In the Classroom edited by
Overhead Projector Demonstrations
Doris K. Kolb Bradley University Peoria, IL 61625
Of Magnets and Mechanism
Edward G. Neeland Department of Chemistry, Okanagan University-College, Kelowna, BC V1V 1V7, Canada;
[email protected] Children (…and adults) experience the wonder of bringing two magnets closer and closer to see if they will stick together—or more mysteriously, push away from each other. Educators have used this interaction of like or opposite poles in magnets to facilitate the understanding of many different chemical concepts: VSEPR theory and bonding/ antibonding (1), visualizing biomolecules (2), solubility (3), orbital filling (4 ), crystal lattices and hydrogen bonding (5), paramagnetism (6 ), oil spill adsorption (7 ), and scanning probe microscopy (8). Recently, we discovered that bar magnets are also tailor-made for demonstrating the electron flow (mechanism) of a chemical reaction.
H S
N
H
N
S
H
Figure 1. The SN2 reaction with bar magnets substituting for OH᎑ and Br᎑ ions. H S
NH H
N
S
Figure 2. Initial nucleophilic attack and subsequent loss of the bromine atom.
Method The demonstration begins with an overhead projection showing the hydroxide anion and bromomethane positioned to undergo a classic SN2 reaction. The class is asked to identify charged or slightly charged atoms (in this case, the hydroxide anion and the dipole between the carbon and bromine atoms). The diagram is then labeled as shown below: H HO−
H
+ Br H δ δ−
Next, it is pointed out that opposites attract in a Lewis base–Lewis acid manner and a reaction is expected between these molecules. The class has no trouble identifying the atoms that will react, and the two-electron flow in the initial nucleophilic attack is represented with the use of an arrow:
Thus the class watches the mechanism of a simple SN2 reaction. The approach of the nucleophile to the electrophilic carbon atom forces the leaving group away. Afterwards we discuss the transition state (by slowly moving the OH᎑ bar magnet toward the electrophilic carbon), and talk about why the bromine was repelled in terms of pentavalency of carbon, stability of the bromide anion, and the overall balance of “electron flow in” and “electron flow out”. A disadvantage of this analogy is that it does not show the Walden inversion at the carbon center. This is discussed later with the class as a separate topic. Alternatively, the hydroxide bar magnet can be used to attack a carbocation in an SN1 reaction, where the carbocation is replaced with an iron disk to show that no accompanying loss of electron density is necessary to complete the electron flow, as was the case for the SN2 reaction.
H HO−
H
+ Br H δ δ−
However, the accompanying loss of the bromine leaving group is not fully grasped by the class, and this is what prompted this demonstration. Figure 1 shows bar magnets laid on top of the hydroxide nucleophile and the bromine leaving group. Since the class understands the first step of the mechanism, we push the hydroxide magnet to the right, and the class is surprised to see that the bromine bar magnet is pushed away quite forcefully (Fig. 2). We repeat this demonstration a few times, go back to the original reaction transparency, and complete it with a second arrow showing the bromine leaving the molecule: H HO−
H
+ Br H δ δ−
No words are spoken during this time, as it is a clear comparison to the magnet reaction. 186
Acknowledgments Many thanks to Kitrin and Kira who first suggested this exercise. Literature Cited 1. Schobert, H. H. J. Chem. Educ. 1973, 50, 651. Shaw, C. F.; Shaw, B. A. J. Chem. Educ. 1991, 68, 861. Hervas, M.; Silverman , L. P. J. Chem. Educ. 1991, 68, 861. 2. Jones, B. L.; Kramer, K. J. J. Chem. Educ. 1981, 58, 72. 3. Kjonaas, R. A. J. Chem. Educ. 1984, 61, 765. Garde, I. B. J. Chem. Educ. 1987, 64, 154. 4. Hill, J. W. J. Chem. Educ. 1990, 67, 320. 5. Davies, W. G. J. Chem. Educ. 1991, 68, 245. 6. Cortel, A. J. Chem. Educ. 1998, 75, 61. Tweldemedhin, Z. S.; Fuller, R. L.; Greenblatt, M. J. Chem. Educ. 1996, 73, 906. 7. Orbell, J. D.; Godhino, L.; Bigger, S.W.; Nguyen, T. M.; Ngeh, L. N. J. Chem. Educ. 1997, 74, 1446. 8. Lorentz, J. K.; Olson, J. A.; Campbell, D. J.; Lisensky, G. C.; Ellis, A. B. J. Chem. Educ. 1997, 74, 1032A.
Journal of Chemical Education • Vol. 79 No. 2 February 2002 • JChemEd.chem.wisc.edu
