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Why Benzene prefers to substitute and an Olefin likes to add?

  • Nicolaos Demetrios Epiotis
Chapter
Part of the Lecture Notes in Chemistry book series (LNC, volume 34)

Abstract

Hückel MQ (HMO) theory,1,2 predicts that pi benzene is more stable than three pi ethylenes. This conclusion seems to be compatible with the fact that the heat of hydrogenation of benzene is much less than the heat of hydrogenation of three cyclohexenes3 and the known unwillingness of benzene to undergo addition reactions, opting for “aromatic” substitution instead. These data have prompted an on-going preoccupation with “resonance energies”, “aromaticity”, and the like. In this chapter, we suggest that, while the experimental facts are indisputable, the concepts which chemists have devised over more than a century are most likely erroneous and that benzene is pi destabilized but operationally “aromatic”.

Keywords

Resonance Energy Configuration Wave Sigma Bond Interelectronic Repulsion Total Excitation Energy 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

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References

  1. 1.
    Hückel, E., Z. Physik. 1930, 60, 423; ibid. 1931, 70, 204; ibid. 1932, 76, 628.CrossRefGoogle Scholar
  2. Hückel, E., Z. Electrochem. 1937, 43, 752.Google Scholar
  3. 2.
    For pedagogical presentation and application of Hückel MO theory, see, inter alia:Google Scholar
  4. (a).
    Streitwieser, Jr., A., “Molecular Orbital Theory for Organic Chemists”; John Wiley and Sons, Inc.: New York, 1961.Google Scholar
  5. (b).
    Heilbronner, E., Bock, H., “Das HMO-Modell und Seine Anwendung”; Verlag Chemie, Gmbh: Weinheim, 1968.Google Scholar
  6. 3.
    From thermochemical data, the heat of hydrogenation of benzene (to form cyclohexane) is 49 kcal/mol and that of cyclohexene (to form cyclohexane) 29 kcal/mol. Thus, the heat of hydrogenation of three cyclohexenes is much larger than that of benzene.Google Scholar
  7. (b).
    Pople, J.A., Untch, K.G., J. Am. Chem. Soc. 1966, 88, 4811.CrossRefGoogle Scholar
  8. 5.
    Glasstone, S., Laidler, K.J., Eyring, H., “The Theory of Rate Processes”; McGraw-Hill: New York, 1941, and references therein.Google Scholar
  9. 6.
    According to DIM theory with neglect of non-neighbor overlap, 3A2 have a total energy of Q + 3T while hexagonal A6 an energy of Q1 + 2.61T, where Q is the coulomb and T the exchange integral of VB theory. For neutral fragments, Q = Q1, which means that 3A2 lie below A6 even if non-neighbor overlap is neglected. For a pedagogical discussion of pi ethylene (A2) and pi benzene (A6) see.Google Scholar
  10. Sandorfy, C. “Electronic Spectra and Quantum Chemistry”; Prentice-Hall: Englewood Cliffs, NJ, 1964.Google Scholar
  11. 7.
    Dixon, D.A., Stevens, R.M., Herschbach, D.R., Faraday Discussion Chem. Soc. 1977, 62, 110. This paper is of central importance as it clearly projects the differing binding mechanisms of nonmetallic and metallic atoms, something having profound chemical implications.CrossRefGoogle Scholar
  12. 8.
    For computational results, see: Borden, W.T., Davidson, E.R., Hart, P., J. Am. Chem. Soc. 1978, 100, 388.CrossRefGoogle Scholar

Copyright information

© Springer-Verlag Berlin Heidelberg 1983

Authors and Affiliations

  • Nicolaos Demetrios Epiotis
    • 1
  1. 1.Department of ChemistryUniversity of WashingtonSeattleUSA

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