In the Classroom
A Simple Mnemonic for Tautomerization Mechanisms in Organic Chemistry Chad E. Stephens Department of Chemistry and Physics, Augusta State University, Augusta, Georgia 30904
[email protected] Mnemonic devices can be very helpful to students as they strive to organize and remember certain types of information (1, 2). Some of the chemistry mnemonics that have appeared in the literature have focused on the periodic table (3), electron configurations (4), the mole concept (5), the electronegativity scale (6), the formula of certain polyatomic ions (7), the stereochemistry of alkenes (8) and carbohydrates (9, 10), and the fate of electrons in redox reactions (11). Apparently, however, no mnemonic related to a standard, “textbook” reaction mechanism of organic chemistry has previously been described. Herein, a simple, one-word mnemonic for recalling the steps of a classical tautomerization mechanism, either under acidic or basic conditions, is presented. Tautomerization is a fundamental mechanistic process in introductory organic chemistry. The process typically involves the interconversion of a carbonyl compound (keto form) to an enol form, but can also involve other interconversions, such as between imines and enamines. The mechanism for tautomerization involves a sequential two-step process regardless of whether it is catalyzed by acid or base. Under acidic conditions, the mechanism involves protonation of one atom (proton on), followed by deprotonation of another atom (proton of f ). Under basic conditions, the mechanism is just the reverse and involves deprotonation of one atom (proton of f ) followed by reprotonation of another (proton on). Importantly, in writing either process, the showing of an alternative resonance structure between the protonation-deprotonation steps can allow for smooth transition between the keto and enol forms (12). Despite the stepwise and predictable nature of tautomerization mechanisms, students seem to have particular difficulty in recognizing when tautomerization has occurred and in being able to show the mechanistic steps for the process. To help them recall this information, the simple, one-word mnemonic OREO (as in the cookie) can be utilized. In this mnemonic, the bookend O's refer to whether the proton is being added on or taken off of the substrate in the mechanism, with the exact definition being adaptable to the catalytic conditions employed. In either case, RE stands for resonance, which can be sandwiched between the on-off steps in the written mechanism. Thus, under acidic conditions, OREO stands for on-resonance-off, whereas under basic conditions, it stands for the reverse process of off-resonanceon. Knowing that a proton is typically added on under acidic conditions and taken off under basic conditions should be enough to allow for correct usage of the mnemonic under either condition. OREO is illustrated with the tautomerization mechanisms in Figure 1. Ultimately, the use of this mnemonic does not, of course, guarantee that a student will be able to correctly show the process of tautomerization using mechanism arrows, nor understand the 1186
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Figure 1. Illustrating OREO with (A) keto-enol tautomerization under acidic conditions and (B) imine-enamine tautomerization under basic conditions. Movement of electrons with arrows has been omitted.
structure and chemistry of keto and enol forms. This will require a deeper understanding. However, through the use of OREO, students can be reminded that the written mechanism is a sequence of fundamental steps rather than some “mystic” transformation. As an additional benefit, the OREO mnemonic can also be used to remind students that tautomerization is more than just resonance, as they sometimes can believe; it also involves relocation of the proton through the on/off steps! Finally, it has been gratifying to hear positive feedback about OREO from some of my students. One recently stated, enthusiastically, that it very much helped solidify his knowledge of tautomerization and that he assumed this mnemonic was wellknown. Another student said that OREO was cited many times at a group study session as a helpful reminder of the steps of the mechanism. These and many other comments have led me to believe that OREO may be helpful to other students of organic chemistry. Acknowledgment The author would like to thank Tom Crute for helpful comments related to this manuscript.
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Vol. 87 No. 11 November 2010 pubs.acs.org/jchemeduc r 2010 American Chemical Society and Division of Chemical Education, Inc. 10.1021/ed100437b Published on Web 08/30/2010
In the Classroom Thomas, C. W. J. Chem. Educ. 1988, 65, 44. Starkey, R. J. Chem. Educ. 2000, 77, 734. Painter, J. J. J. Chem. Educ. 1996, 73, 949–950. The widely known OIL RIG mnemonic is cited in: Timberlake, K. C. General, Organic, and Biological Chemistry: Structures of Life, 3rd ed.; Pearson Education: Upper Saddle River, NJ, 2010; p 221. 12. Smith, J. G. Organic Chemistry, 1st ed.; McGraw-Hill: New York, NY, 2006, p 835.
Literature Cited 1. 2. 3. 4. 5. 6. 7.
8. 9. 10. 11.
Manalo, E. Psychologia 2002, 45, 69–79. Quigley, M. N. J. Chem. Educ. 1992, 69, 138–140. Covey, W. J. Chem. Educ. 1988, 65, 1089. Iza, N; Gil, M. J. Chem. Educ. 1995, 72, 1025. Brown, B. S. J. Chem. Educ. 1991, 68, 1039. Cole, G. M., Jr. J. Chem. Educ. 1985, 62, 230. Marmorino, M. G. Chem. Educator 2005, 10, 415.
r 2010 American Chemical Society and Division of Chemical Education, Inc.
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