Demonstration pubs.acs.org/jchemeduc
Regioselective Hydration of an Alkene and Analysis of the Alcohol Product by Remote Access NMR: A Classroom Demonstration Maureen E. Smith, Sara L. Johnson, and Douglas S. Masterson* Department of Chemistry and Biochemistry, The University of Southern Mississippi, Hattiesburg, Mississippi 39406, United States S Supporting Information *
ABSTRACT: A two-part demonstration was conducted in our firstsemester organic chemistry course designed to introduce students to the formation of alcohols, regioselective reactions, and analysis of organic products by NMR analysis. This demonstration utilized the oxymercuration−demercuration sequence to prepare an alcohol from an alkene in a Markovnikov manner because the reaction is easy to execute and has a dramatic, observable color change during the transformation. The alcohol product produced was then utilized in a classroom demonstration of 1H NMR using a remote accessible NMR spectrometer.
KEYWORDS: Second-Year Undergraduate, Demonstrations, Organic Chemistry, Hands-On Learning/Manipulatives, Constitutional Isomers, NMR Spectroscopy
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producing a secondary alcohol. Metallic mercury is formed in this reaction sequence and is recovered from the mixture by simple decantation (Scheme 1). The oxymercuration−demercuration sequence is an example of a regioselective reaction where the formation of one bond is preferred over other possible locations. Two possible products may arise from the hydration of 1-octene, 1-octanol or 2octanol. In this reaction, Markovnikov’s rule allows one to predict which product will be formed by stating that in the addition of water to a double bond, the nucleophile will be added to the more substituted carbon of the double bond. Therefore, using Markovnikov’s rule, students should predict that the product of this reaction will be 2-octanol. However, formation of this product may not be intuitive to a novice organic chemist, as a student can use correct arrow pushing formalisms to arrive at either product. The two products are structurally different, but there is no difference between their physical properties that would allow for easy identification by the students in a classroom setting. Because the product requires further analysis, the situation presents an opportunity to demonstrate region-specificity, Markovnikov’s rule, and the use of NMR spectroscopy in structural identification. Organic chemistry students are exposed to NMR spectroscopy through both lecture and laboratory; however, because of time and resource limitations, very few students experience more than a theoretical education in the introductory organic course sequence. Unless a student participates in undergraduate research, most student experience is limited to analysis of hypothetical spectra based on the predicted product of a
ydration of an alkene to produce an alcohol, by addition of water across a double bond, is a fundamental reaction taught in organic chemistry courses. Students learn that hydration reactions can be completed in a regioselective manner. One regioselective method is the hydroboration− oxidation sequence. This reaction has been carried out successfully as an undergraduate laboratory experiment.1−3 However, the reaction is not a feasible reaction for an inclassroom demonstration because it must be carried out under a nitrogen atmosphere, requires a long reaction time (>30 min), and does not produce any visible changes during the reaction progress. A second regioselective method of hydration is the oxymercuration−demercuration sequence. This reaction lends itself well to the classroom, as it proceeds quickly without side products and provides clear color changes that are easily observable as the reaction progresses. Additionally, workup is rapid and straightforward, allowing for quick characterization of the product by NMR. Therefore, for this demonstration, a modified undergraduate organic laboratory oxymercuration− demercuration experiment was chosen for use in the classroom.4 This reaction takes place in two stages, the oxymercuration with mercuric acetate, followed by the in situ reduction of the organomercurial intermediate with sodium borohydride. In the oxymercuration step, 1-octene is reacted with a solution of mercuric acetate in deionized water/tetrahydrofuran (THF). The solution of mercuric acetate in H2O/THF begins opaque yellow, and immediately becomes clear and colorless as it reacts with the 1-octene, indicating the formation of the organomercurial intermediate. The intermediate is reduced in situ by treatment with sodium borohydride in basic medium, © 2012 American Chemical Society and Division of Chemical Education, Inc.
Published: November 2, 2012 99
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Scheme 1. Oxymercuration−Demercuration Reaction and Corresponding Color Changes
mercuric acetate was dissolved, THF was added with continuous swirling. This resulted in the solution turning an opaque yellow color and the formation of a fine precipitate. Next, 1-octene was added to the flask with continuous swirling. As the 1-octene was added, the solution quickly changed colors from bright opaque yellow to clear and colorless, indicating that the organomercuric intermediate has been formed (Scheme 1). The organomercurial intermediate was then reduced by slowly adding an aliquot of NaBH4 in NaOH solution to the Erlenmeyer flask. The reaction was exothermic, so the flask was placed in a cool water bath to avoid excessive boiling of the THF. During the addition of the NaBH4/NaOH solution to the flask, the solution turned an opaque gray that separated with time to a clear colorless liquid and a mercury precipitate. After typical workup, a sample was prepared for NMR analysis. A proton NMR spectrum was acquired using a spectrometer that was accessed by remote connection. Students observed the instrument demonstration by a Web camera. Workup of product, setup of the remote network infrastructure, sample preparation, details of NMR data acquisition, and spectra can be found in Supporting Information.
laboratory reaction or analysis of samples produced and purified by the instructor. These spectra do not give students an accurate impression of NMR spectroscopy or the scientific process because hypothetical spectra do not take into account side products, and instructor-prepared samples are less likely to include contaminants and unreacted materials. Therefore, although students learn to interpret spectra, they do not become familiar with the nature of science. Likewise, students gain no experience with the instrument or the processes by which spectra are obtained. Students’ lack of experience is due largely to the cost of NMR spectrometers and their maintenance. Such instrumentation is too costly for most colleges to maintain for the sole purpose of education. NMR spectrometers are common to research universities, but access to these instruments by students is limited by their location and availability. Spectrometers are often located in research departments where it is not feasible for inexperienced undergraduate chemistry students to have open access on an individual basis without major safety concerns. In addition, the increase in traffic would disrupt researchers and place an unnecessarily high demand on persons responsible for maintaining the instrument. In an effort to expand the use of research instrumentation, the National Science Foundation has awarded funds to universities interested in purchasing multiuser cyber-accessible instruments. The remote network infrastructure allows for instrumentation in one location to be accessed by users in a different location. This feature increases the research accessibility of the instrument and creates a new opportunity for educational outreach. Sharing of the instrument allows instructors on the same or nearby campuses to incorporate NMR spectroscopy into their classroom. The remote network infrastructure increases use of the instrument and enriches the educational experience of students involved without increasing safety or traffic concerns. In recent years, several examples of networked NMR spectrometer arrangements have been published.5−8 We describe an organic chemistry demonstration that makes use of remote access to a networked NMR spectrometer equipped with Web camera to demonstrate the identification of the product obtained from the hydration of 1-octene by the oxymercuration−demercuration sequence.
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HAZARDS Safety glasses, gloves, and laboratory coat were all worn during this demonstration. Mercuric acetate is toxic and can cause skin irritation, eye, and lung damage and should be handled with extreme care. The alkene, 1-octene, can cause skin, respiratory tract, and eye irritation and damage. Sodium borohydride and sodium hydroxide are caustic and can cause burns of the skin, eye, and respiratory tract. Tetrahydrofuran can cause skin and eye irritation. Dichloromethane can cause skin and eye irritation and is harmful if swallowed. Deuterated chloroform is a suspected carcinogen and should be handled with care. The dichloromethane and deuterated chloroform were both handled in a well-ventilated hood.
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DISCUSSION This demonstration was presented in two classes of firstsemester introductory organic chemistry: a larger regular lecture section and a smaller honors section. Each demonstration was presented using two people, the instructor and the presenter, over two class periods. The demonstrations were scheduled to coincide with the instructor’s lessons on regioselectivity and NMR spectroscopy, but they were not presented in consecutive classes. In between the two demonstrations, the product was worked up and prepared for NMR analysis. Each demonstration used approximately 15 min
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DEMONSTRATION The oxymercuration−demercuration reaction took place in a single reaction flask. Mercuric acetate was placed in an Erlenmeyer flask and dissolved in deionized water. After the 100
dx.doi.org/10.1021/ed300053z | J. Chem. Educ. 2013, 90, 99−101
Journal of Chemical Education
Demonstration
of the beginning of class. This left the rest of the class period for the instructor to cover class content. The group presentation dynamic worked well, as the presenter was able to focus on presenting the demonstration while the instructor was able to talk about the related course content. This also prevented any downtime when issues arose with the demonstration. In general, most chemistry questions were directed to and addressed by the instructor. However, during the NMR analysis, students did ask the presenter questions about how the instrument was maintained. A number of students were interested in the technical details of the instrument and acquisition process. They asked questions about the sample tubes, the strength of the magnet, and the temperature of fluids inside the magnet. When students compared the acquired spectrum to the two possible products, they were able to identify peaks indicating the presence of 2-octanol. Students also mentioned that a number of other peaks were present (slight amount of 1-octene and trace solvents). This was due to the fact that the product had been worked up, but not purified. Students discussed what these peaks’ presence meant about the reaction and, in the honors section, some students even assigned the peaks to the starting materials.
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ASSOCIATED CONTENT
S Supporting Information *
Product workup, setup of the remote network infrastructure, sample preparation, details of NMR data acquisition, and spectra. This material is available via the Internet at http:// pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS D.S.M. would like to thank the National Science Foundation for a CAREER award (MCB 0844478). M.E.S. would like to thank the National Science Foundation for providing her with a GK-12 Fellowship (0947944). S.L.J. would like to thank the National Science Foundation for the instrumentation grant providing equipment and funding used in this work (CHE 0840390). We would also like to thank Gary Cook for his help in the initial setup of the remote network infrastructure.
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