Nonclassical ion explained

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.^{[1] [2]}

Examples

Historically, nonclassical ions were invoked to explain unusually fast solvolyses of steroidal, norbornyl, and cyclopropyl halides.^{[3] [4]} Explanations for these rates was once controversial.^[5]

The 2-Norbornyl cation is one of the best characterized carbonium ion. It is the prototype for non-classical ions. As indicated first by low-temperature NMR spectroscopy and confirmed by X-ray crystallography, 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.^{[6] [7]}

Further reading

• Brown, H. C. (with commentary by P. v. R. Schleyer) The Nonclassical Ion Problem; Plenum Press:  New York /Springer 1977 https://www.springer.com/de/book/9781461341208
• Moss, R.A. The 2-norbornyl cation: a retrospective J. Phys. Org. Chem. 2014, 27, 374-379 https://onlinelibrary.wiley.com/doi/full/10.1002/poc.3290

See also

• 2-Norbornyl cation
• Neighbouring group participation
• Carbocation
• Steric effects
• Solvation

Notes and References

1. Book: Mechanism and Theory in Organic Chemistry, Second Edition. Thomas H. Lowery. Kathleen Schueller Richardson. Harper and Rowe. 1981. 0-06-044083-X. 396.
2. Anslyn, E.V., Dougherty, D.A Modern Physical Organic Chemistry University Science Books 2005http://www.uscibooks.com/adfm_new.pdf
3. Sykes P. A guide book to mechanism in organic chemistry 6th Ed., New Delhi: Orient Longman, 1986, p. 111 ffhttp://about.mdma.ch/picproxie_docs/000536454-Guidebook_to_Mechanism_in_Organic_Chemistry.pdf
4. Capon, B., McManus, S. P. Neighboring Group Participation Vol. 1, Plenum, New York, 1976 https://link.springer.com/content/pdf/bfm%3A978-1-4684-0826-3%2F1.pdf
5. Schneider, H.-J. The Controversy about Nonclassical Ions – Abandoned too Early? J. Phys. Org. Chem. 2018,https://onlinelibrary.wiley.com/doi/10.1002/poc.3846
6. Saunders, M., Laidig, K.E., Wiberg, K.B., Schleyer Structures, energies, and modes of interconversion of ions J. Am. Chem. Soc., 1988, 110, 7652–7659 https://pubs.acs.org/doi/pdf/10.1021/ja00231a012
7. Siehl, H. U. The Conundrum of the (C4H7) Cation: Bicyclobutonium and Related Carbocations Adv. Phys. Org. Chem., 2018, 52, 1-47 https://www.elsevier.com/books/advances-in-physical-organic-chemistry/williams/978-0-12-815211-9