group. Isolating the product by filtration, rinsing the crystals with chilled water, and allowing the product to dry gives a reasonable yield (45-60%) of fine, offwhite crystals of sufficient purity for 1H NMR, melting point, and optical rotation analyses.1 Recrystallization from water increases the purity of the product, but the recovery from the recrystallization solution is only 55-70%. The 1H NMR of the product is simple but interesting. As recorded on a 60 MHz instrument, the product shows clear coupling in the diastereotopic methylene protons, giving a clean doublet of doublets for each hydrogen as can be seen in Figure 1. The coupling constants can be measured readily, and we found this to be an excellent compound to introduce the effect that a chiral center exerts on the NMR characteristics of prochiral hydrogens. Hazards The major components of this reaction system have relatively low hazard potential. The starting phenylalanine is a naturally occurring, nontoxic amino acid. The product (S)-3phenyl-2-hydroxypropanoic acid (L-3-phenyllactic acid) can cause skin and eye irritation. Sulfuric acid is corrosive and contact can cause severe damage to skin and eyes. Sodium nitrite is a strong oxidizing agent and toxic, and formation of noxious nitrogen
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r 2010 American Chemical Society and Division of Chemical Education, Inc. pubs.acs.org/jchemeduc Vol. 87 No. 6 June 2010 10.1021/ed100167k Published on Web 04/23/2010
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hood, although in our laboratories we are unable to do so and therefore exercise caution during the addition. Conclusion We have developed a simple, environmentally friendly reaction procedure that gives students an excellent and visual experience with a stereospecific SN2 reaction. Additional benefits include the interesting solubilities of reactant and product, a simple example of a relatively advanced topic in 1H NMR spectroscopy, and an introduction to the use of the chiral pool as an inexpensive and abundant optically active starting material. Note
Figure 1. Coupling pattern for diastereotopic protons in (S)-2-hydroxy3-phenylpropanoic acid. Scheme 1. Conversion of L-phenylalanine to (S)-2-Hydroxy-3-phenylpropanoic Acid via Tandem SN2 Reactions
1. The melting point of the product is a few degrees lower than the literature melting point, indicating the presence of impurities. However, the 1H NMR spectrum shows no impurities, and the optical rotation is close to the literature value.
Literature Cited 1. A preparative SN2 reaction yielding a racemic mixture is described in Stabile, R. G.; Dicks, A. P. J. Chem. Educ. 2003, 80, 313. An example of blended SN1/SN2 mechanisms yielding partially racemized products can be found in Mosher, M. D.; Kelly, C. O.; Mosher, M. W. J. Chem. Educ. 1996, 73, 567. A recent lab experiment describing an achiral SN2 reaction can be found in Esteb, J. J.; Magers, J. R.; McNulty, L.A.; Morgan, P.; Wilson, A. M. J. Chem. Educ. 2009, 86, 850. 2. Yoshikoshi, A. Kagaku, Zokan ( Kyoto, Jpn) 1981, 91, 87. Nakahara, Y.; Ogawa, T. Kagaku, Zokan ( Kyoto, Jpn) 1981, 91, 101. 3. Brewster, P.; Hiron, F.; Hughes, E. D.; Ingold, C. K.; Rao, P. A.D. S. Nature 1950, 166, 179–180. The related double-inversion of Lglutamic acid has been reported in this Journal, see Markgraf, J. H.; Davis, H. A. J. Chem. Educ. 1990, 67, 173. Smith, L. R.; Williams, H. J. J. Chem. Educ. 1979, 56, 696. The use of phenylalanine in place of glutamic acid has the advantages of demonstrating both intra- and inter-molecular SN2 reactions (rather than two intramolecular reactions); the product crystallizes from aqueous solution, obviating the need for extraction and concentration steps; and the 1H NMR spectrum of the product gives a clear, readily understood example of the complex splitting seen with diastereotopic protons.
oxide gases is possible under the conditions of the reaction. The NaNO2 solution should be added slowly to the reaction mixture such that formation of the brown nitrogen oxide fumes is minimized. Ideally, this reaction should be carried out in a fume
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Supporting Information Available Student procedure; instructor's notes; product characterization, including NMR spectra. This material is available via the Internet at http://pubs.acs.org.
pubs.acs.org/jchemeduc
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r 2010 American Chemical Society and Division of Chemical Education, Inc.
