In the Classroom
Shake For Sigma, Pray For Pi: Classroom Orbital Overlap Analogies Andrew P. Dicks Department of Chemistry, University of Toronto, Toronto, Ontario, Canada, M5S 3H6
[email protected] The nature of covalent bonding in methane, ethane, and ethyne is routinely discussed in introductory organic chemistry textbooks (1). Orbital hybridization and overlap concepts are correspondingly taught at the commencement of early (and often populous) organic college courses. A significant distinction is conventionally made between properties of sigma (σ) bonds and pi (π) bonds. The former involve “head-on” overlap of hybridized or unhybridized orbitals to generate comparatively stronger bonds in all three hydrocarbons. Relatively weaker π bonds are additionally formed by “sideways” overlap of unhybridized p orbitals in ethene and ethyne (2). As orbitals cannot be directly observed (3), their overlap in bond formation is a somewhat abstract notion for student comprehension. Preceding articles in this Journal have sought to address this by construction of molecular models (4, 5). The orbital overlap analogies described herein simply feature instructor and student hand contact and interactively demonstrate differences between σ and π bonds. The parallels are effective in classes of any size and can be used as an icebreaker for students to meet and greet one another. Use of hands and gloves has been outlined previously to teach stereochemical principles (6, 7). From “Handshake and High-Five” to “Shake for Sigma, Pray for Pi” I outline orbital overlap analogies during the opening lecture to a first-year life science cohort (typically >500 undergraduates). A student volunteer is requested to come on stage, and a handshake performed by way of introduction. I then explain to the class that a handshake is an excellent analogy for σ bond formation (head-on overlap of two hands forming a strong bond that is difficult to break). Following this, the student and I perform a “high-five” as an analogy for π bond formation (sideways overlap of two hands that are easier to separate, thus, making a weaker bond). The audience is subsequently encouraged to meet their neighbors by shaking hands and “highfiving”, thereby making the demonstration more vivid. This additionally serves to alleviate formality and decreases tension in the lecture room. Finally, I press my hands together as if praying as an identical analogy for unhybridized p orbital overlap in π bond formation. This leads to the mnemonic “shake for sigma, pray for pi” as a catchphrase to recall the different bond types and their attributes (Figure 1). Feedback End-of-semester feedback1 solicited from students has been overwhelmingly positive. When asked “Do the analogies help 426
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Figure 1. Orbital overlap analogies: (A) shake for sigma and (B) pray for pi.
you remember the different types of orbital overlap and the relative bond strengths (sigma = strong, pi = weak)?” representative written comments were as follows: • The way you physically showed how much harder it is to break apart two hands that are placed in a way as if they are shaking compared to two hands that are only touching meant now I fully understand, and do not need to memorize. • The shake for sigma and pray for pi analogy is one that I'm still using and think back to, even though so much time has passed since you first mentioned it. • I didn't really have a problem distinguishing their difference to begin with, but I do think it was a very good visual and analogy that reinforced their meanings. It was great to be able to think of it in a “real world” context. • Although I learned about sigma and pi bonds in high school, the analogy you provided helped give me a better understanding of the material. • When writing term tests, these analogies come to mind (literally, I was hearing your voice in my head). A concept may be difficult to understand when in words and theory, but analogies make the burden lighter! • Yes, it really helped me understand, and it is catchy.
When posed the question “Did it help to have audience participation (a student on stage) to demonstrate these analogies?” typical student responses were as follows: • Demonstration of the analogies could have been done with or without student participation, but calling a student on stage makes the class feel more involved with the lecture. • I found the handshake demonstration a pleasant surprise during that lecture. Although I did not volunteer, it was exciting for me
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In the Classroom
to see the interactions on the stage. Not only was it helpful for me to understand the bonding concept, but the overall lecture felt livelier. Having students going up on the stage feels like breaking a barrier between the professor and us. • It was such a simple and clear analogy that I don't believe the audience participation was necessary to grasp the concept. However I would still recommend using the audience participation as it caught my attention, and helped me focus when things were shaken up a bit.
Interestingly, one student extended the analogies to resonance principles discussed later in the course: • Overall they were useful analogies that helped me to remember the different kinds of overlap and relative bond strengths. They helped me remember, especially, that sigma bonds are never moved when drawing resonance structures (you can move pi bonds because there is nothing holding two praying hands together, but you can't move sigma bonds because the hands are interlocked).
Summary The use of hands to demonstrate orbital overlap principles is a novel, communal, and captivating approach to teaching bonding in organic molecules. The analogies are imparted without need for external equipment, can be employed in any class venue, and extended to general chemistry courses if desired. Students
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appreciate their interactive nature and acknowledge their pedagogical benefit and memorable delivery. Note 1. Written feedback gathered from 96 students enrolled in the course Introductory Organic Chemistry I at the end of the fall 2009 semester.
Literature Cited 1. For examples, see (a) Brown, W. H.; Foote, C. S.; Iverson, B. L.; Anslyn, E. V. Organic Chemistry, 5th ed.; Thomson Higher Education: Belmont, CA, 2009; pp 36-41. (b) Solomons, T. W. G.; Fryhle, C. Organic Chemistry, 9th ed.; Wiley: Hoboken, NJ, 2008; pp 24-36. (c) McMurry, J. Organic Chemistry, 7th ed.; Thomson Higher Education: Belmont, CA, 2008; pp 12-18. 2. McMurry, J. Organic Chemistry, 7th ed.; Thomson Higher Education: Belmont, CA, 2008; pp 15-18. 3. (a) Scerri, E. R. J. Chem. Educ. 2000, 77, 1492–1494. (b) Spence, J. C. H.; O'Keeffe, M.; Zuo, J. M. J. Chem. Educ. 2001, 78, 877. (c) Scerri, E. R. J. Chem. Educ. 2002, 79, 310. 4. Hoffman, K. B. J. Chem. Educ. 1960, 37, 637–638. 5. Rothchild, R. J. Chem. Educ. 1981, 58, 757. 6. Silversmith, E. F. J. Chem. Educ. 1988, 65, 70–73. 7. For examples, see (a) Lujan-Upton, H. J. Chem. Educ. 2001, 78, 475–477.(b) McMurry, J. Organic Chemistry, 7th ed.; Thomson Higher Education: Belmont, CA, 2008; pp 289-291.
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