Nonclassical ion

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Structure of the 2-norbornyl cation, an iconic nonclassical ion. The structure was deduced from X-ray crystallography. C-C bond lengths not labeled are normal (ca. 1.5 A). CSD CIF HIGNAOhires.png
Structure of the 2-norbornyl cation, an iconic nonclassical ion. The structure was deduced from X-ray crystallography. C-C bond lengths not labeled are normal (ca. 1.5 Å).

In chemistry, a nonclassical ion usually refers to carbonium ions, a family of organic cations. They are characterized by delocalized three-center, two-electron bonds. The more stable members are often bi- or polycyclic. [2] [3]

Contents

Examples

Historically, nonclassical ions were invoked to explain unusually fast solvolyses of steroidal, norbornyl, and cyclopropyl halides. [4] [5] Explanations for these rates was once controversial. [6]

The 2-norbornyl cation is one of the best characterized carbonium ions:

C7H10 + H+ → C7H+11

In fact, it has emerged as the prototype for non-classical ions. As indicated first by low-temperature NMR spectroscopy and confirmed by X-ray crystallography, [1] it has a symmetric structure with an RCH2+ group bonded to an alkene group, stabilized by a bicyclic structure.

Solvolyses of cyclopropylcarbinyl, cyclobutyl, and homoallyl esters are also characterized by very large rates, and have been shown to occur via a common nonclassical ion structure in the form of a bicyclobutonium ion. [7] [8]

Further reading

See also

References

  1. 1 2 Scholz, F.; Himmel, D.; Heinemann, F. W.; Schleyer, P. v R.; Meyer, K.; Krossing, I. (2013-07-05). "Crystal Structure Determination of the Nonclassical 2-Norbornyl Cation". Science. 341 (6141): 62–64. Bibcode:2013Sci...341...62S. doi:10.1126/science.1238849. ISSN   0036-8075. PMID   23828938. S2CID   206549219.
  2. ↑ Thomas H. Lowery; Kathleen Schueller Richardson (1981). Mechanism and Theory in Organic Chemistry, Second Edition. Harper and Rowe. p. 396. ISBN   0-06-044083-X.
  3. ↑ Anslyn, E.V., Dougherty, D.A Modern Physical Organic Chemistry University Science Books 2005
  4. ↑ Sykes P. A guide book to mechanism in organic chemistry 6th Ed., New Delhi: Orient Longman, 1986, p. 111 ff
  5. ↑ Capon, B., McManus, S. P. Neighboring Group Participation Vol. 1, Plenum, New York, 1976
  6. ↑ Schneider, H.-J. The Controversy about Nonclassical Ions – Abandoned too Early? J. Phys. Org. Chem. 2018,
  7. ↑ Saunders, M., Laidig, K.E., Wiberg, K.B., Schleyer Structures, energies, and modes of interconversion of C4H+7 ions J. Am. Chem. Soc., 1988, 110, 7652–7659
  8. ↑ Siehl, H. U. The Conundrum of the (C4H7) Cation: Bicyclobutonium and Related Carbocations Adv. Phys. Org. Chem., 2018, 52, 1-47
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