In the Classroom
Making Organic Concepts Visible Robert S. H. Liu* and Alfred E. Asato Department of Chemistry, University of Hawaii, 2545 The Mall, Honolulu, HI 96822 Organic chemistry, a basic science that serves as the cornerstone for many disciplines, can prove to be quite difficult to many non-scientifically oriented students. Through many years of teaching the introductory organic chemistry class, we found that the use of familiar analogies is frequently a very effective way of explaining abstract concepts. For that, it could be quite appropriate to add a little local flavor into the discussion. Here we would like to mention five examples intended to explain terms such as functional groups, resonance structures, polarizability, ionization in mass spectrometry, and difference in reactivities between alkyl and vinyl halides. While these examples were intended for students in Hawaii and may not be applicable elsewhere, they nevertheless should serve the purpose in demonstrating our belief that there are more ways than one to communicate with students. The important part is to be able to talk to your students. “That big pineapple blows my mind!” Functional group is a useful and simple concept. But yet, for some strange reasons, many students have difficulties in applying a generalized equation to a specific example. Thus, while many of them are quite willing to put in the effort to learn reactions such as the Hofmann rearrangement O + Br2, NaOH
NH2
R
RNH2
and its reaction mechanism, when a related reaction was asked subsequently,
+? NH2 NH2 O
“Does it make sense to complain that the pineapple is so big that it blows your mind?”
Hapa-haole Resonance hybrid is an important concept in organic chemistry. But now and then a few students in the class have problems grasping its true meaning. How does a double-headed arrow (describing resonance) differ from a two-arrow symbol (equilibrium)? Or, why are certain resonance contributing structures more important than others (major and minor contributing structures) while none accurately represents the compound itself? The following locally relevant analogy appears appropriate. Benzene is a hapa-haole (a mixed-blood in Hawaiian), which does not look like a haole (Caucasian) or a Japanese, or wot-evah, but rather an average of the two. For that, one certainly never would think of a hapa-haole as one periodically jumping back and forth between a haole and a Japanese (or wot-evah). The latter equilibrating situation could only correspond to a two-room mixer in a U.S.–Japan binational conference. Sometimes a hapa-haole may be one-half Irish, onequarter Japanese, and one-quarter Chinese. So can the molecules. It may resemble one resonance hybrid (major contributing structure) more than the others (minor contributing structures).
+ Br2, NaOH H2N
Kamikaze Fly
O
students frequently reacted with a complete silence. “You are probably thinking ‘that’s not fair. It’s just like exams with unfamiliar big molecules. They blew our minds. Unfair! Sadistic! Don’t know how to teach!’—and all that stuff.” A few students actually nodded in agreement. Then, they were reminded that organic chemistry is the chemistry of functional groups. All they had to do was to cover up that great big chunk of molecule that sometimes seemed to serve as a smoke screen to confuse them. (The only problem is that it usually works too well.) Then, the reaction became so simple. “What if the question is changed to the following form?”
*Corresponding author.
In discussing mass spectrometry one starts with the equation: M + e{ → M + + 2e{ But, it may not be immediately obvious to beginning organic students why a neutral molecule, when hit by a speeding electron, loses another negatively charged electron. Let’s think about the alternative. The nucleus is the other spot the speeding electron may hit. But because of its compact size, the probability of hitting it is low. Furthermore, one should consider the mass difference. A proton is 1840 times heavier than an electron, and a nucleus many more times of that. So when an electron hits the nucleus, it is like a fly committing kamikaze on Konishiki.1 I think the natural reaction by the mammoth Sumo wrestler would be: “It doesn’t hurt,” and nothing would happen. If, instead, that speeding electron hits an
Vol. 74 No. 7 July 1997 • Journal of Chemical Education
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In the Classroom electron, the billiard ball effect takes over—out come two electrons at a certain deflected angle.
“You’re Repulsive!” “Why is a vinyl (or an aryl) halide much less reactive than an alkyl halide in an SN2 reaction? Why is halogenation of an alkene classified as an electrophilic addition reaction?” “I don’t know. Just memorize them.” Nu– Nu δ+
Michael Jordan & Konishiki Polarizability is a somewhat difficult, abstract concept to many students. With eight valence electrons in both (seven in atoms), it is not immediately obvious why a fluoride ion is not polarizable whereas an iodide ion is. Well, the main difference is the degree of compactness. The valence electrons in a fluoride are in the second shell, close to the nucleus, very compact. When it moves, it moves like the lean and silky Michael Jordan. Smooth. But for an iodide ion, the valence electrons are in the fifth shell with additional d and f electrons underneath. So, when it moves, it moves like Konishiki, the same mammoth Sumo wrestler. That is, his frame moves first and the rest of his body (and there is quite a lot of that) will have to catch up. Then, they over-shoot with the result of all those things oscillating around. This means that for a few seconds the center of gravity of his bones does not coincide with the center of gravity of the rest of his bulk. When that happens to an atom or a molecule, an oscillating dipole results. Movement of a single ion is only one way to cause the displacement of center of gravity. Sometimes there could be induced dipoles. Just imagine what happens when two giant Sumo wrestlers bump into each other.
784
δ+
X
+
X
X–
Perhaps the situation can be clarified in a different way. “When an electron-rich nucleophile attempts to react with an alkyl halide, it first seeks out the δ+ carbon to initiate the reaction. When the same electron-rich nucleophile attempts to seek out the δ + sp2 hybridized carbon, it first comes into contact with the negatively charged π-cloud. Under the circumstance, what do you think the two electron-rich reagents will tell each other? ‘You’re repulsive!’ A reaction between the two? Certainly not!” For hydrohalogenation of alkenes, there is no doubt as to which part of the reagent is electrophilic: the electron deficient proton. But for the halogenation reaction, which one of the two halogen atoms is electron deficient? Such a question is usually greeted with open-palm shrugs from the students. However, “Let us visualize the reaction again. As the molecular bromine (Br–Br) with its big balls of negatively charged electron cloud approaches the negatively charged π-cloud of the alkene, what do you think the two reagents will tell each other? ‘You’re repulsive!’ But this time the Konishiki effect takes over. The fluffy electron cloud of the molecular bromine backs off first in a way that the two atoms will no longer share equally all the electrons between them. Obviously the one closer to the alkene will have less (thus becoming δ+) and the other one more (δ{), leading to the eventual formation of a bromonium and a bromide ion.” -
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Br -
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Br
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Br
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Br
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%$@#&*†!!! d-
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Br -
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d+
Br -
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Br-
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+
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Br+
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Note 1. An immense (>600 lb.) Sumo wrestling star in Japan who, incidentally, was born and raised in Hawaii.
Journal of Chemical Education • Vol. 74 No. 7 July 1997
