Chemistry for Everyone edited by
Applications and Analogies
Arthur M. Last University College of the Fraser Valley Abbotsford, BC, Canada
Aromatic Bagels: An Edible Resonance Analogy Shirley Lin Department of Chemistry, United States Naval Academy, Annapolis, MD 21402;
[email protected] The Lewis structure formalism for representing covalently bonded molecules is not only a mandatory subject in organic chemistry but also commonly taught in general chemistry. While the Lewis structure formalism does an excellent job of describing the bonding in many different molecules, there are some molecules where one conventional Lewis structure does not adequately describe the actual structure of a species (1). In these cases, two or more structures, resonance structures, are used. Benzene is a familiar example of a molecule whose structure can be explained in terms of resonance. Two Lewis structures, resonance contributors, are commonly used to describe benzene (the Kekulé structures are shown in Figure 1). Many students struggle with resonance theory for various reasons (2) and chemical educators have responded with a variety of approaches to teach this difficult concept. Some visual aids have depended on overlaying the two structures (2) or the electron clouds of the π system (3). Other teachers have relied upon analogies involving cartoon characters (4), colors (5), and animals (6). The activity or analogy described below addresses two common misconceptions: (i) structures A and B in Figure 1 are identical since B can be achieved by a 60⬚ rotation of A and (ii) benzene is sometimes in form A and sometimes in form B and the molecule oscillates between the two. The activity emphasizes that benzene is one discrete, non-fluxional entity even though it must be depicted using two resonance contributors. This analogy is intended for use in smaller classes where every student can participate directly. However, it may be done as a demonstration by the instructor for larger classes where the logistics of equipping the students and the cost of materials are a concern. The analogy begins with students being paired and each pair receiving one bagel or mini-bagel that has been cut in half. Each student numbers 6 flat-end toothpicks from 1–6 by sandwiching each toothpick between (3兾4)-in. round color coding labels. The toothpicks are placed around the periphery of the half-bagel with the numbers increasing in order in the clockwise direction (Figure 2). These toothpicks represent the carbon atoms of benzene. Using a plastic knife, one student spreads a thick layer of cream cheese, representing pairs of π electrons, between toothpicks 1–2, 3–4, and 5–6. The other student spreads cream cheese between toothpicks 2–3, 4–5, and 6–1. These two bagel halves are now analogous to the two Kekulé structures of benzene. Have the students note that at this point the two half-bagels are not identical. Rotating one half-bagel relative to the other, in an attempt to align the regions of cream cheese, results in a mismatch between the numbered toothpicks. Next, the students spread the regions of cream cheese on each half-bagel evenly over the entire surface. This step is www.JCE.DivCHED.org
•
analogous to saying that the electrons can delocalize over the 6 carbon atoms in the ring. Have the students note that at this point the two half-bagels are identical. The two halves now represent the true structure of benzene, one discrete entity. It is helpful to show students that chemists have created a variation on the Lewis structure formalism to depict the πelectron delocalization in benzene using a single structure (Figure 3). The circle in this structure is analogous to the circle of cream cheese on the bagel. The reaction of 60 students in 3 different sections of general chemistry to this exercise was favorable although one student did not participate fully owing to his dislike of cream
A
B
Figure 1. The two Kekulé structures of benzene.
Figure 2. Two bagel halves representing the Kekulé structures of benzene. The numbered toothpicks correspond to the carbon atoms in the two structures and the π electrons are represented by regions of cream cheese.
Figure 3. Delocalization of π electrons in benzene.
Vol. 84 No. 5 May 2007
•
Journal of Chemical Education
779
Chemistry for Everyone
cheese. Students especially enjoyed eating the bagel halves (see below for safety precautions). This analogy may be extended to resonance-stabilized molecules other than benzene provided that food items with the appropriate shape and symmetry can be obtained. Any thick, spreadable substance can be substituted for the cream cheese. Hazards This activity or demonstration should be conducted in an area separate from the laboratory and should not employ any laboratory equipment. Students should be provided with a clean working surface (paper plates or paper towels). To minimize hazard to students and time spent on the activity, presliced bagels are recommended. Refrigeration is required for
780
Journal of Chemical Education
•
storage of cream cheese; individual cream cheese containers (such as Kraft Philadelphia Mini Tubs) may be preferable to having many students share larger containers. The toothpicks should be removed before consumption of the bagels. Students with allergies to wheat or dairy should not eat the bagels. Literature Cited 1. Streitwieser, A.; Heathcock, C. H.; Kosower, E. M. Introduction to Organic Chemistry, 4th ed.; Prentice Hall: Upper Saddle River, NJ, 1992; p 12. 2. Richardson, W. S. J. Chem. Educ. 1986, 63, 518–519. 3. Delvigne, F. J. Chem. Educ. 1989, 66, 461–462. 4. Starkey, R. J. Chem. Educ. 1995, 72, 542. 5. Abel, K. B.; Hemmerlin, W. M. J. Chem. Educ. 1991, 68, 834. 6. Silverstein, T. P. J. Chem. Educ. 1999, 76, 206–208.
Vol. 84 No. 5 May 2007
•
www.JCE.DivCHED.org
