Commentary pubs.acs.org/jchemeduc
Hyperconjugation: A More Coherent Approach Joseph J. Mullins* Department of Chemistry and Physics, Le Moyne College, Syracuse, New York 13214, United States ABSTRACT: This article is a discussion of hyperconjugation and a proposal for introducing the idea within the framework of other important concepts such as conjugation and resonance. KEYWORDS: Second-Year Undergraduate, Upper-Division Undergraduate, Curriculum, History/Philosophy, Organic Chemistry, Misconceptions/Discrepant Events, Nomenclature/Units/Symbols, Reactive Intermediates, Resonance Theory
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electron-density delocalization that occurs, for example, when sigma-bonded electrons contribute electron density to a p orbital (e.g., carbocation or radical) or a pi system (Figure 1).
he object of chemical educators in is to train students in the concepts, terminology, and applications of the field. One such important term is hyperconjugation. In a previous article, I presented the importance of outlining key concepts of organic chemistry.1 Hyperconjugation was not given its own position but was mentioned: “Hyperconjugation can be considered within the resonance pillar of organic chemistry, and as such can illustrate to students the overlap of these concepts in many cases.”1 In this article, the topic of hyperconjugation is revisited. Alabugin remarked: “Stereoelectronic interactions involving pi-bonds (conjugation) are generally regarded as being among the most important chemical phenomena. Interactions between sigma orbitals (hyperconjugation) have received less attention, although as early as 1941 Robert Mulliken pointed out its importance and, indeed, hyperconjugative stereoelectronic effects were later found to be ubiquitous in chemistry.”2 In a book review on the topic in this Journal many years ago, V. J. Shriner noted:3 “It is an interesting, stimulating and useful exercise to re-examine various concepts extremely critically to see how well we can do without them.” Only a handful of references have been made to hyperconjugation in this Journal since the middle previous century. This article considers hyperconjugation in the context of resonance and conjugation. Resonance receives much attention and has been a subject of discussion and debate among chemical educators.4 Similar debate is appropriate for hyperconjugation: how terminology can be employed to help integrate this idea into the topic of organic chemistry and how we may relate it meaningfully to other topics such as resonance.
Figure 1. Hyperconjugation in the ethyl cation.
Hyperconjugation can lead to student confusion because the previous meanings can be contradictory in the context of chemistry. Many examples of concepts or names in chemistry have sources unrelated to the currently understood chemical reality. Resonance has been mentioned. Aromaticity is another example: this name arose from the observation of early chemists that certain uniquely stable hydrocarbons had a fragrant aroma. Our approach in chemistry should be both true to the current understanding and accessible to students. Hyperconjugation, in the context of the generally accepted definition6 of the prefix “hyper-”, would be taken as abnormally great conjugation, excessive conjugation, or superconjugation (hyper- is used appropriately in chemical terms such as hypervalent). The tendency of this term to confuse students has been recognized and reinforces the call to introduce to students the more appropriate term σ,π-conjugation7 because hyperconjugation has less effect than conjugation.8 For example, an allylic C−H bond is weaker than a primary ethyl C−H bond. This implies that sigma delocalization is not as potent as pi delocalization (interpreted as resonance but certainly arising from the conjugative overlap of p orbitals).9 We should remedy this situation by referring to hyperconjugation as scientists already have for years. In his 1941 article “Hyperconjugation”, Mulliken10 suggested the terms hyperconjugation (second-order conjugation) for the interaction of sigma and pi bonds, and second-order hyperconjugation (third-order conjugation) for sigma−sigma interactions (ordinary pi−pi interactions being “first-order conjugation”). Instructors of organic chemistry should simplify their approach by introducing to students the terms π,π-
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TERMINOLOGY It is known that students learn well when the new material is based upon previous knowledge. Therefore, topics should be introduced in a way that is likely to make use of students’ prior understanding of terms. That is not to say that we should try to eliminate words that are entrenched in past and modern scientific vocabulary. For example, not using the word resonance with students (as has been suggested)4a is not practical, given such widespread use. We should rather address and explain these terms appropriately, placing them in context. Hyperconjugation (originally the Baker−Nathan Effect)5 is a form of © 2012 American Chemical Society and Division of Chemical Education, Inc.
Published: May 15, 2012 834
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conjugation, σ,π-conjugation, and σ,σ-conjugation in the classroom. Such clearly descriptive terms would aid student understanding and reinforce the belief of Mulliken who noted that “the differences in conjugating power among these groups are quantitative rather than qualitative.” Such designations have appeared commonly in research literature;11 however, this article suggests textbook and classroom usage. One could further use such descriptive terms as σ,p-conjugation, σ,π*conjugation, and σ,σ*-conjugation (for carbocation stabilization, stabilization of substituted alkenes, and stabilization of the staggered conformation of ethane, respectively). It is appropriate for undergraduate chemistry because students will have a thorough understanding of s and p orbitals and sigma and pi bonds prior to encountering this subject, thereby ensuring this terminology is introduced in a known context.
between two atoms. I propose that the delocalization in hyperconjugation therefore be referred to as sigma resonance. Such usage would preserve the integrity of the accepted definition of resonance while differentiating between the “normal” resonance that takes place in pi systems. A key proposal of this article is to unify for students the idea that orbital overlap and interaction (conjugation) is the central idea in many stabilizing, delocalizing effects such as resonance and hyperconjugation and that the known terms π,π-conjugation, σ,π-conjugation, and σ,σ-conjugation become the context for presentation.
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NEGATIVE HYPERCONJUGATION Negative hyperconjugation occurs when filled π or p orbitals interact with adjacent antibonding σ* orbitals (contrasted with “positive” hyperconjugation as seen in the ethyl carbocation). An example of this effect can be seen in the trifluoromethoxy anion and in the anomeric effect. Alabugin addresses this issue convincingly.21 This stabilization is often additionally explained using the inductive effect, or the field effect (a through-space effect),22 and in fact these are also likely. The case of the stability of species such as alkyl carbanions and alkyl oxides is noteworthy. The stability of such anions in the gas phase (free from solvent effects, counterion effects, etc.) increases as methyl substitution increases. This might seem unexpected considering that alkyl groups are considered to be electron donating. These observations are understood to be the result of polarization effects.23 Such results would not be expected if only electron-donating hyperconjugative (or electron-donating field effects) predominated.
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CONJUGATION BEFORE HYPERCONJUGATION Consider the relative position of conjugation and hyperconjugation in organic chemistry textbooks: hyperconjugation occurs in many textbooks well before the term conjugation (Table 1). This is similar to asking a student to learn the term Table 1. First Mention of Hyperconjugation and Conjugation in Selected Textbooks First Mention of Textbook
Conjugation/page no.
Hyperconjugation/page no.
McMurry, eighth ed12 Solomons, 10th ed13 Jones, fourth ed14 Carroll,15
500 585 512 850
235 158 377 262
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DELOCALIZATION IS THE KEY Finally, the key to these stabilizing factors is delocalization. Delocalization happens in the presence of conjugation. This gives the term conjugation a central position. Ingold has stated: “Of course, delocalization in the allyl radical is generally called resonance [in comparison to hyperconjugation]. However, the different names we bestow on conceptually different mechanisms of delocalization of an unpaired electron should not mislead us into thinking that one mechanism has different energetic consequences from another mechanism. Delocalizing is stabilizing...”.24,25 These mechanisms are all types of conjugation, but different in the type of bonds or orbitals that are conjugated. In this way one can unify these effects of electron delocalization and distribution. This presentation of conjugation (orbital overlap) can be extended to other examples in organic chemistry such as the anomeric effect (see “Negative Hyperconjugation”).
superconductivity before they understand conductivity. We should define conjugation for our students before hyperconjugation; this can be done, for example, in the context of discussing resonance (resonance is presented in general chemistry and early in organic chemistry texts).
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RESONANCE AND HYPERCONJUGATION Ferreira juxtaposed hyperconjugation and resonance in 1952:16 “...the following treatment may be useful to many organic chemistry teachers who wish to introduce the concept of hyperconjugation together with that of ‘normal’ resonance... ”. The IUPAC definition of resonance17 has no requirement that contributing forms differ only in the placement of pi bonding or nonbonding electrons, and in this light hyperconjugation is a form of resonance. But is hyperconjugation is a form of “double bond−no bond” resonance? The term “double bond−no bond” resonance is found in the literature18 and in the IUPAC definition. 19 Such a presentation is inappropriate and misleading, particularly for presentation in second-year organic chemistry. One reason is that the rules for “normal” resonance allow for the movement only of pi bonding and nonbonding electrons (one example: McMurry, 8th ed., p. 43, “Rule 2: Resonance forms differ only in the placement of their pi or nonbonding electrons.”12) Another text incorrectly states that the sigma bond must remain intact:20 “Rules For Drawing Resonance Contributors...2. Only pi-electrons (electrons in pibonds) and lone-pair electrons can move; sigma-electrons never move.” Second, although resonance forms need not be identical in energy or stability to one another, one structure unrealistically has no sigma bond (i.e., no electron density)
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CONCLUSION In conclusion, there are three termsconjugation, hyperconjugation, and resonancethat involve delocalization of electron density (I am always careful to say that electron density is delocalized, not electrons). Perhaps a better term is electron distribution. Electrons can be more-or-less widely distributed leading to important differences in stability that are crucial for understanding organic chemistry. Appropriate terms should be presented in a logical order as they relate to conjugation, because whatever else is true, the relevant orbitals are properly aligned and overlapping. 835
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electronic structure of a molecular entity in terms of contributing structures. Resonance among contributing structures means that the wavefunction is represented by ’mixing’ the wavefunctions of the contributing structures. The concept is the basis of the quantum mechanical valence bond methods. The resulting stabilization is linked to the quantum mechanical concept of ’resonance energy’. The term resonance is also used to refer to the delocalization phenomenon itself.” (18) Hine, J. J. Am. Chem. Soc. 1963, 85, 3239−3244. Davies, A. G. J. Chem. Soc., Perkin Trans. 2 1999, 2461−2467. (19) IUPAC Compendium of Chemical Terminology, 2nd ed. (the “Gold Book”). Compiled by McNaught, A. D.; Wilkinson, A.; Blackwell Scientific Publications: Oxford, 1997. XML online corrected version: http://goldbook.iupac.org (2006-) created by M. Nic, J. Jirat, B. Kosata; updates compiled by A. Jenkins. DOI: 10.1351/goldbook. “Hyperconjugation: In the formalism that separates bonds into σ and π types, hyperconjugation is the interaction of σ-bonds (e.g. C−H, C−C, etc.) with a π network. This interaction is customarily illustrated by contributing structures, e.g. for toluene (below), sometimes said to be an example of ‘heterovalent’ or ‘sacrificial hyperconjugation’, so named because the contributing structure contains one two-electron bond less than the normal Lewis formula for toluene... At present, there is no evidence for sacrificial hyperconjugation in neutral hydrocarbons. The concept of hyperconjugation is also applied to carbenium ions and radicals, where the interaction is now between σ-bonds and an unfilled or partially filled π- or p-orbital. ... This latter example is sometimes called an example of ‘isovalent hyper-conjugation’ (the contributing structure containing the same number of two-electron bonds as the normal Lewis formula). Both structures shown on the right hand side are also examples of ‘double bond-no-bond resonance’. The interaction between filled π- or p- orbitals and adjacent antibonding σ* orbitals is referred to as ‘negative hyperconjugation’, as for example in the fluoroethyl anion...” (20) Bruice, P. Y. Organic Chemistry, 5th ed.; Pearson Education, Inc.: Upper Saddle River, NJ, 2007; p 293. (21) Alabugin, I. V.; Zeidan, T. A. J. Am. Chem. Soc. 2002, 124 (12), 3175−3185. “The C-H bond is a mediocre sigma donor. It is interesting to compare the acceptor ability of C-Hal bonds toward a considerably better donor, a lone pair at nitrogen....a carbanionic center is even a better donor ... Therefore, the hyperconjugative donoracceptor interactions in beta-halogen anions is expected to be very strong.” (22) Bunnet, J. F. Bull. Hist. Chem. 1996, 19, 33−42. “...experimental evidence indicating operation of what is now generally termed the f ield ef fect has been obtained...the effect stems from the interaction of the substituent-to-carbon bond with an electrical charge or dipole...” (23) Brauman, J. I.; Blair, L. K. J. Am. Chem. Soc. 1968, 90 (23), 6561−6562. “‘The stabilization observed is consistent with a model in which the alkyl groups, being polarizable, stabilize the charge by an induced dipole.’ It is to be expected that methyl or methylene groups will be more effective than hydrogen in stabilizing both positive and negative charges and that such stabilization will be of more importance in the gas phase than in solution.” Streitwieser, A.; Keevil, T. A.; Taylor, D. R.; Dart, E. C. J. Am. Chem. Soc. 2005, 127 (25), 9290− 9297. “Similarly the gas-phase acidity of the tertiary position in isobutane is greater than that of the secondary position in propane. It is now well understood that polarization plays an important role in the stabilization of anions in the gas phase.” (24) See Ingold, K. U., ref 9. (25) Not all forms of delocalization are stabilizing, as in antiaromaticity (the author thanks a manuscript reviewer for advising this inclusion).
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected].
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ACKNOWLEDGMENTS The author thanks C. Giunta, M. Masingale, and T. Morrill for helpful discussions and review of this manuscript and reviewers who made beneficial suggestions.
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REFERENCES
(1) Mullins, J. J. J. Chem. Educ. 2008, 85 (1), 83−87. (2) Alabugin, I. V.; Zeidan, T. A. J. Am. Chem. Soc. 2002, 124 (12), 3175−3185. (3) Shriner, V. J. J. Chem. Educ. 1963, 40 (4), 232. (4) (a) Kerber, R. C. J. Chem. Educ. 2006, 83, 223−227. (b) Jensen, W. B. J. Chem. Educ. 2006, 83, 1290. Kerber, R. C. J. Chem. Educ. 2006, 83, 1291. (5) Baker, J. W.; Nathan, W. S. J. Chem. Soc. 1935, 1844. (6) Webster’s Revised Unabridged Dictionary, Porter, N., Ed.; G. & C. Merriam Co: Springfield, MA, 1913. Ref.: Hyper- Hy″per- [Gr. ″ypèr over, above; akin to L. super, E. over. See Over, and cf. Super-.] 1. A prefix signifying over, above; as, hyperphysical, hyperthyrion; also, above measure, abnormally great, excessive; as, hyper[ae]mia, hyperbola, hypercritical, hypersecretion. 2. (Chem.) A prefix equivalent to super- or per-; as hyperoxide, or peroxide. [Obs.] See Per-. (7) Traylor, T. G.; Berwin, H. J.; Jerkunica, J.; Hall, M. L. Pure Appl. 1972, 30 (3), 599−606. “The term sigm-pi conjugation as used in this and our other papers is taken to be synonymous with hyperconjugation as described by Mulliken. Curiously, although the term ’hyperconjugation’, meaning very large conjugation, was a misnomer in its original application to C−H and C−C−x conjugation, it now takes on a correct meaning in that sigm-pi conjugation can be very large indeed. However, the term sigm−pi conjugation is preferable because it does not contain the reference to magnitude implied by the prefix ’hyper’.” (8) Alabugin, I. V. WIREs Comput. Mol. Sci. 2011, 1, 110. DOI: 10.1002/wcms.6. “...hyperconjugation with strong σ-donors rival stabilization due to the conventional conjugation patterns, but the two effects are sometimes difficult to distinguish.” (9) As given in bond dissociation energies in: Ingold, K. U.; DiLabio, G. A. Org. Lett. 2006, 8 (26), 5923−5925. (10) Mulliken, R. S.; Rieke, C. A.; Brown, W. G. J. Am. Chem. Soc. 1941, 63, 41−56. (11) (a) Shishkina, S. V.; Shishkin, O. V.; Desenko, S. M.; Leszczynski, J. J. Phys. Chem. A 2008, 112, 7080−7089. (b) Roberts, D. D. J. Org. Chem. 1984, 49, 2521−2526. (c) Kitching, W.; Drew, G. J. Org. Chem. 1981, 46, 2252−2260. “σ - π Conjugation in benzyl derivatives of tin and mercury as probed by tin-119 and mercury-199 resonance” “The interaction of a polarized carbon-metal sigma bound with an adjacent pi system (carbon-metal hyperconjugation or sigmapi conjugation) can be manifested by impressive changes in chemical reactivity’ ” (12) McMurry, J. M. Organic Chemistry, 8th ed.; Brooks/Cole Cengage Learning: Independence, KY,2012. (13) Solomons, T. W. G., Fryhle, C. Organic Chemistry, 10th ed.; John Wiley & Sons, Inc.: New York, 2009. (14) Jones Jr., M., Fleming, S. A. Organic Chemistry, 4th ed.; W. W. Norton & Company, Inc: New York, 2009. (15) Carroll, F. A. Perspectives on Structure and Mechanism in Organic Chemistry; Brooks/Cole: Pacific Grove, CA, 1997. (16) Ferreira, R. C. J. Chem. Educ. 1952, 29 (11), 554. (17) IUPAC Compendium of Chemical Terminology, 2nd ed. (the “Gold Book”). Compiled by McNaught, A. D.; Wilkinson, A.; Blackwell Scientific Publications; Oxford, 1997. XML online corrected version: http://goldbook.iupac.org (2006-) created by M. Nic, J. Jirat, B. Kosata; updates compiled by A. Jenkins. DOI: 10.1351/goldbook. “ In the context of chemistry, the term refers to the representation of the 836
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