In the Laboratory
Environmentally Responsible Redox Chemistry: An Example of Convenient Oxidation Methodology without Chromium Waste
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Robyn L. Crumbie School of Science, Food and Horticulture, University of Western Sydney, Campbelltown Campus, NSW 1797, Australia;
[email protected] Redox reactions play an extensive role in organic synthesis. As a consequence, undergraduate chemistry laboratory courses usually contain a number of examples, many of which use chromium(VI) as the oxidant. However, there is increasing awareness that the chromium waste from these reactions may have harmful impacts on the environment (1, 2). The recent strides in “green chemistry”, coupled with the need to make the general public realize that chemistry actually benefits humanity and is not necessarily an environmental and health ogre, mean that simple, environmentally responsible experiments need to be introduced into our undergraduate curricula. The reactions described below use recyclable Magtrieve as the oxidant in a simple reaction sequence illustrating the reciprocity of oxidation and reduction processes. The Laboratory Exercise Both the oxidation of benzoin to benzil and the reduction of benzil to meso-hydrobenzoin (Scheme I) are common junior-level undergraduate organic chemistry experiments (3). The difference in our approach is two-fold. Firstly, we perform both reactions in the same laboratory class (typically 2.5 h). This serves to emphasize the reciprocal relationship between oxidation and reduction, a concept sometimes lost in the teaching of organic chemistry where we do not often look at gain or loss of electrons, or change in oxidation number (4). The exercise also provides a good example of the fact that many organic compounds can be oxidized or reduced to more than one product, the nature of which depends on the particular reagent and reaction conditions, and hence redox potentials. In this case, benzoin is oxidized to benzil, not benzoic acid, and the benzil is reduced to meso-hydrobenzoin, not benzoin. Secondly, we use the magnetically retrievable oxidant Magtrieve. Magtrieve is a selective, heterogeneous form of CrO2, whose reduced form stays on the crystal surface (5). It can be easily removed after the reaction by a simple magnetic separation, because only the surface of the CrO2 is reduced. This has significant environmental and cost advantages over traditional chromium reagents that require aqueous work-up and consequent appropriate disposal of the chromium waste. In addition, the reduced chromium surface can be simply reconverted to CrO2 by heating in air, thus adding to its recyclability and cost-effectiveness; indeed, reactions were found to be as successful after the third recycling as with fresh Magtrieve (5). Reactions with Magtrieve are typically performed in chlorinated solvents or toluene. We have chosen to do this experiment in toluene because it is cheaper, more environmentally friendly than chlorinated solvents and easier to recover because of its higher boiling point and lower volatility. The sol268
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vent can then be reused in this experiment with no purification. Consequently, the Magtrieve methodology is cheap, environmentally friendly, and easily performed by students. Common experimental techniques involved in this experiment include refluxing, filtration under vacuum, recrystallization, melting point determination and running and interpreting infrared spectra. In particular, the experiment provides a good introduction to thin layer chromatography. Chromatographic techniques play a major role in chemistry and chemically related areas such as biochemistry and forensic and medical science. There are a large number of chromatographic techniques, some of which require large and expensive equipment. However, all kinds of chromatography involve the differential adsorption of compounds onto a stationary phase and this feature allows mixtures to be separated, either as a means of purification or simply to analyze the components. Thin-layer chromatography is a cheap and easy method of demonstrating this process, and since benzoin and benzil have quite different Rf values and are both visible under ultraviolet light, monitoring the progress of this oxidation reaction by TLC is easily achieved. O
OH benzoin oxidation
O
O benzil reduction
OH
OH meso-hydrobenzoin Scheme I. Oxidation of benzoin to benzil and the reduction of benzil to meso-hydrobenzoin.
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In the Laboratory
The experimental procedure involves adding the Magtrieve to the toluene solution of benzoin in a 100-mL roundbottom flask and refluxing the mixture. The reaction is monitored by TLC; the first sample is taken 15 minutes after refluxing has commenced. The total reaction time is generally around 25 minutes. When the reaction is complete, the cool solution is decanted from the Magtrieve using a bar magnet to keep the oxidant in the flask. The residue is washed several times with small aliquots of toluene. Benzil is yellow, so it is easy to see when most of the product has been washed off the oxidant. The combined toluene solution is then filtered through Celite on a Büchner funnel to remove powdery traces of Magtrieve. The solvent is removed on a rotary evaporator and the residue recrystallized and characterized by melting point determination and IR. Typical percentage yields are of the order of 80%. Low recovery is usually due to insufficient or poor washing of the Magtrieve (most common) or occasionally refluxing the solution for a very long time (when the student disappears for coffee). The second half of the experiment involves dissolving benzil in ethanol and adding sodium borohydride. The color of the solution disappears as the reaction progresses and the diketone is fully reduced after 10 minutes. Isolation of the product involves dilution with a given amount of water, heating the solution for several minutes on a steambath and allowing the solution to cool slowly. Under these experimental conditions, only the meso isomer should precipitate. This provides an inroad into a number of stereochemical questions, such as how many isomers can theoretically form, are they all formed (why or why not) and if all are formed, are they formed in the same amounts and why is only one isomer isolated. The product is again characterized by melting point and IR, with typical yields of 30–35%. Equipment and Chemicals The experiment makes use of glassware commonly found in undergraduate laboratories. Other equipment required includes a good magnetic stirrer, a hairdryer, an ultraviolet light (to visualize the TLC plates), a rotary evaporator, a steam bath, melting point apparatus and an infrared spectrometer. Again, these are all common undergraduate laboratory items. The chemicals involved may be obtained from a large number of chemical suppliers, with the exception of Magtrieve, which is marketed through the Sigma-Aldrich Chemical Company.
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Hazards Toluene, light petroleum, acetone, and ethanol are flammable and should not be used near naked flames or electrical sparks. They may be harmful by inhalation, ingestion, or skin absorption and on contact may irritate the eyes or dry the skin. Magtrieve, Celite, benzil, and benzoin may all be harmful by inhalation, ingestion, or skin absorption and on contact may irritate the eyes or skin. The compounds may emit toxic fumes under fire conditions. Sodium borohydride liberates a flammable gas when exposed to water. It is toxic in contact with the skin and if swallowed, and can cause burns. Ultraviolet lights can burn the eyes. Summary This laboratory exercise allows students to explore the reciprocity of oxidation and reduction reactions while undertaking the reactions in an environmentally friendly manner. The oxidation with Magtrieve eliminates the potential human and environmental hazards created by oxidants in alternative procedures. While this oxidant is quite expensive, it is completely recyclable for virtually only the costs of the labor, and so is actually inexpensive when used on a long term basis. W
Supplemental Material
Instructions for the students and notes for the instructor are available in this issue of JCE Online. Literature Cited 1. Pellerin, Cheryl; Booker, Susan M. Environ. Health Persp. 2000, 108, A4002–A407. 2. Steinpress, Martin G.; Ward, Anthony C. Groundwater 2001, 39, 321–322. 3. See, for example, Fieser, Louis F.; Williamson, Kenneth L. Organic Experiments, 8th ed.; Houghton Mifflin: Boston, 1998; pp 497–499, 502–503. 4. Smith, Michael B.; March, Jerry. March’s Advanced Organic Chemistry: Reactions, Mechanisms, and Structure, 5th ed.; Wiley-Interscience: New York, 2001; p 1506. 5. Lee, Ross A.; Donald, Dennis S. Tetrahedron Lett. 1997, 38, 3857–3860.
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