In the Classroom
"You’re Repulsive!"—Teaching VSEPR in a Not-So-Elegant Way Robert S. H. Liu Department of Chemistry, University of Hawaii at Manoa, Honolulu, HI 96822;
[email protected] Valence shell electron pair repulsion is a useful concept that has appeared in many general chemistry (1) and introductory organic chemistry texts (2). The acronym VSEPR is often used and pronounced as Ves-Per. But, is such an acronym a help to the students for retaining the concept? In the first place, Ves-Per does not correspond to the order of the five letters. How likely then will the students be able to associate the acronym with the correct phrase (and the concept) a year later? Is it VESPER or VESPR or VSPER? But, not likely VSEPR. To retain that information they must learn to say VVV-Sep-RRR! I approach the topic in a different way in my organic chemistry classes where VSEPR is usually used only with the hybridized orbitals of carbon. An additional goal of mine is to show that the concept of repulsion among valence electrons can also be used in discussions of organic reactivity and spectroscopic properties. I shall illustrate my approach with a few examples that I have used in introductory and physical organic chemistry classes.
SIVE!” The electrons of the fluffy Br2 back up as if in a state of shock, creating one bromine atom with a δ+ charge and the other one with a δ− charge. This polarization amplifies as the two reactants get closer. Eventually a Br+ is produced, which adds to the double bond to form a bromonium ion (hence the electrophilic addition). At the same time, a bromide ion is expelled.
Br
Br H
Br
H
H
H
H #&$?%†!!
δ−
δ+
Br
H H
H
ⴚ
Br
Halogenation Reaction A hydrohalogenation reaction (3) is an electrophilic addition reaction because it starts with the addition of an electron deficient proton (an electrophile) to an electron rich π-bond (a nucleophile).
H H
Brⴙ H
H H
H
H
H
Br
H
H
H
+ H
H ⴙ Brⴚ
H
ⴙ
H
H
H
Br ⴚ
+
Br
H
Nucleophilic Substitution Reactions H
H
H
H
Br
H
But why is the halogenation reaction also considered an electrophilic addition reaction? Which of the two bromine atoms (for example) is positively charged or electron deficient? I usually received shrugs from students when I asked this question, and for good reasons. H
H
H
+ H
Br
Br
H
Br H
Let us examine the reaction in a stepwise manner. As the bromine molecule approaches the π-bond, the first things coming in contact with each other are the negatively charged valence electrons of Br2 and the negatively charged π-cloud. What do you think they tell each other? “YOU’RE REPUL-
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Br
Nuⴚ
Nu
+ Brⴚ
+
H
Br H
Nucleophilic substitution reactions (3) of aryl halides (e.g., bromobenzene) or vinyl halides (e.g., 1-bromocyclohexene) are extremely slow reactions while substitutions of comparable saturated structures (e.g., bromocyclohexane) proceed at reasonably fast rates. Why?
Br ⴚ
Nu
+
no reaction
Br ⴚ
Nu
+
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no reaction
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In the Classroom
The answer becomes obvious when we examine the reactions more closely. When an electron rich nucleophile approaches bromocyclohexane, it seeks out the partially positively charged carbon and attacks from the back side to displace the bromide ion in a manner typical of the Walden inversion. When the negatively charged nucleophile attempts to seek out the δ+ carbon in bromobenzene or bromocyclohexene, it first bumps into the negatively charged π-cloud. Under the circumstance, what do you think they tell each other? “YOU’RE REPULSIVE!” Are they going to react? Not a chance! They back off from each other. Instead, the nucleophile could change the direction of attack to that perpendicular to the p orbitals, thereby initiating an E2 reaction.
#&$?%†!!
ⴚ
Figure 1. Electron distribution in HOMO and LUMO of azulene and naphthalene.
Br δ−
Nu
Why Is Azulene Blue While the Isomeric Naphthalene Is Colorless?
δ+
Fluorine Chemical Shifts 19
F is not only a sensitive nucleus for NMR studies (4) but also has a huge range of chemical shifts (∼300 ppm) (5). Hence it is an ideal label for detecting intramolecular or intermolecular perturbations. When a F-label is introduced into a protein binding site, it often exhibits a 3–8 ppm downfield shift commonly known among protein chemists as the protein shift (5). In small organic compounds, steric crowding can cause a substantial downfield shift. This is best represented by the series of 1,8-substituted fluoronaphthalenes (F chemical shifts of ᎑123.9, ᎑112.8, and ᎑96.4 ppm for 8-H, 8-methyl, and 8-t-butyl compounds, respectively, giving a downfield F-shift of 11.1 and 27.5 ppm for the two substituted naphthalenes) (6). What is the origin of all these downfield shifts? F
R
#&$?%†!! F
R
R = H, Me, t-Bu
Above on the right is a cartoon figure for the 1,8-substituted fluoronaphthalenes. As the R group increases in size, it comes in close van der Waals contact with the fluoro-substituent. When the negatively charged valence electrons of F and R stare at each other at a short distance, what do you think they tell each other? “YOU’RE REPULSIVE!” The net result is that the valence electrons on the fluorine atom back away, dispersing toward the bonded C-atom. Hence, the F-atom becomes deshielded while the C-atom is more shielded. The protein shift results from a similar interaction between the nonmobile protein residue(s) and the juxtaposed F-label. www.JCE.DivCHED.org
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Azulene and naphthalene have the same molecular formulas and the same total of five conjugated π-bonds. However, one compound is intensely colored (azure or blue) while the other is colorless (7). The reason is due largely to the fact that one is an alternant hydrocarbon (naphthalene) and the other, a nonalternant hydrocarbon (azulene). 8
1
α
7
2
6
α β
β β
3
4 azulene
5
β α
α
naphthalene
For the lowest excited states, distributions of the electron densities in the HOMO and LUMO orbitals of azulene and naphthalene are shown in Figure 1 (8). In the excited state, the HOMO and LUMO are singly occupied. However, the HOMO–LUMO gap is not the only factor determining the transition energy between the S0 and S1 states (Figure 1). The two electrons in the two orbitals of azulene (representative of nonalternant hydrocarbons) occupy different spaces. The HOMO electron is localized largely at C-1 and C-3 and less so at C-5 and C-7 while the LUMO electron largely is located at C-4, C-6, and C-8 and less so at C-2. A different situation exists in excited naphthalene (representative of alternant hydrocarbons). The HOMO and LUMO electrons occupy exactly the same space: mostly at α-carbons and less so at β-carbons. This means in the latter case the two negatively charged electrons are enclosed in a small “room” (the sigma framework). While staring at each other all the time, what do you think the two negatively charged electrons tell each other? “YOU’RE REPULSIVE!” This results in an increase of the S1-level owing to electron–electron repulsive interaction, making the transition occur in the UV region, hugely higher in energy than that of azulene (7, 9).
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In the Classroom
Why Is the Triplet State Lower in Energy Than the Corresponding Singlet State? In discussing photochemistry, we customarily start with the Jablonski diagram in which the triplet state is always placed below the corresponding singlet state. Why? For a twoelectron system, there are four possible spin wave functions: α (1) α (2)
Acknowledgments
β (1) β (2)
I thank the students for providing me the opportunity to try out different ways of teaching chemical concepts during my 36 years at the University of Hawaii, and the Hawaii chapter of the ACS (10) for the opportunities to share my experiences.
α (1) β (2) + β (1) α (2 ) α (1) β (2) − β (1) α (2 )
The top three are symmetric wave functions (no change of signs upon interchanging electron coordinates, 1 and 2). The last one is antisymmetric (a change of signs upon changing electron coordinates). Therefore, the top three are similar, labeled as a group, the triplet states; and the last, that of the singlet state. The wave functions for the entire molecule must include the space portion of the wave functions. Because of antisymmetric properties of matter, only the following four are acceptable wave functions: φ (1) φ (2) − φ (2) φ (1) α (1) α (2) φ (1) φ (2) − φ (2) φ (1) β (1) β (2) φ (1) φ (2) − φ (2) φ (1) α (1) β (2) + β (1) α (2) φ (1) φ (2) + φ (2) φ (1) α (1) β (2) − β (1) α (2 )
Of these, we already mentioned that the top three are those of the triplet states and the last that of the singlet state. They show that the space portion of the triplet wave functions is antisymmetric while that of the singlet state is symmetric. This means that in the latter case interchanging electrons does not change signs of the wave function. It also says that only in the singlet state are the two electrons close to each other. In the triplet state the two electrons tend to avoid each other or occupy different spaces. Therefore, in the singlet state, the two negatively charged electrons always stare at each other in close proximity. What do you think they tell each other? “YOU’RE REPULSIVE!” That is why it is generally said that the electron–electron correlation term makes the singlet state higher in energy than the triplet states (9). Summary All these examples involve repulsive interactions between valence electrons although not all of them involve valence shell electron pairs. They are clear extensions of the concept valence shell electron pair repulsion. It might be of interest
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to mention that some of my former students, while in reiterating “You’re repulsive!” demonstrated varying degrees of retention of the chemical concept. Would that be possible if the students were told about Ves-Per only? One likely criticism of my method is that it is not elegant. Perhaps so. But since the method is effective as an instructional tool, at least to my students, lack of elegance is a sacrifice that I am ready to accept.
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Literature Cited 1. (a) Chang, R. Chemistry, 6th ed.; McGraw-Hill: Boston, 1998; pp 386–388. (b) Whitten, K. W.; Davis, R. E.; Peck, M. L.; Stanley, G. G. General Chemistry, 7th ed.; Thomson Brooks/ Cole: Belmont, CA, 2004; pp 305–306. (c) Zumdahl, S. S.; Zumdahl, S. A. Chemistry, 6th ed.; Houghton Mifflin Co.: Boston, 2003; pp 389–392. 2. (a) Brown, W. H.; Foote, C. S. Organic Chemistry, 3rd ed.; Harcourt College Publishers: Fort Worth, TX, 2002; pp 20– 23. (b) Bruice, P. Y. Organic Chemistry, 4th ed.; Prentice Hall: Upper Saddle River, NJ, 2004; p 24. (c) Carey, F. A. Organic Chemistry, 3rd ed.; McGraw-Hill Co., Inc.: New York, 1996; pp 26–28. (d) Solomons, T. W. G.; Fryhle, C. B. Organic Chemistry, 8th ed.; John Wiley & Sons, Inc.: New York, 2003; pp 37–40. (e) Wade, L. G., Jr. Organic Chemistry, 5th ed.; Pearson Education, Inc.: Upper Saddle River, NJ, 2003; p 44. (f ) Vollhardt, K. P. C.; Schore, N. E. Organic Chemistry, 3rd ed.; W. H. Freeman, Co.: New York, 1999; p 14. 3. The first two examples were discussed elsewhere for a different purpose: Liu, R. S. H.; Asato, A. E. J. Chem. Educ. 1997, 74, 783–784. 4. (a) Colmenares, L. U.; Niemczura, W. P.; Asato, A. E.; Liu, R. S. H. J. Phys. Chem. 1996, 100, 9175–9180; (b) Colmenares, L. U.; Liu, R. S. H. J. Photosci. 2003, 10, 81– 86. 5. Gerig, J. T. Biol. Magn. Reson. 1978, 1, 139–203. 6. Gribble, G. W.; Keavy, D. J.; Olsen, E. R.; Rae, I. D.; Staffia, A.; Herr, T. E.; Ferraro, M. B.; Contreras, R. H. Magn. Reson. Res. Chem. 1991, 29, 422–432. 7. Liu, R. S. H. J. Chem. Educ. 2002, 79, 183–185. 8. (a) Lemal, D. M.; Goldman, G. D. J. Chem. Educ. 1988, 65, 923–925; (b) Liu, R. S. H.; Muthyala, R.; Wang, X.-S.; Asato, A. E.; Wang, P.; Ye, C. Org. Lett. 2000, 2, 269–271. 9. (a) McGlynn, S. P.; Azumi, T.; Kinoshita, M. Molecular Spectroscopy of the Triplet State; Prentice Hall, Inc.: Upper Saddle River, NJ, 1969; Chapter 3. (b) Klessinger, M.; Michl, J. Excited States and Photochemistry of Organic Molecules; VCH Publishers: New York, 1995; Chapter 1 and p 234. 10. This paper is a condensed version of a lecture presented at the ACS Hawaii Sectional Meeting in August 2002.
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