In the Laboratory edited by
The Microscale Laboratory
R. David Crouch Dickinson College Carlisle, PA 17013-2896
Student Empowerment through “Mini-microscale” Reactions: The Epoxidation of 1 mg of Geraniol W Thomas R. Hoye* and Christopher S. Jeffrey Department of Chemistry, University of Minnesota, Minneapolis, MN 55455; *
[email protected] It is fair to guess that fewer than 1% of organic chemists entering graduate school or assuming an entry level bench-chemist position in a corporate laboratory has the ability and confidence to perform a chemical transformation on 1 mg of substrate.1 While the extent to which this specific skill is necessary can be debated, experience of the corresponding author suggests that the ability to design and execute reactions and to isolate, purify, and characterize products on a very small scale (called here, mini-microscale1) correlates strongly with experimental prowess across the entire range of preparative organic chemistry techniques. One specific setting where mini-microscale skills are essential is in the late stages of complex molecule synthesis, where the material at the forefront of exploration is necessarily (i) limited in quantity and (ii) precious. The working hypothesis at the outset of the study described here was that it is highly advantageous for novice researchers to gain these skills at an early stage of their learning, long before the expertise is critically needed and with materials that have little inherent value. We have identified an exercise that permits one to gain this experience and confidence. While there are many different reactions that could serve this role, we selected the title epoxidation for reasons discussed in the following paragraph. We now ask every new student entering our laboratory to go through this training drill (1). The benefits of this exercise have become clear, which prompts us to provide the details and to make the recommendation that others consider adopting this exercise. It is time well spent. This experiment is appropriate for both undergraduate and graduate students starting research work in organic chemistry or for use in an advanced organic chemistry laboratory or research methods course. While it is not well-suited for routine use in the introductory organic laboratory course, the exercise could be considered as a candidate for a capstone experience for those students in an introductory course who are up to the challenge.
enes in 1 react at a similar rate, giving the proximal and distal monoepoxides 2 and 3, respectively, in comparable amounts (3, 4). We selected this epoxidation reaction because there is no critical need to control the atmosphere (insensitive to moisture or oxygen), the reaction is rapid at ambient temperature (∼1 h), the reaction apparatus is exceptionally simple, the workup is straightforward, and the starting material and products have appropriate volatility characteristics and are well behaved chromatographically and spectroscopically (4). While excess MCPBA can be used to cleanly obtain the diepoxide 4 from 1, a more challenging and instructive experiment is to limit the quantity of MCPBA to ∼1 equivalent, which gives rise to a product mixture containing all of 1–4. One milligram of geraniol is 0.007 mmol (or 7 µmol). We typically perform the reaction at a concentration of ∼0.1 M, which means ∼70 µL of solvent (CH2Cl2, DCM) is used. To prevent evaporative loss of the solvent, the reaction vessel should have a small headspace and be kept tightly closed. We have used either a 300 µL mini-vial or a one dram (1.8 mL), screw-capped vial as the reaction “flask”. We typically have prepared a stock solution of MCPBA in DCM and added 1–1.2 equiv to the geraniol兾DCM solution. Assessing the titer of the MCPBA, which is sold containing varying quantities of m-chlorobenzoic acid and water2 and which slowly loses its peroxidic content upon storage, is important. All four of compounds 1–4 have distinct retention factors (Rf) on silica gel and are readily amenable to separation through either a micro-chromatography column (e.g., using a disposable pipet with ∼1 g of SiO2) or a prepacked medium pressure chromatography (MPLC) column. Each pure product is then characterized by GC–MS and 1H NMR spectroscopy.
The Experiment Geraniol (1, Scheme I) is a readily available monoterpene containing two similar (trisubstituted) alkenes, one of which bears an allylic alcohol substituent. Whereas the allylic hydroxyl group inductively deactivates the proximal alkene toward electrophilic attack, by virtue of hydrogen bonding to a peracid reagent, it promotes attack at that adjacent alkene (2). These opposing effects nearly balance one another for the case where m-chloroperoxybenzoic acid (MCPBA) is the epoxidizing agent. Consequently, both alk-
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Scheme I. Products (2–4) from epoxidation of geraniol (1) by mchloroperoxybenzoic acid (MCPBA).
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In the Laboratory
The experiment can be tailored in a variety of ways [we have done (i)–(iii)] to better fit local preferences and needs. For example, (i) the experiment could first be performed on a larger scale (e.g., 25 mg of 1) and then taken down to the 1 mg level. It is important to emphasize that the entire process from reaction setup to isolation of pure products can be effected in an afternoon. Thus, the barrier to repeating the experiment is relatively low. (ii) An excess of MCPBA could be used to cleanly provide diepoxide 4 as the sole product, thereby deemphasizing the separation aspects. (iii) The rate or progress of the reaction could be conveniently monitored by in situ 1H NMR analysis using CDCl3 as the reaction solvent and an NMR tube as the reaction vessel. Finally and to repeat, (iv) the conceptual and pedagogical essence of the exercise could be retained by use of a different reaction altogether but still on an ∼1 mg scale. Hazards m-Chloroperoxybenzoic acid is an oxidizing agent and irritant. It should be stored separately from other combustible chemicals and has a longer shelf-life when refrigerated. Users should avoid breathing particles when handling. It is recommended that the material safety data sheet (MSDS) for MCPBA be read prior to use. Geraniol shows the following toxicity: acute oral toxicity (LD50): 3600 mg兾kg [Rat]; acute dermal toxicity (LD50): 5001 mg兾kg [Rabbit]. Toxicity data for the epoxide products (2–4) are not available. Users should avoid contact with the skin. The solvents (CH2Cl2, EtOAc, hexanes, and CDCl3) are volatile and should be used in a ventilated work area. Appropriate personal protection should be worn during the experiment. Outcomes Valuable skills that are gained include: the added importance of using pure solvents and of effecting quantitative transfers in small-scale experiments; the concept of using a stock solution; an appreciation for the impact of reactant concentration on an observed reaction rate3; the ability to evaluate reagent purity and to quantify small quantities of material by 1H NMR spectroscopy; and the importance of repeating a failed experimental procedure after incorporating changes to remove an observed impediment. Summary Presented here is an easily mastered exercise that provides researchers with valuable skills and accompanying confidence in their ability to perform a class of experiments that at the outset may have been deemed impossible. The combined bottom line is empowerment. To reiterate, the ability
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to design and execute reactions and to isolate, purify, and characterize products on a 1 mg scale correlates strongly with experimental abilities across the entire range of preparative organic chemistry techniques. Acknowledgments This study was supported, in part, by a grant awarded by the DHHS, GM-65597. We thank Eric E. Buck, Junha Jeon, Brandon S. Lange, Kyle A. Marten, Bongjin Moon, and Liansheng Su for feedback and insight. W
Supplemental Material
A handout for the students, notes for the instructor, and H NMR, TLC, GC, and EI–MS data for the reactants and products are available in this issue of JCE Online. 1
Notes 1. In this article we refer to a preparative reaction using ≤ 10 µmol of substrate (which is ≤ 1.54 mg of geraniol) as “mini-microscale.” Common usage of the term “microscale” in the context of experiments relevant to laboratory instruction exercises suggests scales 10–100 times larger (i.e., 0.1–1 mmol, 15–150 mg in the case of geraniol). 2. For example, in the current Aldrich catalog, 3-chloroperoxybenzoic acid is specified to contain “77% max.” of MCPBA with the “remainder 3-chlorobenzoic acid and water”. We have found it convenient to determine the ratio of the two aromatic materials using proton NMR spectroscopy. The spectrum of any sample of MCPBA (CDCl3) is recorded and the region from δ 7.4-8.1 expanded and integrated. Unique resonances for the peracid and acid are easily observed in the spectrum recorded at 300 or 500 MHz. An example is included in the Supplemental Material.W 3. In our experience, even seasoned researchers often need to be reminded that a rate is a rate constant times a concentration.
Literature Cited 1. Hoye, T. R.; Jeffrey, C. S. A ‘Boot Camp’ Training Exercise in the Hoye Group: The Epoxidation of 1.0 mg of Geraniol. Presented as a poster at the 36th National Organic Symposium, Bloomington, IN, June 10, 2003. 2. (a) Henbest, H. B.; Wilson, R. A. L. J. Chem. Soc. 1957, 1958– 1965. (b) Adam, W.; Wirth, T. Acc. Chem. Res. 1999, 32, 703– 710. 3. Klein, E.; Rojahn, W.; Henneberg, D. Tetrahedron 1964, 20, 2025–2035. 4. Bradley, L. M.; Springer, J. W.; Delate, G. M.; Goodman, A. J. Chem. Educ. 1997, 74, 1336–1338.
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